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