Converting a EF CodeFirst Base class to a Inherited class (using table-per-type)

Question

I am using EF Code First and have two classes defined as follows:

public class User
{
    public int Id { get; set; }
    public string Username { get; set;          


        

Answer 1

You were almost there... The key is to detach the existing entity, then attach the new one.

Here's an example:

using System.Data;
using System.Data.Entity;
using System.Diagnostics;

public class Animal
{
    public long Id { get; set; }
}

public class Dog : Animal
{
}

public class AnimalsContext : DbContext
{
    public DbSet Animals { get; set; }
}


public class Tester
{
    public void Test()
    {
        var context = new AnimalsContext();


        var genericAnimal = new Animal();
        context.Animals.Add(genericAnimal);
        context.SaveChanges();


        // Make a new clean entity, but copy the ID (important!)
        var dog = new Dog { Id = genericAnimal.Id, };

        // Do the old switch-a-roo -- detach the existing one and attach the new one
        // NOTE: the order is important!  Detach existing FIRST, then attach the new one
        context.Entry(genericAnimal).State = EntityState.Detached;
        context.Entry(dog).State = EntityState.Modified;
        context.SaveChanges();


        var thisShouldBeADog = context.Animals.Find(genericAnimal.Id);

        // thisShouldBeADog is indeed a Dog!
        Debug.Assert(thisShouldBeADog is Dog);

        // And, of course, all the IDs match because it's the same entity
        Debug.Assert((genericAnimal.Id == dog.Id) && (dog.Id == thisShouldBeADog.Id));
    }
}