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Efcore Principal Engineer Guide

Advanced 5 min read 1000 words

Entity Framework Core - Principal Engineer Deep Dive

A comprehensive guide covering EF Core internals, query optimization, advanced modeling, and production patterns for Principal/Staff Engineers.

Table of Contents

  1. Loading Strategies Deep Dive
  2. Query Optimization
  3. Advanced Modeling
  4. Concurrency & Transactions
  5. Production Patterns
  6. Raw SQL & Advanced Operations

1. Loading Strategies Deep Dive

1.1 Eager Loading

Eager loading uses the Include() and ThenInclude() methods to load related data in a single query.

Basic Usage

// Single level Include
var orders = await context.Orders
    .Include(o => o.Customer)
    .Include(o => o.ShippingAddress)
    .ToListAsync();

// Multi-level Include with ThenInclude
var orders = await context.Orders
    .Include(o => o.OrderItems)
        .ThenInclude(oi => oi.Product)
            .ThenInclude(p => p.Category)
    .Include(o => o.Customer)
        .ThenInclude(c => c.Address)
    .ToListAsync();

[SQL] Generated SQL Analysis

// This query:
var order = await context.Orders
    .Include(o => o.OrderItems)
        .ThenInclude(oi => oi.Product)
    .Include(o => o.Customer)
    .FirstOrDefaultAsync(o => o.Id == orderId);

// Generates SQL like:
/*
SELECT [o].[Id], [o].[CustomerId], [o].[OrderDate], [o].[Total],
       [c].[Id], [c].[Name], [c].[Email],
       [t].[Id], [t].[OrderId], [t].[ProductId], [t].[Quantity],
       [t].[Id0], [t].[Name], [t].[Price]
FROM [Orders] AS [o]
LEFT JOIN [Customers] AS [c] ON [o].[CustomerId] = [c].[Id]
LEFT JOIN (
    SELECT [oi].[Id], [oi].[OrderId], [oi].[ProductId], [oi].[Quantity],
           [p].[Id] AS [Id0], [p].[Name], [p].[Price]
    FROM [OrderItems] AS [oi]
    INNER JOIN [Products] AS [p] ON [oi].[ProductId] = [p].[Id]
) AS [t] ON [o].[Id] = [t].[OrderId]
WHERE [o].[Id] = @orderId
ORDER BY [o].[Id], [c].[Id], [t].[Id]
*/

The Cartesian Explosion Problem

// PROBLEM: Multiple collection Includes cause Cartesian explosion
var orders = await context.Orders
    .Include(o => o.OrderItems)      // 10 items per order
    .Include(o => o.Payments)         // 3 payments per order
    .Include(o => o.StatusHistory)    // 5 statuses per order
    .ToListAsync();

// For 100 orders: 100 × 10 × 3 × 5 = 15,000 rows returned!
// Each order data is duplicated 150 times

// SOLUTION: Use AsSplitQuery or separate queries
var orders = await context.Orders
    .Include(o => o.OrderItems)
    .Include(o => o.Payments)
    .Include(o => o.StatusHistory)
    .AsSplitQuery()  // Generates separate queries
    .ToListAsync();

[BENCHMARK] Performance with Different Relationship Depths

// Benchmark: Loading 100 orders with varying Include depths
// Database: SQL Server, 10,000 orders, 5 items/order average

| Depth | Includes | Rows Returned | Query Time | Memory |
|-------|----------|---------------|------------|--------|
| 1     | Customer | 100           | 5ms        | 2MB    |
| 2     | +Items   | 500           | 12ms       | 8MB    |
| 3     | +Product | 500           | 18ms       | 12MB   |
| 4     | +Category| 500           | 25ms       | 15MB   |

// With Cartesian explosion (multiple collections):
| Collections | Rows Returned | Query Time | Memory |
|-------------|---------------|------------|--------|
| 1 (Items)   | 500           | 12ms       | 8MB    |
| 2 (+Payments)| 1,500        | 45ms       | 25MB   |
| 3 (+History)| 7,500         | 180ms      | 95MB   |

1.2 Explicit Loading

Explicit loading allows you to load related data on demand after the parent entity is loaded.

// Load the order first
var order = await context.Orders.FindAsync(orderId);

// Then explicitly load related data
await context.Entry(order)
    .Collection(o => o.OrderItems)
    .LoadAsync();

await context.Entry(order)
    .Reference(o => o.Customer)
    .LoadAsync();

// Conditional loading with Query()
await context.Entry(order)
    .Collection(o => o.OrderItems)
    .Query()
    .Where(oi => oi.Quantity > 5)
    .LoadAsync();

When to Use Explicit Loading

public class OrderService
{
    public async Task<OrderDto> GetOrderWithDetailsAsync(int orderId, bool includeItems)
    {
        var order = await _context.Orders
            .Include(o => o.Customer)
            .FirstOrDefaultAsync(o => o.Id == orderId);

        if (order == null) return null;

        // Only load items if needed
        if (includeItems)
        {
            await _context.Entry(order)
                .Collection(o => o.OrderItems)
                .Query()
                .Include(oi => oi.Product)
                .LoadAsync();
        }

        return MapToDto(order);
    }
}

[INTERNALS] Change Tracker Interaction

// Explicit loading uses the Change Tracker to avoid duplicate queries
var order = await context.Orders.FindAsync(orderId);

// First load - executes query
await context.Entry(order).Collection(o => o.OrderItems).LoadAsync();

// Second load - checks IsLoaded flag, skips query
await context.Entry(order).Collection(o => o.OrderItems).LoadAsync();

// Check if already loaded
var entry = context.Entry(order);
bool itemsLoaded = entry.Collection(o => o.OrderItems).IsLoaded;

// Force reload even if already loaded
await context.Entry(order)
    .Collection(o => o.OrderItems)
    .Query()
    .LoadAsync();  // Query() creates new query, ignores IsLoaded

1.3 Lazy Loading

Lazy loading automatically loads related data when the navigation property is accessed.

Setup with Proxies

// Package: Microsoft.EntityFrameworkCore.Proxies
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .UseLazyLoadingProxies());

// Entities must have virtual navigation properties
public class Order
{
    public int Id { get; set; }
    public virtual Customer Customer { get; set; }  // virtual required
    public virtual ICollection<OrderItem> Items { get; set; }
}

Setup with ILazyLoader (No Proxies)

public class Order
{
    private readonly ILazyLoader _lazyLoader;
    private Customer _customer;
    private ICollection<OrderItem> _items;

    public Order() { }

    public Order(ILazyLoader lazyLoader)
    {
        _lazyLoader = lazyLoader;
    }

    public int Id { get; set; }

    public Customer Customer
    {
        get => _lazyLoader.Load(this, ref _customer);
        set => _customer = value;
    }

    public ICollection<OrderItem> Items
    {
        get => _lazyLoader.Load(this, ref _items);
        set => _items = value;
    }
}

The N+1 Problem

// PROBLEM: Each iteration triggers a new query
var orders = await context.Orders.ToListAsync(); // Query 1

foreach (var order in orders)
{
    // Each access triggers a query
    Console.WriteLine(order.Customer.Name);  // Query 2, 3, 4...
    foreach (var item in order.Items)        // More queries!
    {
        Console.WriteLine(item.Product.Name);
    }
}
// Result: 1 + N + N*M queries for N orders with M items each

// SOLUTION: Use Eager Loading when you know you need the data
var orders = await context.Orders
    .Include(o => o.Customer)
    .Include(o => o.Items)
        .ThenInclude(i => i.Product)
    .ToListAsync();

[DEBUGGING] Identifying Lazy Loading Issues

// Enable sensitive data logging in development
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .EnableSensitiveDataLogging()
           .EnableDetailedErrors()
           .LogTo(Console.WriteLine, LogLevel.Information));

// Or use interceptors for production monitoring
public class LazyLoadingInterceptor : DbCommandInterceptor
{
    private readonly ILogger<LazyLoadingInterceptor> _logger;
    private int _queryCount = 0;

    public override InterceptionResult<DbDataReader> ReaderExecuting(
        DbCommand command,
        CommandEventData eventData,
        InterceptionResult<DbDataReader> result)
    {
        _queryCount++;
        if (_queryCount > 10)
        {
            _logger.LogWarning(
                "Potential N+1 detected. Query #{Count}: {Sql}",
                _queryCount,
                command.CommandText);
        }
        return base.ReaderExecuting(command, eventData, result);
    }
}

[PRODUCTION] When to Use Lazy Loading (Rarely!)

