Entity Framework Core - Quick Reference

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DbContext Setup

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options) { }

    public DbSet<User> Users => Set<User>();
    public DbSet<Order> Orders => Set<Order>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);
    }
}

// Registration
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));

Entity Configuration (Fluent API)

public class UserConfiguration : IEntityTypeConfiguration<User>
{
    public void Configure(EntityTypeBuilder<User> builder)
    {
        builder.HasKey(u => u.Id);
        builder.Property(u => u.Name).IsRequired().HasMaxLength(100);
        builder.Property(u => u.Email).IsRequired().HasMaxLength(255);
        builder.HasIndex(u => u.Email).IsUnique();

        // Relationships
        builder.HasMany(u => u.Orders)
            .WithOne(o => o.User)
            .HasForeignKey(o => o.UserId)
            .OnDelete(DeleteBehavior.Cascade);

        // Value conversion
        builder.Property(u => u.Status)
            .HasConversion<string>();

        // Default values
        builder.Property(u => u.CreatedAt)
            .HasDefaultValueSql("GETUTCDATE()");
    }
}

Migrations

# Add migration
dotnet ef migrations add InitialCreate

# Update database
dotnet ef database update

# Remove last migration (not applied)
dotnet ef migrations remove

# Generate SQL script
dotnet ef migrations script

# Revert to specific migration
dotnet ef database update MigrationName

LINQ Queries

// Basic queries
var user = await context.Users.FindAsync(id);
var users = await context.Users.Where(u => u.IsActive).ToListAsync();
var user = await context.Users.FirstOrDefaultAsync(u => u.Email == email);

// Projection
var dtos = await context.Users
    .Where(u => u.IsActive)
    .Select(u => new UserResponse(u.Id, u.Name, u.Email))
    .ToListAsync();

// Include related data
var usersWithOrders = await context.Users
    .Include(u => u.Orders)
    .ThenInclude(o => o.OrderItems)
    .ToListAsync();

// Pagination
var page = await context.Users
    .OrderBy(u => u.Name)
    .Skip((pageNumber - 1) * pageSize)
    .Take(pageSize)
    .ToListAsync();

// Aggregation
var count = await context.Users.CountAsync(u => u.IsActive);
var avgAge = await context.Users.AverageAsync(u => u.Age);

Repository Pattern

public interface IRepository<T> where T : class
{
    Task<T?> GetByIdAsync(int id);
    Task<IEnumerable<T>> GetAllAsync();
    Task AddAsync(T entity);
    void Update(T entity);
    void Remove(T entity);
    Task<bool> ExistsAsync(Expression<Func<T, bool>> predicate);
    Task SaveChangesAsync();
}

public class Repository<T> : IRepository<T> where T : class
{
    protected readonly AppDbContext _context;
    protected readonly DbSet<T> _dbSet;

    public Repository(AppDbContext context)
    {
        _context = context;
        _dbSet = context.Set<T>();
    }

    public async Task<T?> GetByIdAsync(int id) => await _dbSet.FindAsync(id);
    public async Task<IEnumerable<T>> GetAllAsync() => await _dbSet.ToListAsync();
    public async Task AddAsync(T entity) => await _dbSet.AddAsync(entity);
    public void Update(T entity) => _dbSet.Update(entity);
    public void Remove(T entity) => _dbSet.Remove(entity);
    public async Task<bool> ExistsAsync(Expression<Func<T, bool>> predicate)
        => await _dbSet.AnyAsync(predicate);
    public async Task SaveChangesAsync() => await _context.SaveChangesAsync();
}

Performance Tips

| Tip | Implementation | |-----|----------------| | Use AsNoTracking() for read-only | context.Users.AsNoTracking().ToListAsync() | | Use Select to project | Avoid loading full entities | | Use AsSplitQuery() | Prevent cartesian explosion with includes | | Use compiled queries | EF.CompileAsyncQuery(...) for hot paths | | Batch operations | ExecuteUpdateAsync / ExecuteDeleteAsync (EF Core 7+) |

// Bulk update (EF Core 7+)
await context.Users
    .Where(u => u.LastLoginAt < cutoff)
    .ExecuteUpdateAsync(u => u.SetProperty(x => x.IsActive, false));

// Bulk delete
await context.Users
    .Where(u => u.IsDeleted)
    .ExecuteDeleteAsync();

Anti-Patterns

| Anti-Pattern | Why It's Bad | Correct Approach | |--------------|--------------|------------------| | Loading full entities for display | Memory waste, slow | Use Select projections | | N+1 queries | Performance killer | Use Include or projections | | Not using AsNoTracking | Unnecessary overhead | Use for read-only queries | | Calling SaveChanges per entity | Slow batch operations | Call once after all changes | | Using DbContext as singleton | Thread-safety issues | Use AddDbContext (Scoped) |

Quick Troubleshooting

| Issue | Likely Cause | Solution | |-------|--------------|----------| | Tracking conflict | Entity already tracked | Use AsNoTracking or detach | | Migration fails | Model mismatch | Check pending changes, rebuild | | Slow query | Missing index | Add HasIndex in configuration | | Lazy loading fails | Not configured | Use Include (explicit loading) | | Concurrency conflict | Stale data | Add [ConcurrencyCheck] or RowVersion |