Entity Framework Code First: Setup and Usage

Entity Framework Code First is an approach to building database-driven applications where you define your data model using C# classes first, and Entity Framework automatically creates the database schema based on those models. This approach provides flexibility, version control friendliness, and a code-centric development experience.

Prerequisites

Before you start, ensure you have:

• Visual Studio or Visual Studio Code
• A .NET project (Framework or .NET Core/8)
• NuGet Package Manager installed

Step 1: Install Entity Framework

Install Entity Framework via NuGet Package Manager. Open the Package Manager Console and run:

Install-Package EntityFramework

For .NET Core/.NET 5+, use:

Install-Package Microsoft.EntityFrameworkCore

Step 2: Define Your Model Classes

Create C# classes that represent your database entities. Each class will become a table in the database.

using System;
using System.Collections.Generic;

public class Customer
{
    public int CustomerId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public DateTime CreatedDate { get; set; }

    // Navigation property for related orders
    public virtual ICollection Orders { get; set; }
}

public class Order
{
    public int OrderId { get; set; }
    public int CustomerId { get; set; }
    public decimal Total { get; set; }
    public DateTime OrderDate { get; set; }

    // Foreign key and navigation property
    public virtual Customer Customer { get; set; }
}

Step 3: Create a DbContext

The DbContext is the main class that coordinates Entity Framework functionality. It represents a session with the database and allows you to query and save data.

using Microsoft.EntityFrameworkCore;

public class MyApplicationContext : DbContext
{
    public DbSet Customers { get; set; }
    public DbSet Orders { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=YOUR_SERVER;Database=MyAppDb;Trusted_Connection=true;");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        // Configure relationships, indexes, and constraints here
        modelBuilder.Entity()
            .HasOne(o => o.Customer)
            .WithMany(c => c.Orders)
            .HasForeignKey(o => o.CustomerId);
    }
}

Step 4: Create and Apply Migrations

Migrations allow you to version control your database schema. Enable migrations and create the initial migration:

Enable-Migrations
Add-Migration InitialCreate

This creates a migration file in your Migrations folder. To apply the migration to your database:

Update-Database

Your database and tables are now created!

Step 5: Query and Manipulate Data

Now you can use your context to query and save data:

using (var context = new MyApplicationContext())
{
    // Create a new customer
    var customer = new Customer
    {
        FirstName = "John",
        LastName = "Doe",
        Email = "john.doe@example.com",
        CreatedDate = DateTime.Now
    };

    context.Customers.Add(customer);
    context.SaveChanges();

    // Query customers
    var customers = context.Customers.Where(c => c.LastName == "Doe").ToList();

    // Update a customer
    var existingCustomer = context.Customers.FirstOrDefault(c => c.CustomerId == 1);
    if (existingCustomer != null)
    {
        existingCustomer.Email = "newemail@example.com";
        context.SaveChanges();
    }

    // Delete a customer
    context.Customers.Remove(existingCustomer);
    context.SaveChanges();
}

Step 6: Working with Relationships

Entity Framework handles relationships between entities:

// Load customer with related orders (Eager Loading)
var customerWithOrders = context.Customers
    .Include(c => c.Orders)
    .FirstOrDefault(c => c.CustomerId == 1);

// Create an order for a customer
var newOrder = new Order
{
    CustomerId = 1,
    Total = 150.00m,
    OrderDate = DateTime.Now
};
context.Orders.Add(newOrder);
context.SaveChanges();

Common Configurations

Customize your model behavior using Fluent API in OnModelCreating:

modelBuilder.Entity(entity =>
{
    entity.HasKey(c => c.CustomerId);
    entity.Property(c => c.Email).IsRequired().HasMaxLength(255);
    entity.Property(c => c.FirstName).IsRequired().HasMaxLength(100);
    entity.HasIndex(c => c.Email).IsUnique();
});

modelBuilder.Entity(entity =>
{
    entity.Property(o => o.Total).HasPrecision(10, 2);
});

Best Practices

• Use async/await: Use ToListAsync(), SaveChangesAsync() for better performance
• Leverage migrations: Always track schema changes with migrations
• Use dependency injection: Inject your DbContext rather than hardcoding connection strings
• Handle relationships carefully: Use Include() for eager loading to avoid N+1 queries
• Dispose DbContext: Always dispose your context using using statements or dependency injection
• Enable lazy loading judiciously: Be aware of performance implications

Troubleshooting

Issue: "No DbContext was found in assembly"
Solution: Ensure your DbContext is in the startup project and accessible to the migrations project.

Issue: "The connection string is not valid"
Solution: Verify your connection string in OnConfiguring or appsettings.json.