// AVOID lazy loading in:
// - Web applications (unpredictable query count)
// - High-traffic scenarios
// - When you know you'll need related data

// CONSIDER lazy loading in:
// - Desktop applications with connected scenarios
// - Exploration/admin tools
// - When related data is rarely needed

// Better alternatives:
// 1. Eager loading with Include
// 2. Projection with Select
// 3. Explicit loading for conditional cases

1.4 Loading Strategy Comparison

Decision Matrix

Scenario Recommended Strategy Reason
Always need related data Eager Loading Single query, predictable
Sometimes need related data Explicit Loading Load on demand
Rarely need related data Lazy Loading* Auto-load when accessed
Read-only scenarios Projection (Select) Best performance
Multiple collections Split Query Avoid Cartesian explosion
Mobile/Low bandwidth Projection Minimal data transfer

*Use with caution - prefer Explicit Loading in most cases

[BENCHMARK] Real-World Scenarios

// Scenario: Load 1000 orders with customer and 5 items each

| Strategy | Queries | Time | Memory | Best For |
|----------|---------|------|--------|----------|
| Eager (Single) | 1 | 45ms | 50MB | Small datasets |
| Eager (Split) | 3 | 35ms | 40MB | Multiple collections |
| Lazy | ~6000 | 2.5s | 80MB | Never for lists |
| Explicit | 3 | 40ms | 42MB | Conditional loading |
| Projection | 1 | 15ms | 15MB | Read-only scenarios |

// Projection example (fastest):
var orders = await context.Orders
    .Select(o => new OrderDto
    {
        Id = o.Id,
        CustomerName = o.Customer.Name,
        ItemCount = o.Items.Count,
        Total = o.Items.Sum(i => i.Price * i.Quantity)
    })
    .ToListAsync();

2. Query Optimization Deep Dive

2.1 AsSplitQuery

Single Query vs Split Query

// SINGLE QUERY (default) - One query with JOINs
var orders = await context.Orders
    .Include(o => o.OrderItems)
    .Include(o => o.Payments)
    .ToListAsync();

// Generated SQL: One query with LEFT JOINs
// Problem: Cartesian explosion with multiple collections

// SPLIT QUERY - Separate queries per Include
var orders = await context.Orders
    .Include(o => o.OrderItems)
    .Include(o => o.Payments)
    .AsSplitQuery()
    .ToListAsync();

// Generated SQL: Three separate queries
// Query 1: SELECT * FROM Orders
// Query 2: SELECT * FROM OrderItems WHERE OrderId IN (...)
// Query 3: SELECT * FROM Payments WHERE OrderId IN (...)

[BENCHMARK] Performance Comparison

// Scenario: 100 orders, 10 items each, 3 payments each

| Query Type | Rows Returned | DB Round Trips | Time | Memory |
|------------|---------------|----------------|------|--------|
| Single     | 3,000         | 1              | 85ms | 45MB   |
| Split      | 1,400         | 3              | 55ms | 25MB   |

// When Split Query wins:
// - Multiple collection navigations
// - Large result sets
// - High item count per parent

// When Single Query wins:
// - Simple queries (1-2 includes)
// - Low network latency to DB
// - Small datasets

When Split Queries Help/Hurt

// HELPS: Multiple collections
var orders = await context.Orders
    .Include(o => o.Items)       // Collection
    .Include(o => o.Payments)    // Collection
    .Include(o => o.AuditLogs)   // Collection
    .AsSplitQuery()              // Avoids 3-way Cartesian explosion
    .ToListAsync();

// DOESN'T HELP: Reference navigations (no Cartesian explosion)
var orders = await context.Orders
    .Include(o => o.Customer)     // Reference (1:1)
    .Include(o => o.ShipAddress)  // Reference (1:1)
    .ToListAsync();  // Single query is fine

// HURTS: When data consistency matters
// Split queries may see different data if changes occur between queries
var orders = await context.Orders
    .Include(o => o.Items)
    .AsSplitQuery()
    .ToListAsync();
// If an item is added between Query 1 and Query 2, you might get inconsistent data

Configure as Default

// Set split query as default for all queries
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .UseQuerySplittingBehavior(QuerySplittingBehavior.SplitQuery));

// Override per query if needed
var orders = await context.Orders
    .Include(o => o.Customer)
    .AsSingleQuery()  // Override default
    .ToListAsync();

2.2 Compiled Queries

EF.CompileQuery Usage

// Define compiled query as static field
private static readonly Func<ApplicationDbContext, int, Task<Order?>> GetOrderById =
    EF.CompileAsyncQuery(
        (ApplicationDbContext context, int id) =>
            context.Orders
                .Include(o => o.Customer)
                .Include(o => o.Items)
                .FirstOrDefault(o => o.Id == id));

// Usage
public async Task<Order?> GetOrderAsync(int id)
{
    return await GetOrderById(_context, id);
}

// More complex example with multiple parameters
private static readonly Func<ApplicationDbContext, int, DateTime, DateTime, IAsyncEnumerable<Order>>
    GetOrdersByCustomerInDateRange = EF.CompileAsyncQuery(
        (ApplicationDbContext context, int customerId, DateTime start, DateTime end) =>
            context.Orders
                .Where(o => o.CustomerId == customerId &&
                           o.OrderDate >= start &&
                           o.OrderDate <= end)
                .OrderByDescending(o => o.OrderDate));

// Usage with IAsyncEnumerable
public async IAsyncEnumerable<Order> GetOrdersAsync(
    int customerId, DateTime start, DateTime end)
{
    await foreach (var order in GetOrdersByCustomerInDateRange(
        _context, customerId, start, end))
    {
        yield return order;
    }
}

Limitations and Gotchas

// LIMITATION 1: No dynamic filters
// WRONG - Can't compile with dynamic Where clauses
// var query = EF.CompileQuery((context, filter) =>
//     context.Orders.Where(filter));  // Error!

// LIMITATION 2: No navigation fixup
var getOrder = EF.CompileAsyncQuery(
    (ApplicationDbContext ctx, int id) =>
        ctx.Orders.FirstOrDefault(o => o.Id == id));

var order1 = await getOrder(_context, 1);
var order2 = await getOrder(_context, 1);
// order1 != order2 (different instances)

// LIMITATION 3: Limited to simple return types
// IEnumerable<T>, IAsyncEnumerable<T>, T, T?

// WORKAROUND for dynamic queries: Use expression building
public async Task<List<Order>> GetOrdersAsync(OrderFilter filter)
{
    var query = _context.Orders.AsQueryable();

    if (filter.CustomerId.HasValue)
        query = query.Where(o => o.CustomerId == filter.CustomerId);

    if (filter.MinTotal.HasValue)
        query = query.Where(o => o.Total >= filter.MinTotal);

    // Can't be compiled, but at least optimized with query caching
    return await query.ToListAsync();
}

[BENCHMARK] Compilation Overhead vs Query Speedup

// Benchmark: Execute same query 10,000 times

| Query Type | First Execution | Subsequent | Total Time |
|------------|-----------------|------------|------------|
| Regular    | 15ms            | 0.8ms      | 8.0s       |
| Compiled   | 0.5ms           | 0.3ms      | 3.0s       |

// Compiled queries skip:
// - LINQ expression parsing
// - Query plan generation
// - Parameter binding setup

// Best use cases:
// 1. Hot paths executed frequently
// 2. Simple queries with known parameters
// 3. High-throughput scenarios

[PRODUCTION] When to Use Compiled Queries

// USE compiled queries for:
// - Queries executed > 1000 times/minute
// - Simple CRUD operations (GetById, GetAll)
// - Queries in hot loops

// DON'T USE for:
// - Dynamic/filterable queries
// - Queries executed rarely
// - Complex queries that change often

// Example: Repository with compiled queries
public class OrderRepository
{
    private static readonly Func<ApplicationDbContext, int, Task<Order?>> _getById =
        EF.CompileAsyncQuery((ApplicationDbContext ctx, int id) =>
            ctx.Orders.FirstOrDefault(o => o.Id == id));

    private static readonly Func<ApplicationDbContext, int, IAsyncEnumerable<Order>> _getByCustomer =
        EF.CompileAsyncQuery((ApplicationDbContext ctx, int customerId) =>
            ctx.Orders.Where(o => o.CustomerId == customerId));

    public Task<Order?> GetByIdAsync(int id) => _getById(_context, id);

    public IAsyncEnumerable<Order> GetByCustomerAsync(int customerId) =>
        _getByCustomer(_context, customerId);
}

