Question

I have two classes that each implement an interface. One of the classes contains an ICollection of the other’s interfaces.

Now I want to map this to my database using EF but get an exception (below). Is this supposed to be possible somehow?

Entity definitions for my classes (products and categories):

public interface IProduct
{
    string ProductId { get; set; }
    string CategoryId { get; set; }
}

public interface ICategory
{
    string CategoryId { get; set; }
    ICollection<IProduct> Products  { get; set; };
}

public class ProductImpl : IProduct
{
    public string ProductId { get; set; }
    public string CategoryId { get; set; }
}

public class CategoryImpl : ICategory
{
    public string CategoryId { get; set; }
    public ICollection<IProduct> Products { get; set; }
}

I want to map the relationship between CategoryImpl and ProductImpl so I’m using the following OnModelCreating method in my DbContext:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    var a = modelBuilder.Entity<CategoryImpl>();
    a.ToTable("Categories");
    a.HasKey(k => k.CategoryId);
    a.Property(p => p.CategoryId);
    a.HasMany(p => p.Products).WithOptional().HasForeignKey(p => p.CategoryId);

    var b = modelBuilder.Entity<ProductImpl>();
    b.ToTable("Products");
    b.HasKey(k => k.ProductId);
    b.Property(p => p.ProductId);
}

The exception I get is below. Am I supposed to somehow specify that the concrete type to be used for IProduct is ProductImpl?

    System.InvalidOperationException: The navigation property 'Products' 
is not a declared property on type 'CategoryImpl'. Verify that it has 
not been explicitly excluded from the model and that it is a valid navigation property.

Answer 1

It’s not possible to do it with interfaces in EF. The type of a navigation property must be mapped for the property to be mapped. And for a type to be mapped it needs to be a concrete type among other things.

If you need to have different types of products and categories you could instead use a base class for them:

public class ProductBase
{
    public string ProductId { get; set; }
    public string CategoryId { get; set; }
}

public class CategoryBase
{
    public string CategoryId { get; set; }
    public virtual ICollection<ProductBase> Products { get; set; }
}

public class DerivedProduct : ProductBase
{
}

public class DerivedCategory : CategoryBase
{
}