Entity Framework Core

After this lesson, you will be able to: Use Entity Framework Core: code-first migrations, DbContext, LINQ queries, relationships, performance traps.

EF Core is the standard .NET ORM. Code-first migrations + LINQ queries that translate to SQL = productive but with footguns around N+1 and lazy loading.

Prerequisites:ASP.NET Core

Define models + DbContext

Models are POCOs; DbContext is your unit of work.

html

// Models
public class User {
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public string Email { get; set; } = "";
    public List<Post> Posts { get; set; } = new();
}

public class Post {
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public int UserId { get; set; }
    public User User { get; set; } = null!;
}

// DbContext
public class AppDbContext : DbContext {
    public DbSet<User> Users => Set<User>();
    public DbSet<Post> Posts => Set<Post>();

    protected override void OnConfiguring(DbContextOptionsBuilder options) {
        options.UseNpgsql("Host=localhost;Database=mydb;Username=postgres;Password=...");
        // Or: options.UseSqlServer(connStr) / UseSqlite(connStr)
    }
}

// In Program.cs
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseNpgsql(builder.Configuration.GetConnectionString("Default")));

Migrations + database create

Code-first workflow.

tsx

# Install EF tool (one-time)
dotnet tool install --global dotnet-ef

# Add EF Core packages
dotnet add package Microsoft.EntityFrameworkCore.Design
dotnet add package Microsoft.EntityFrameworkCore.Npgsql

# Add a migration
dotnet ef migrations add InitialCreate

# Apply to database
dotnet ef database update

# Roll back
dotnet ef migrations remove

# Generate SQL script (don't apply)
dotnet ef migrations script

Querying with LINQ → SQL

LINQ queries translate to SQL automatically.

tsx

using var db = new AppDbContext();

// Simple find
var user = await db.Users.FindAsync(1);

// Where + Select (translates to SQL)
var names = await db.Users
    .Where(u => u.Name.StartsWith("A"))
    .Select(u => u.Name)
    .ToListAsync();

// Eager loading (avoids N+1)
var usersWithPosts = await db.Users
    .Include(u => u.Posts)
    .ToListAsync();

// Async paging
var page = await db.Users
    .OrderBy(u => u.Name)
    .Skip(10).Take(10)
    .ToListAsync();

// Insert
db.Users.Add(new User { Name = "Sam", Email = "[email protected]" });
await db.SaveChangesAsync();

// Update, change tracker auto-detects changes
var u = await db.Users.FindAsync(1);
u!.Email = "[email protected]";
await db.SaveChangesAsync();

// Delete
db.Users.Remove(u);
await db.SaveChangesAsync();

Common mistakes

Forgetting Include() → N+1 queries in prod. Singleton DbContext → corrupted state. Loading entire table for a count (use `.CountAsync()` not `.ToListAsync().Count`). Saving inside a loop (call SaveChangesAsync ONCE at the end of a batch).

←ASP.NET Core REST APIs

Back to C# and .NET

C# for Game Dev: Unity Intro→