2.3 LINQ to SQL Translation

Expression Tree to SQL Conversion

// LINQ expression
var expensiveOrders = context.Orders
    .Where(o => o.Total > 1000 && o.Customer.Country == "USA")
    .OrderByDescending(o => o.OrderDate)
    .Select(o => new { o.Id, o.Total, CustomerName = o.Customer.Name })
    .Take(10);

// EF Core builds an expression tree, then translates to SQL:
/*
SELECT TOP(10) [o].[Id], [o].[Total], [c].[Name] AS [CustomerName]
FROM [Orders] AS [o]
INNER JOIN [Customers] AS [c] ON [o].[CustomerId] = [c].[Id]
WHERE [o].[Total] > 1000 AND [c].[Country] = N'USA'
ORDER BY [o].[OrderDate] DESC
*/

Client vs Server Evaluation

// SERVER EVALUATION (Good) - Runs in database
var orders = await context.Orders
    .Where(o => o.Total > 1000)
    .Where(o => o.OrderDate.Year == 2024)  // Translated to SQL
    .ToListAsync();

// CLIENT EVALUATION (Bad) - Loads all data, filters in memory
var orders = await context.Orders
    .Where(o => MyCustomMethod(o.Total))  // Can't translate!
    .ToListAsync();
// Warning: This will load ALL orders, then filter in memory

// Mixed evaluation (Partial client evaluation)
var orders = await context.Orders
    .Where(o => o.Total > 1000)           // Server
    .AsEnumerable()                        // Switch to client
    .Where(o => MyCustomMethod(o.Total))   // Client
    .ToList();

[GOTCHAS] Client Evaluation Detection

// EF Core 3.0+ throws by default on client evaluation
// Configure to warn instead (not recommended for production):
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .ConfigureWarnings(w => w.Throw(RelationalEventId.QueryPossibleUnintendedUseOfEqualsWarning)));

// Common patterns that can't be translated:
// 1. Custom methods
.Where(o => CalculateDiscount(o.Total) > 100)  // Error

// 2. Complex string operations
.Where(o => Regex.IsMatch(o.Name, pattern))  // Error

// 3. Collection methods not supported
.Where(o => o.Tags.Any(t => customList.Contains(t)))  // May error

// SOLUTIONS:

// 1. Use EF.Functions for DB operations
.Where(o => EF.Functions.Like(o.Name, "%search%"))

// 2. Compute before query
var discount = CalculateDiscount(1000);
.Where(o => o.Total > discount)

// 3. Use raw SQL for complex operations
context.Orders.FromSqlRaw("SELECT * FROM Orders WHERE ...")

Query Tags for Debugging

// Add tags to identify queries in logs and query store
var orders = await context.Orders
    .TagWith("GetRecentOrders - OrderService.cs:42")
    .TagWith($"User: {userId}, Date: {DateTime.UtcNow}")
    .Where(o => o.OrderDate > DateTime.UtcNow.AddDays(-30))
    .ToListAsync();

// Generated SQL includes comments:
/*
-- GetRecentOrders - OrderService.cs:42
-- User: 123, Date: 2024-01-15

SELECT [o].[Id], [o].[OrderDate], [o].[Total]
FROM [Orders] AS [o]
WHERE [o].[OrderDate] > @date
*/

[DEBUGGING] Logging Generated SQL

// Method 1: Configure logging in DbContext
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .LogTo(Console.WriteLine, LogLevel.Information)
           .EnableSensitiveDataLogging());  // Shows parameter values

// Method 2: Use ToQueryString() for specific queries
var query = context.Orders
    .Where(o => o.Total > 1000)
    .Include(o => o.Customer);

var sql = query.ToQueryString();
Console.WriteLine(sql);

// Method 3: Interceptor for production logging
public class QueryLoggingInterceptor : DbCommandInterceptor
{
    private readonly ILogger _logger;

    public override ValueTask<InterceptionResult<DbDataReader>> ReaderExecutingAsync(
        DbCommand command,
        CommandEventData eventData,
        InterceptionResult<DbDataReader> result,
        CancellationToken ct = default)
    {
        _logger.LogDebug(
            "Executing query: {Query}\nParameters: {Parameters}",
            command.CommandText,
            string.Join(", ", command.Parameters.Cast<DbParameter>()
                .Select(p => $"{p.ParameterName}={p.Value}")));

        return base.ReaderExecutingAsync(command, eventData, result, ct);
    }
}

2.4 Global Query Filters

Multi-Tenancy Implementation

public class ApplicationDbContext : DbContext
{
    private readonly ITenantService _tenantService;

    public ApplicationDbContext(
        DbContextOptions options,
        ITenantService tenantService) : base(options)
    {
        _tenantService = tenantService;
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Apply tenant filter to all tenant entities
        modelBuilder.Entity<Order>()
            .HasQueryFilter(o => o.TenantId == _tenantService.CurrentTenantId);

        modelBuilder.Entity<Customer>()
            .HasQueryFilter(c => c.TenantId == _tenantService.CurrentTenantId);

        modelBuilder.Entity<Product>()
            .HasQueryFilter(p => p.TenantId == _tenantService.CurrentTenantId);
    }
}

// All queries automatically filtered:
var orders = await context.Orders.ToListAsync();
// SQL: SELECT * FROM Orders WHERE TenantId = @currentTenantId

Soft Delete with Global Filters

public interface ISoftDeletable
{
    bool IsDeleted { get; set; }
    DateTime? DeletedAt { get; set; }
}

public class Order : ISoftDeletable
{
    public int Id { get; set; }
    public bool IsDeleted { get; set; }
    public DateTime? DeletedAt { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // Apply to all soft-deletable entities
    foreach (var entityType in modelBuilder.Model.GetEntityTypes())
    {
        if (typeof(ISoftDeletable).IsAssignableFrom(entityType.ClrType))
        {
            var parameter = Expression.Parameter(entityType.ClrType, "e");
            var property = Expression.Property(parameter, nameof(ISoftDeletable.IsDeleted));
            var condition = Expression.Equal(property, Expression.Constant(false));
            var lambda = Expression.Lambda(condition, parameter);

            modelBuilder.Entity(entityType.ClrType).HasQueryFilter(lambda);
        }
    }
}

Bypassing Filters (IgnoreQueryFilters)

// Normal query - filter applied
var activeOrders = await context.Orders.ToListAsync();
// SQL: SELECT * FROM Orders WHERE IsDeleted = 0

// Bypass filter for admin operations
var allOrders = await context.Orders
    .IgnoreQueryFilters()
    .ToListAsync();
// SQL: SELECT * FROM Orders

// Bypass for specific scenarios
public async Task<Order?> GetOrderIncludingDeletedAsync(int id)
{
    return await _context.Orders
        .IgnoreQueryFilters()
        .FirstOrDefaultAsync(o => o.Id == id);
}

// Note: IgnoreQueryFilters removes ALL filters, not just soft delete
// For selective bypass, use conditional logic:
public async Task<List<Order>> GetOrdersAsync(bool includeDeleted = false)
{
    var query = _context.Orders.AsQueryable();

    if (includeDeleted)
    {
        query = query.IgnoreQueryFilters();
    }

    return await query.ToListAsync();
}

Filter Composition and Conflicts

// Multiple filters are combined with AND
modelBuilder.Entity<Order>()
    .HasQueryFilter(o => !o.IsDeleted && o.TenantId == _tenantId);

// Or apply multiple filters separately (they stack)
modelBuilder.Entity<Order>()
    .HasQueryFilter(o => !o.IsDeleted);

// In a second configuration:
modelBuilder.Entity<Order>()
    .HasQueryFilter(o => o.TenantId == _tenantId);
// WARNING: Second filter REPLACES the first one!

// SOLUTION: Combine in single filter or use interface approach

[CODE] Full Multi-Tenant Implementation

// Tenant service
public interface ITenantService
{
    Guid CurrentTenantId { get; }
}

public class TenantService : ITenantService
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public TenantService(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    public Guid CurrentTenantId =>
        Guid.Parse(_httpContextAccessor.HttpContext?
            .User.FindFirst("tenant_id")?.Value
            ?? throw new UnauthorizedAccessException("No tenant"));
}

// Base entity
public abstract class TenantEntity
{
    public Guid TenantId { get; set; }
}

// DbContext
public class MultiTenantDbContext : DbContext
{
    private readonly Guid _tenantId;

    public MultiTenantDbContext(
        DbContextOptions options,
        ITenantService tenantService) : base(options)
    {
        _tenantId = tenantService.CurrentTenantId;
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        foreach (var entityType in modelBuilder.Model.GetEntityTypes())
        {
            if (typeof(TenantEntity).IsAssignableFrom(entityType.ClrType))
            {
                modelBuilder.Entity(entityType.ClrType)
                    .HasQueryFilter(CreateTenantFilter(entityType.ClrType));
            }
        }
    }

    private LambdaExpression CreateTenantFilter(Type entityType)
    {
        var parameter = Expression.Parameter(entityType, "e");
        var tenantProperty = Expression.Property(parameter, nameof(TenantEntity.TenantId));
        var tenantValue = Expression.Constant(_tenantId);
        var condition = Expression.Equal(tenantProperty, tenantValue);
        return Expression.Lambda(condition, parameter);
    }

    public override int SaveChanges()
    {
        SetTenantId();
        return base.SaveChanges();
    }

    public override Task<int> SaveChangesAsync(CancellationToken ct = default)
    {
        SetTenantId();
        return base.SaveChangesAsync(ct);
    }

    private void SetTenantId()
    {
        foreach (var entry in ChangeTracker.Entries<TenantEntity>()
            .Where(e => e.State == EntityState.Added))
        {
            entry.Entity.TenantId = _tenantId;
        }
    }
}

3. Advanced Modeling

3.1 Shadow Properties

Definition and Configuration

// Shadow properties exist in the model but not in the entity class
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // Define shadow property
    modelBuilder.Entity<Order>()
        .Property<DateTime>("CreatedAt");

    modelBuilder.Entity<Order>()
        .Property<DateTime>("ModifiedAt");

    modelBuilder.Entity<Order>()
        .Property<string>("CreatedBy")
        .HasMaxLength(100);
}

Audit Fields Implementation

public interface IAuditable
{
    // Interface doesn't need the properties - they're shadow properties
}

public class Order : IAuditable
{
    public int Id { get; set; }
    public decimal Total { get; set; }
    // CreatedAt, ModifiedAt, CreatedBy, ModifiedBy are shadow properties
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    foreach (var entityType in modelBuilder.Model.GetEntityTypes())
    {
        if (typeof(IAuditable).IsAssignableFrom(entityType.ClrType))
        {
            modelBuilder.Entity(entityType.ClrType)
                .Property<DateTime>("CreatedAt")
                .HasDefaultValueSql("GETUTCDATE()");

            modelBuilder.Entity(entityType.ClrType)
                .Property<DateTime>("ModifiedAt");

            modelBuilder.Entity(entityType.ClrType)
                .Property<string>("CreatedBy")
                .HasMaxLength(100);

            modelBuilder.Entity(entityType.ClrType)
                .Property<string>("ModifiedBy")
                .HasMaxLength(100);
        }
    }
}

// Set values on SaveChanges
public override Task<int> SaveChangesAsync(CancellationToken ct = default)
{
    var entries = ChangeTracker.Entries()
        .Where(e => e.Entity is IAuditable &&
                   (e.State == EntityState.Added || e.State == EntityState.Modified));

    foreach (var entry in entries)
    {
        entry.Property("ModifiedAt").CurrentValue = DateTime.UtcNow;
        entry.Property("ModifiedBy").CurrentValue = _currentUser.Id;

        if (entry.State == EntityState.Added)
        {
            entry.Property("CreatedAt").CurrentValue = DateTime.UtcNow;
            entry.Property("CreatedBy").CurrentValue = _currentUser.Id;
        }
    }

    return base.SaveChangesAsync(ct);
}

Accessing Shadow Properties

// Read shadow property value
var order = await context.Orders.FindAsync(id);
var createdAt = context.Entry(order).Property<DateTime>("CreatedAt").CurrentValue;

// Query using shadow property
var recentOrders = await context.Orders
    .Where(o => EF.Property<DateTime>(o, "CreatedAt") > DateTime.UtcNow.AddDays(-7))
    .OrderByDescending(o => EF.Property<DateTime>(o, "CreatedAt"))
    .ToListAsync();

// Set shadow property value
context.Entry(order).Property("ModifiedAt").CurrentValue = DateTime.UtcNow;

// Include in projection
var orders = await context.Orders
    .Select(o => new
    {
        o.Id,
        o.Total,
        CreatedAt = EF.Property<DateTime>(o, "CreatedAt"),
        CreatedBy = EF.Property<string>(o, "CreatedBy")
    })
    .ToListAsync();

3.2 Owned Types

Value Objects with Owned Types

// Value object (no identity)
public class Address
{
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string ZipCode { get; set; }
    public string Country { get; set; }
}

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Address BillingAddress { get; set; }
    public Address ShippingAddress { get; set; }
}

// Configuration
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Customer>(builder =>
    {
        builder.OwnsOne(c => c.BillingAddress, address =>
        {
            address.Property(a => a.Street).HasColumnName("BillingStreet");
            address.Property(a => a.City).HasColumnName("BillingCity");
            address.Property(a => a.State).HasColumnName("BillingState");
            address.Property(a => a.ZipCode).HasColumnName("BillingZipCode");
            address.Property(a => a.Country).HasColumnName("BillingCountry");
        });

        builder.OwnsOne(c => c.ShippingAddress, address =>
        {
            address.Property(a => a.Street).HasColumnName("ShippingStreet");
            // ... other properties
        });
    });
}

// Result: Single Customers table with columns:
// Id, Name, BillingStreet, BillingCity, ..., ShippingStreet, ShippingCity, ...

Table Splitting Configuration

// Store owned type in separate table
modelBuilder.Entity<Customer>(builder =>
{
    builder.OwnsOne(c => c.BillingAddress, address =>
    {
        address.ToTable("CustomerBillingAddresses");
    });
});

// Result: Two tables:
// Customers: Id, Name
// CustomerBillingAddresses: CustomerId, Street, City, State, ...

Owned Collections

public class Order
{
    public int Id { get; set; }
    public List<OrderLine> Lines { get; set; }  // Owned collection
}

public class OrderLine  // No Id - owned by Order
{
    public string ProductName { get; set; }
    public int Quantity { get; set; }
    public decimal UnitPrice { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Order>()
        .OwnsMany(o => o.Lines, line =>
        {
            line.ToTable("OrderLines");
            line.WithOwner().HasForeignKey("OrderId");
            line.Property<int>("Id");  // Shadow property for PK
            line.HasKey("Id");
        });
}

JSON Column Mapping (.NET 7+)

// Store complex type as JSON column
public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ProductMetadata Metadata { get; set; }
}

public class ProductMetadata
{
    public List<string> Tags { get; set; }
    public Dictionary<string, string> Attributes { get; set; }
    public ProductDimensions Dimensions { get; set; }
}

public class ProductDimensions
{
    public decimal Width { get; set; }
    public decimal Height { get; set; }
    public decimal Depth { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>()
        .OwnsOne(p => p.Metadata, metadata =>
        {
            metadata.ToJson();  // Store as JSON column
            metadata.OwnsOne(m => m.Dimensions);
        });
}

// Query JSON properties
var products = await context.Products
    .Where(p => p.Metadata.Tags.Contains("featured"))
    .Where(p => p.Metadata.Dimensions.Width > 10)
    .ToListAsync();

3.3 Inheritance Strategies

Table-per-Hierarchy (TPH)

// Base class
public abstract class Payment
{
    public int Id { get; set; }
    public decimal Amount { get; set; }
    public DateTime ProcessedAt { get; set; }
}

// Derived classes
public class CreditCardPayment : Payment
{
    public string CardNumber { get; set; }
    public string CardHolderName { get; set; }
}

public class BankTransferPayment : Payment
{
    public string AccountNumber { get; set; }
    public string RoutingNumber { get; set; }
}

public class CryptoPayment : Payment
{
    public string WalletAddress { get; set; }
    public string CryptoCurrency { get; set; }
}

// TPH Configuration (default)
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Payment>()
        .HasDiscriminator<string>("PaymentType")
        .HasValue<CreditCardPayment>("CreditCard")
        .HasValue<BankTransferPayment>("BankTransfer")
        .HasValue<CryptoPayment>("Crypto");
}

// Result: Single table with discriminator column
// Payments: Id, Amount, ProcessedAt, PaymentType, CardNumber, CardHolderName,
//           AccountNumber, RoutingNumber, WalletAddress, CryptoCurrency
// NULL values for columns not applicable to each type

Table-per-Type (TPT)

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Payment>().ToTable("Payments");
    modelBuilder.Entity<CreditCardPayment>().ToTable("CreditCardPayments");
    modelBuilder.Entity<BankTransferPayment>().ToTable("BankTransferPayments");
    modelBuilder.Entity<CryptoPayment>().ToTable("CryptoPayments");
}

// Result: Separate tables
// Payments: Id, Amount, ProcessedAt
// CreditCardPayments: Id, CardNumber, CardHolderName (FK to Payments)
// BankTransferPayments: Id, AccountNumber, RoutingNumber (FK to Payments)
// CryptoPayments: Id, WalletAddress, CryptoCurrency (FK to Payments)

Table-per-Concrete-Type (TPC) - .NET 7+

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Payment>().UseTpcMappingStrategy();

    modelBuilder.Entity<CreditCardPayment>().ToTable("CreditCardPayments");
    modelBuilder.Entity<BankTransferPayment>().ToTable("BankTransferPayments");
    modelBuilder.Entity<CryptoPayment>().ToTable("CryptoPayments");
}

// Result: Separate tables, each with ALL columns
// CreditCardPayments: Id, Amount, ProcessedAt, CardNumber, CardHolderName
// BankTransferPayments: Id, Amount, ProcessedAt, AccountNumber, RoutingNumber
// CryptoPayments: Id, Amount, ProcessedAt, WalletAddress, CryptoCurrency
// No base Payments table!

[BENCHMARK] Performance Comparison

// Query: Get all payments over $1000

| Strategy | Tables | JOINs | Rows Scanned | Query Time |
|----------|--------|-------|--------------|------------|
| TPH      | 1      | 0     | All rows     | 15ms       |
| TPT      | 4      | 3     | Base + matched| 45ms      |
| TPC      | 3      | 0     | All 3 tables | 25ms       |

// TPH: Best for polymorphic queries, simple schema
// TPT: Best for many derived types with many properties
// TPC: Best for querying single concrete types

// INSERT performance:
| Strategy | Tables Written | Insert Time |
|----------|---------------|-------------|
| TPH      | 1             | 2ms         |
| TPT      | 2             | 8ms         |
| TPC      | 1             | 2ms         |

[PRODUCTION] Migration Between Strategies

// Migrating from TPH to TPT requires:
// 1. Create new tables
// 2. Migrate data with SQL
// 3. Update EF configuration
// 4. Drop discriminator column

// Migration SQL example (TPH to TPT):
migrationBuilder.Sql(@"
    -- Create derived tables
    CREATE TABLE CreditCardPayments (
        Id INT PRIMARY KEY REFERENCES Payments(Id),
        CardNumber NVARCHAR(20),
        CardHolderName NVARCHAR(100)
    );

    -- Migrate data
    INSERT INTO CreditCardPayments (Id, CardNumber, CardHolderName)
    SELECT Id, CardNumber, CardHolderName
    FROM Payments
    WHERE PaymentType = 'CreditCard';

    -- Clean up base table
    ALTER TABLE Payments DROP COLUMN CardNumber, CardHolderName;
    ALTER TABLE Payments DROP COLUMN PaymentType;
");

4. Concurrency & Transactions

4.1 Optimistic Concurrency

Concurrency Tokens Configuration

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public int StockQuantity { get; set; }

    [Timestamp]
    public byte[] RowVersion { get; set; }  // SQL Server rowversion
}

// Or via Fluent API
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>()
        .Property(p => p.RowVersion)
        .IsRowVersion();

    // Alternative: Use specific property as concurrency token
    modelBuilder.Entity<Product>()
        .Property(p => p.Price)
        .IsConcurrencyToken();
}

RowVersion/Timestamp Implementation

// SQL Server automatically updates rowversion on each UPDATE
// Generated SQL:
/*
UPDATE [Products]
SET [Name] = @Name, [Price] = @Price
WHERE [Id] = @Id AND [RowVersion] = @OriginalRowVersion;

SELECT [RowVersion]
FROM [Products]
WHERE @@ROWCOUNT = 1 AND [Id] = @Id;
*/

Handling DbUpdateConcurrencyException

public async Task<bool> UpdateProductPriceAsync(int id, decimal newPrice)
{
    var maxRetries = 3;
    var retryCount = 0;

    while (retryCount < maxRetries)
    {
        try
        {
            var product = await _context.Products.FindAsync(id);
            if (product == null) return false;

            product.Price = newPrice;
            await _context.SaveChangesAsync();
            return true;
        }
        catch (DbUpdateConcurrencyException ex)
        {
            retryCount++;

            if (retryCount >= maxRetries)
            {
                _logger.LogError(ex,
                    "Concurrency conflict for product {Id} after {Retries} retries",
                    id, retryCount);
                throw;
            }

            // Get the current database values
            var entry = ex.Entries.Single();
            var databaseValues = await entry.GetDatabaseValuesAsync();

            if (databaseValues == null)
            {
                // Entity was deleted
                throw new InvalidOperationException("Product was deleted");
            }

            // Refresh with database values
            entry.OriginalValues.SetValues(databaseValues);

            _logger.LogWarning(
                "Concurrency conflict for product {Id}, retry {Retry}/{Max}",
                id, retryCount, maxRetries);
        }
    }

    return false;
}

[CODE] Full Conflict Handling Implementation

public class ConflictResolutionService<T> where T : class
{
    public enum ResolutionStrategy
    {
        ClientWins,      // Overwrite database with client values
        DatabaseWins,    // Discard client changes
        Merge,           // Custom merge logic
        ThrowException   // Let caller handle
    }

    public async Task<T> ResolveConflictAsync(
        DbContext context,
        DbUpdateConcurrencyException exception,
        ResolutionStrategy strategy,
        Func<T, T, T>? mergeFunc = null)
    {
        var entry = exception.Entries.Single();
        var clientValues = entry.CurrentValues;
        var databaseEntry = await entry.GetDatabaseValuesAsync();

        if (databaseEntry == null)
        {
            throw new InvalidOperationException("Entity was deleted");
        }

        switch (strategy)
        {
            case ResolutionStrategy.ClientWins:
                // Keep client values, update original to match DB
                entry.OriginalValues.SetValues(databaseEntry);
                await context.SaveChangesAsync();
                return (T)entry.Entity;

            case ResolutionStrategy.DatabaseWins:
                // Discard client changes, reload from DB
                entry.CurrentValues.SetValues(databaseEntry);
                entry.OriginalValues.SetValues(databaseEntry);
                return (T)entry.Entity;

            case ResolutionStrategy.Merge:
                if (mergeFunc == null)
                    throw new ArgumentNullException(nameof(mergeFunc));

                var clientEntity = entry.CurrentValues.ToObject() as T;
                var databaseEntity = databaseEntry.ToObject() as T;
                var merged = mergeFunc(clientEntity!, databaseEntity!);

                entry.CurrentValues.SetValues(merged);
                entry.OriginalValues.SetValues(databaseEntry);
                await context.SaveChangesAsync();
                return merged;

            case ResolutionStrategy.ThrowException:
            default:
                throw exception;
        }
    }
}

// Usage
public async Task UpdateInventoryAsync(int productId, int quantityChange)
{
    try
    {
        var product = await _context.Products.FindAsync(productId);
        product.StockQuantity += quantityChange;
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException ex)
    {
        // Merge strategy: Add the change to whatever the current stock is
        await _conflictService.ResolveConflictAsync(
            _context,
            ex,
            ConflictResolutionService<Product>.ResolutionStrategy.Merge,
            (client, database) =>
            {
                var originalStock = database.StockQuantity;
                database.StockQuantity = originalStock + quantityChange;
                return database;
            });
    }
}

4.2 Cascade Delete Behavior

DeleteBehavior Enum Explained

public enum DeleteBehavior
{
    // When principal is deleted:
    Cascade,       // Dependents are also deleted
    Restrict,      // Prevent deletion if dependents exist
    SetNull,       // Set FK to null (requires nullable FK)
    NoAction,      // Database decides (usually Restrict)
    ClientCascade, // EF deletes dependents in memory only
    ClientSetNull, // EF sets FK to null in memory only
    ClientNoAction // EF does nothing, DB may error
}

Configuring Relationships

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // Cascade delete (default for required relationships)
    modelBuilder.Entity<Order>()
        .HasMany(o => o.OrderItems)
        .WithOne(oi => oi.Order)
        .HasForeignKey(oi => oi.OrderId)
        .OnDelete(DeleteBehavior.Cascade);

    // Restrict (prevent delete if children exist)
    modelBuilder.Entity<Customer>()
        .HasMany(c => c.Orders)
        .WithOne(o => o.Customer)
        .HasForeignKey(o => o.CustomerId)
        .OnDelete(DeleteBehavior.Restrict);

    // SetNull (orphan the children)
    modelBuilder.Entity<Department>()
        .HasMany(d => d.Employees)
        .WithOne(e => e.Department)
        .HasForeignKey(e => e.DepartmentId)
        .OnDelete(DeleteBehavior.SetNull);
}

[GOTCHAS] Common Cascade Delete Issues

// ISSUE 1: Multiple cascade paths
// SQL Server doesn't allow multiple cascade paths to the same table
public class Order
{
    public int Id { get; set; }
    public int BillingAddressId { get; set; }
    public int ShippingAddressId { get; set; }
    public Address BillingAddress { get; set; }   // Cascade
    public Address ShippingAddress { get; set; }  // Cascade - ERROR!
}

// SOLUTION: Use Restrict or ClientCascade for one path
modelBuilder.Entity<Order>()
    .HasOne(o => o.BillingAddress)
    .OnDelete(DeleteBehavior.Cascade);

modelBuilder.Entity<Order>()
    .HasOne(o => o.ShippingAddress)
    .OnDelete(DeleteBehavior.Restrict);

// ISSUE 2: Circular cascade
public class Employee
{
    public int Id { get; set; }
    public int? ManagerId { get; set; }
    public Employee Manager { get; set; }
    public ICollection<Employee> DirectReports { get; set; }
}

// SOLUTION: Use SetNull or handle in code
modelBuilder.Entity<Employee>()
    .HasOne(e => e.Manager)
    .WithMany(e => e.DirectReports)
    .OnDelete(DeleteBehavior.SetNull);

[PRODUCTION] Safe Deletion Patterns

public class SafeDeleteService
{
    public async Task<DeleteResult> SafeDeleteCustomerAsync(int customerId)
    {
        var customer = await _context.Customers
            .Include(c => c.Orders)
            .FirstOrDefaultAsync(c => c.Id == customerId);

        if (customer == null)
            return DeleteResult.NotFound;

        if (customer.Orders.Any())
        {
            return DeleteResult.HasDependents(
                $"Customer has {customer.Orders.Count} orders");
        }

        _context.Customers.Remove(customer);
        await _context.SaveChangesAsync();
        return DeleteResult.Success;
    }

    public async Task<DeleteResult> CascadeDeleteCustomerAsync(int customerId)
    {
        using var transaction = await _context.Database.BeginTransactionAsync();

        try
        {
            // Delete in correct order
            await _context.OrderItems
                .Where(oi => oi.Order.CustomerId == customerId)
                .ExecuteDeleteAsync();

            await _context.Orders
                .Where(o => o.CustomerId == customerId)
                .ExecuteDeleteAsync();

            await _context.Customers
                .Where(c => c.Id == customerId)
                .ExecuteDeleteAsync();

            await transaction.CommitAsync();
            return DeleteResult.Success;
        }
        catch (Exception ex)
        {
            await transaction.RollbackAsync();
            return DeleteResult.Error(ex.Message);
        }
    }
}

5. Production Patterns

5.1 Migrations in Production

Migration Scripts Generation

# Generate SQL script from migrations
dotnet ef migrations script --output migrations.sql

# Generate script from specific migration
dotnet ef migrations script InitialCreate AddOrderTable --output upgrade.sql

# Generate idempotent script (can run multiple times safely)
dotnet ef migrations script --idempotent --output idempotent-migrations.sql

Idempotent Migrations

// Generated idempotent migration SQL checks if already applied:
/*
IF NOT EXISTS (
    SELECT * FROM [__EFMigrationsHistory]
    WHERE [MigrationId] = N'20240115_AddOrderTable'
)
BEGIN
    CREATE TABLE [Orders] (...)

    INSERT INTO [__EFMigrationsHistory] ([MigrationId], [ProductVersion])
    VALUES (N'20240115_AddOrderTable', N'8.0.0');
END;
GO
*/

Data Migrations with SQL

public partial class SplitFullName : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        // Step 1: Add new columns
        migrationBuilder.AddColumn<string>(
            name: "FirstName",
            table: "Customers",
            nullable: true);

        migrationBuilder.AddColumn<string>(
            name: "LastName",
            table: "Customers",
            nullable: true);

        // Step 2: Migrate data
        migrationBuilder.Sql(@"
            UPDATE Customers
            SET FirstName = SUBSTRING(FullName, 1, CHARINDEX(' ', FullName) - 1),
                LastName = SUBSTRING(FullName, CHARINDEX(' ', FullName) + 1, LEN(FullName))
            WHERE FullName IS NOT NULL AND CHARINDEX(' ', FullName) > 0;

            UPDATE Customers
            SET FirstName = FullName, LastName = ''
            WHERE FullName IS NOT NULL AND CHARINDEX(' ', FullName) = 0;
        ");

        // Step 3: Make columns required
        migrationBuilder.AlterColumn<string>(
            name: "FirstName",
            table: "Customers",
            nullable: false,
            defaultValue: "");

        migrationBuilder.AlterColumn<string>(
            name: "LastName",
            table: "Customers",
            nullable: false,
            defaultValue: "");

        // Step 4: Drop old column
        migrationBuilder.DropColumn(
            name: "FullName",
            table: "Customers");
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        // Reverse migration
        migrationBuilder.AddColumn<string>(
            name: "FullName",
            table: "Customers",
            nullable: true);

        migrationBuilder.Sql(@"
            UPDATE Customers
            SET FullName = CONCAT(FirstName, ' ', LastName)
        ");

        migrationBuilder.DropColumn(name: "FirstName", table: "Customers");
        migrationBuilder.DropColumn(name: "LastName", table: "Customers");
    }
}

[PRODUCTION] CI/CD Pipeline Integration

# Azure DevOps pipeline example
stages:
- stage: Build
  jobs:
  - job: BuildAndTest
    steps:
    - task: DotNetCoreCLI@2
      displayName: 'Generate Migration Script'
      inputs:
        command: 'custom'
        custom: 'ef'
        arguments: 'migrations script --idempotent --output $(Build.ArtifactStagingDirectory)/migrations.sql --project src/MyApp.Data'

    - task: PublishBuildArtifacts@1
      inputs:
        PathtoPublish: '$(Build.ArtifactStagingDirectory)/migrations.sql'
        ArtifactName: 'migrations'

- stage: Deploy
  jobs:
  - deployment: DeployDatabase
    environment: 'production'
    strategy:
      runOnce:
        deploy:
          steps:
          - task: SqlAzureDacpacDeployment@1
            displayName: 'Apply Migrations'
            inputs:
              azureSubscription: 'Production'
              ServerName: 'prod-sql.database.windows.net'
              DatabaseName: 'MyAppDb'
              SqlFile: '$(Pipeline.Workspace)/migrations/migrations.sql'

Rollback Strategies

// Strategy 1: Use Down() migration (risky in production)
dotnet ef database update PreviousMigration

// Strategy 2: Generate rollback script beforehand
dotnet ef migrations script CurrentMigration PreviousMigration --output rollback.sql

// Strategy 3: Blue-green deployment
// - Deploy new version to "green" environment
// - Run migrations on green database
// - Test thoroughly
// - Switch traffic to green
// - Keep blue as fallback

// Strategy 4: Forward-only migrations (recommended)
// Instead of rolling back, create a new migration that undoes changes
public partial class RevertSplitFullName : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        // Re-add the old column
        migrationBuilder.AddColumn<string>(
            name: "FullName",
            table: "Customers",
            nullable: true);

        // Migrate data back
        migrationBuilder.Sql(@"
            UPDATE Customers
            SET FullName = CONCAT(FirstName, ' ', LastName)
        ");
    }
}

5.2 Connection Pooling

How Pooling Works Internally

// Connection pooling is handled by ADO.NET, not EF Core
// Connections are pooled by connection string

// Each unique connection string gets its own pool
// Default: Min pool size = 0, Max pool size = 100

// Lifecycle:
// 1. First request: Create new connection, add to pool
// 2. Request completes: Return connection to pool (not closed)
// 3. Next request: Reuse connection from pool
// 4. Pool full: Wait for available connection or timeout

Connection String Configuration

// Configure pooling in connection string
var connectionString = new SqlConnectionStringBuilder
{
    DataSource = "server",
    InitialCatalog = "database",
    IntegratedSecurity = true,

    // Pooling settings
    Pooling = true,           // Enable pooling (default: true)
    MinPoolSize = 5,          // Minimum connections to keep open
    MaxPoolSize = 100,        // Maximum connections allowed
    ConnectionLifetime = 300, // Max age in seconds before recycling
    ConnectTimeout = 30       // Timeout when waiting for connection
}.ConnectionString;

Pool Size Optimization

// Rule of thumb: MaxPoolSize = (Number of CPU cores * 2) + Disk spindles
// For cloud databases: Start with 20-30 and adjust based on metrics

// Monitor pool usage
// SQL Server:
/*
SELECT
    DB_NAME(dbid) as DatabaseName,
    COUNT(*) as ConnectionCount,
    loginame as LoginName,
    status
FROM sys.sysprocesses
WHERE dbid > 0
GROUP BY dbid, loginame, status
ORDER BY COUNT(*) DESC
*/

// .NET Core metrics (via EventCounters)
using var listener = new EventListener();
listener.EnableEvents(
    EventSource.GetSourceByName("Microsoft.Data.SqlClient.EventSource"),
    EventLevel.Informational,
    EventKeywords.All);

[DEBUGGING] Connection Pool Exhaustion

// Symptom: "Timeout expired. The timeout period elapsed prior to obtaining
// a connection from the pool."

// Common causes and fixes:

// 1. Connection leak - forgetting to dispose
// BAD:
public void ProcessData()
{
    var context = new ApplicationDbContext();  // Never disposed!
    var data = context.Orders.ToList();
}

// GOOD:
public void ProcessData()
{
    using var context = new ApplicationDbContext();
    var data = context.Orders.ToList();
}

// 2. Long-running queries blocking pool
// Use async methods and timeouts
context.Database.SetCommandTimeout(TimeSpan.FromSeconds(30));

// 3. Too many concurrent requests for pool size
// Increase MaxPoolSize or reduce concurrency

// 4. Debug with connection string
var connectionString = "...;Application Name=MyApp;Min Pool Size=0;Max Pool Size=10";
// Application Name helps identify in SQL Server

// 5. Clear pool on errors
SqlConnection.ClearAllPools();  // Nuclear option - clears all pools

5.3 High-Traffic Optimization

Read Replicas Configuration

// Configure separate contexts for read and write
services.AddDbContext<WriteDbContext>(options =>
    options.UseSqlServer(config["ConnectionStrings:Write"]));

services.AddDbContext<ReadDbContext>(options =>
    options.UseSqlServer(config["ConnectionStrings:Read"])
           .UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking));

// Repository pattern with read/write separation
public class OrderRepository
{
    private readonly WriteDbContext _writeContext;
    private readonly ReadDbContext _readContext;

    public async Task<Order?> GetByIdAsync(int id)
    {
        // Read from replica
        return await _readContext.Orders
            .AsNoTracking()
            .FirstOrDefaultAsync(o => o.Id == id);
    }

    public async Task<Order> CreateAsync(Order order)
    {
        // Write to primary
        _writeContext.Orders.Add(order);
        await _writeContext.SaveChangesAsync();
        return order;
    }
}

Projection-Only Queries

// Instead of loading full entities:
var orders = await context.Orders
    .Include(o => o.Customer)
    .Include(o => o.Items)
    .ToListAsync();  // Loads all columns, tracks entities

// Use projection for read-only scenarios:
var orderSummaries = await context.Orders
    .Select(o => new OrderSummaryDto
    {
        Id = o.Id,
        CustomerName = o.Customer.Name,
        ItemCount = o.Items.Count,
        Total = o.Items.Sum(i => i.Price * i.Quantity),
        OrderDate = o.OrderDate
    })
    .ToListAsync();  // Only fetches needed columns, no tracking

No-Tracking Queries

// Per-query no tracking
var orders = await context.Orders
    .AsNoTracking()
    .ToListAsync();

// Context-level no tracking
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking));

// AsNoTrackingWithIdentityResolution - no tracking but fixes duplicates
var orders = await context.Orders
    .Include(o => o.Customer)
    .AsNoTrackingWithIdentityResolution()
    .ToListAsync();
// If multiple orders have same customer, they'll reference same Customer instance

Streaming Large Result Sets

// Instead of loading all into memory:
var orders = await context.Orders.ToListAsync();  // Loads all at once

// Stream with IAsyncEnumerable:
public async IAsyncEnumerable<Order> StreamOrdersAsync(
    [EnumeratorCancellation] CancellationToken ct = default)
{
    await foreach (var order in context.Orders
        .AsNoTracking()
        .AsAsyncEnumerable()
        .WithCancellation(ct))
    {
        yield return order;
    }
}

// Usage
await foreach (var order in StreamOrdersAsync(cancellationToken))
{
    await ProcessOrderAsync(order);
}

[BENCHMARK] Optimization Impact Numbers

// Benchmark: Load 100,000 orders

| Technique | Time | Memory | CPU |
|-----------|------|--------|-----|
| Full entity load | 12s | 2.1GB | High |
| With AsNoTracking | 8s | 1.4GB | Medium |
| Projection (Select) | 3s | 400MB | Low |
| Streaming | 4s | 50MB | Low |

// Best practices combination:
var orders = await context.Orders
    .AsNoTracking()                    // No change tracking
    .AsSplitQuery()                    // Avoid Cartesian explosion
    .Where(o => o.Date > cutoffDate)   // Filter early
    .Select(o => new OrderDto {...})   // Project only needed data
    .ToListAsync();

6. Raw SQL & Advanced Operations

6.1 Raw SQL Queries

FromSqlRaw vs FromSqlInterpolated

// FromSqlRaw - Use when you have dynamic SQL or need full control
// WARNING: Vulnerable to SQL injection if using string concatenation!
var tableName = "Orders";  // NEVER concatenate user input!
var orders = await context.Orders
    .FromSqlRaw($"SELECT * FROM {tableName} WHERE Total > {{0}}", minTotal)
    .ToListAsync();

// FromSqlInterpolated - SAFE with interpolation (parameterized)
var minTotal = 1000m;
var orders = await context.Orders
    .FromSqlInterpolated($"SELECT * FROM Orders WHERE Total > {minTotal}")
    .ToListAsync();
// Parameters are properly escaped

// Can chain with LINQ
var orders = await context.Orders
    .FromSqlInterpolated($"SELECT * FROM Orders WHERE Total > {minTotal}")
    .Include(o => o.Customer)
    .Where(o => o.Status == OrderStatus.Pending)
    .OrderBy(o => o.OrderDate)
    .ToListAsync();

[SECURITY] Safe SQL Patterns

// SAFE: Using parameters
var search = userInput;  // Could be malicious
var products = await context.Products
    .FromSqlInterpolated($"SELECT * FROM Products WHERE Name LIKE {'%' + search + '%'}")
    .ToListAsync();

// SAFE: Using SqlParameter for complex scenarios
var param = new SqlParameter("@search", $"%{userInput}%");
var products = await context.Products
    .FromSqlRaw("SELECT * FROM Products WHERE Name LIKE @search", param)
    .ToListAsync();

// UNSAFE: String concatenation
var products = await context.Products
    .FromSqlRaw($"SELECT * FROM Products WHERE Name LIKE '%{userInput}%'")  // SQL INJECTION!
    .ToListAsync();

// For dynamic column/table names (can't parameterize):
// Use whitelist validation
var validColumns = new[] { "Name", "Price", "CreatedAt" };
if (!validColumns.Contains(sortColumn))
    throw new ArgumentException("Invalid sort column");

var sql = $"SELECT * FROM Products ORDER BY {sortColumn}";

Stored Procedure Execution

// Simple stored procedure
var orders = await context.Orders
    .FromSqlRaw("EXEC GetOrdersByCustomer @CustomerId = {0}", customerId)
    .ToListAsync();

// With output parameters
var customerIdParam = new SqlParameter("@CustomerId", customerId);
var totalParam = new SqlParameter
{
    ParameterName = "@Total",
    SqlDbType = SqlDbType.Decimal,
    Direction = ParameterDirection.Output
};

await context.Database.ExecuteSqlRawAsync(
    "EXEC CalculateCustomerTotal @CustomerId, @Total OUTPUT",
    customerIdParam, totalParam);

var total = (decimal)totalParam.Value;

// Stored procedure returning multiple result sets
using var command = context.Database.GetDbConnection().CreateCommand();
command.CommandText = "EXEC GetOrderWithDetails @OrderId";
command.Parameters.Add(new SqlParameter("@OrderId", orderId));

await context.Database.OpenConnectionAsync();
using var reader = await command.ExecuteReaderAsync();

// First result set - Order
var orders = new List<Order>();
while (await reader.ReadAsync())
{
    orders.Add(new Order
    {
        Id = reader.GetInt32(0),
        Total = reader.GetDecimal(1)
    });
}

// Second result set - OrderItems
await reader.NextResultAsync();
var items = new List<OrderItem>();
while (await reader.ReadAsync())
{
    items.Add(new OrderItem
    {
        Id = reader.GetInt32(0),
        ProductName = reader.GetString(1)
    });
}

6.2 Soft Delete Implementation

Global Query Filter Approach

public interface ISoftDeletable
{
    bool IsDeleted { get; set; }
    DateTime? DeletedAt { get; set; }
    string? DeletedBy { get; set; }
}

public class Order : ISoftDeletable
{
    public int Id { get; set; }
    public decimal Total { get; set; }
    public bool IsDeleted { get; set; }
    public DateTime? DeletedAt { get; set; }
    public string? DeletedBy { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // Apply filter to all soft-deletable entities
    foreach (var entityType in modelBuilder.Model.GetEntityTypes())
    {
        if (typeof(ISoftDeletable).IsAssignableFrom(entityType.ClrType))
        {
            var param = Expression.Parameter(entityType.ClrType, "e");
            var prop = Expression.Property(param, nameof(ISoftDeletable.IsDeleted));
            var condition = Expression.Equal(prop, Expression.Constant(false));
            var lambda = Expression.Lambda(condition, param);

            modelBuilder.Entity(entityType.ClrType).HasQueryFilter(lambda);
        }
    }
}

Shadow Property Approach

// Don't pollute entity with soft delete properties
public class Order
{
    public int Id { get; set; }
    public decimal Total { get; set; }
    // No IsDeleted property!
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Order>()
        .Property<bool>("IsDeleted")
        .HasDefaultValue(false);

    modelBuilder.Entity<Order>()
        .Property<DateTime?>("DeletedAt");

    modelBuilder.Entity<Order>()
        .HasQueryFilter(o => EF.Property<bool>(o, "IsDeleted") == false);
}

// Override SaveChanges to handle soft delete
public override Task<int> SaveChangesAsync(CancellationToken ct = default)
{
    foreach (var entry in ChangeTracker.Entries()
        .Where(e => e.State == EntityState.Deleted))
    {
        if (entry.Metadata.FindProperty("IsDeleted") != null)
        {
            entry.State = EntityState.Modified;
            entry.Property("IsDeleted").CurrentValue = true;
            entry.Property("DeletedAt").CurrentValue = DateTime.UtcNow;
        }
    }

    return base.SaveChangesAsync(ct);
}

Cascade Soft Delete

public class SoftDeleteService
{
    public async Task SoftDeleteOrderAsync(int orderId)
    {
        using var transaction = await _context.Database.BeginTransactionAsync();

        try
        {
            var order = await _context.Orders
                .Include(o => o.OrderItems)
                .FirstOrDefaultAsync(o => o.Id == orderId);

            if (order == null) return;

            // Soft delete order items
            foreach (var item in order.OrderItems)
            {
                item.IsDeleted = true;
                item.DeletedAt = DateTime.UtcNow;
            }

            // Soft delete order
            order.IsDeleted = true;
            order.DeletedAt = DateTime.UtcNow;

            await _context.SaveChangesAsync();
            await transaction.CommitAsync();
        }
        catch
        {
            await transaction.RollbackAsync();
            throw;
        }
    }

    // Restore soft-deleted entities
    public async Task RestoreOrderAsync(int orderId)
    {
        var order = await _context.Orders
            .IgnoreQueryFilters()
            .Include(o => o.OrderItems)
            .FirstOrDefaultAsync(o => o.Id == orderId);

        if (order == null || !order.IsDeleted) return;

        order.IsDeleted = false;
        order.DeletedAt = null;

        foreach (var item in order.OrderItems)
        {
            item.IsDeleted = false;
            item.DeletedAt = null;
        }

        await _context.SaveChangesAsync();
    }
}

[CODE] Complete Soft Delete Implementation

// Base class for soft-deletable entities
public abstract class SoftDeletableEntity
{
    public bool IsDeleted { get; set; }
    public DateTime? DeletedAt { get; set; }
    public string? DeletedBy { get; set; }
}

// DbContext with full soft delete support
public class ApplicationDbContext : DbContext
{
    private readonly ICurrentUserService _currentUser;

    public override int SaveChanges(bool acceptAllChangesOnSuccess)
    {
        HandleSoftDelete();
        return base.SaveChanges(acceptAllChangesOnSuccess);
    }

    public override Task<int> SaveChangesAsync(
        bool acceptAllChangesOnSuccess,
        CancellationToken ct = default)
    {
        HandleSoftDelete();
        return base.SaveChangesAsync(acceptAllChangesOnSuccess, ct);
    }

    private void HandleSoftDelete()
    {
        var deletedEntries = ChangeTracker.Entries<SoftDeletableEntity>()
            .Where(e => e.State == EntityState.Deleted);

        foreach (var entry in deletedEntries)
        {
            entry.State = EntityState.Modified;
            entry.Entity.IsDeleted = true;
            entry.Entity.DeletedAt = DateTime.UtcNow;
            entry.Entity.DeletedBy = _currentUser.UserId;
        }
    }

    // Hard delete method for when you really need to remove data
    public async Task HardDeleteAsync<T>(T entity) where T : SoftDeletableEntity
    {
        Entry(entity).State = EntityState.Deleted;
        await SaveChangesAsync();
    }

    // Query including deleted
    public IQueryable<T> QueryIncludingDeleted<T>() where T : SoftDeletableEntity
    {
        return Set<T>().IgnoreQueryFilters();
    }

    // Query only deleted
    public IQueryable<T> QueryOnlyDeleted<T>() where T : SoftDeletableEntity
    {
        return Set<T>()
            .IgnoreQueryFilters()
            .Where(e => e.IsDeleted);
    }
}

Interview Questions & Answers

Q1: What’s the difference between Eager, Explicit, and Lazy loading?

Answer:

  • Eager Loading: Related data loaded in the same query using Include()/ThenInclude(). Best when you know you’ll need the related data.
  • Explicit Loading: Related data loaded on demand after parent entity using Entry().Collection().Load() or Entry().Reference().Load(). Best for conditional loading.
  • Lazy Loading: Related data automatically loaded when navigation property is accessed. Requires proxies or ILazyLoader. Generally avoided due to N+1 problem.

Q2: What is the Cartesian explosion problem and how do you solve it?

Answer: When using multiple Include() for collection navigations, EF generates a single query with JOINs that produces a Cartesian product. For 100 orders with 10 items and 3 payments each, you get 100 × 10 × 3 = 3,000 rows instead of 1,400.

Solutions:

  1. Use AsSplitQuery() to generate separate queries
  2. Use projection with Select() to only fetch needed data
  3. Load collections separately with explicit loading

Q3: How do you handle optimistic concurrency in EF Core?

Answer: Use row versioning or concurrency tokens:

  1. Add [Timestamp] attribute or configure IsRowVersion() on a byte[] property
  2. EF includes the original value in the WHERE clause on updates
  3. If no rows are updated, EF throws DbUpdateConcurrencyException
  4. Handle the exception by refreshing data or implementing merge strategies

Q4: Explain the difference between TPH, TPT, and TPC inheritance strategies.

Answer:

  • TPH (Table-per-Hierarchy): Single table with discriminator column. Best performance for polymorphic queries but has nullable columns for type-specific properties.
  • TPT (Table-per-Type): Separate table for each type with foreign keys. Clean schema but requires JOINs for queries.
  • TPC (Table-per-Concrete-Type): Separate table for each concrete type with all columns. Best for querying single types but complex for polymorphic queries.

Q5: How would you implement soft delete with cascade?

Answer:

  1. Add IsDeleted, DeletedAt, and DeletedBy properties (or shadow properties)
  2. Configure global query filter to exclude deleted entities
  3. Override SaveChangesAsync() to convert Delete operations to updates
  4. For cascade, load related entities and soft delete them in a transaction
  5. Use IgnoreQueryFilters() for admin operations that need to see deleted data

Summary

This guide covered essential EF Core concepts for Principal Engineers:

  1. Loading Strategies: Eager vs Explicit vs Lazy loading with benchmarks and the Cartesian explosion problem
  2. Query Optimization: Split queries, compiled queries, LINQ translation, and global query filters
  3. Advanced Modeling: Shadow properties, owned types, JSON columns, and inheritance strategies
  4. Concurrency: Optimistic concurrency with row versioning and conflict resolution
  5. Production Patterns: Migrations, connection pooling, read replicas, and high-traffic optimization
  6. Raw SQL: Safe SQL execution, stored procedures, and soft delete implementation

Each section includes internals, benchmarks, production patterns, and debugging tips to help you build efficient data access layers.

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