Question

In my project there are 2 classes (this will increase) Post and Product, both can have comments below is the structure i have created but its no working

I have created an interface for entities which can have comments

public interface ICommentable
{
    int Id { get; set; }
}

Post and Product entities may have comments

public class Post : ICommentable
{
    public int Id { get; set; }
    public string Text { get; set; }
    public ICollection<Comment> Comments { get; set; }
}

public class Product : ICommentable
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Rate { get; set; }
    public ICollection<Comment> Comments { get; set; }
}

Comment Entity Itself

public class Comment
{
    public int Id { get; set; }
    public string Message { get; set; }

    public int ItemId { get; set; }
    public ICommentable Item{ get; set; }
}

Data Contxet

public class DataBaseContext:DbContext
{
    public DbSet<Comment> Comments { get; set; }
    public DbSet<Post> Posts { get; set; }
    public DbSet<Product> Products { get; set; }
}

Answer 1

Entity framework doesn’t support interfaces so it is your first problem. The second problem is that navigation property cannot be normally “polymorphic” = you need separate navigation property (and FK) for every ICommentable you want to relate to your Comment:

public class Comment
{
    public int Id { get; set; }
    public string Message { get; set; }

    public int PostId { get; set; }
    public Post Post { get; set; }

    public int ProductId { get; set; }
    public Product Product { get; set; }
}

The only scenario where something similar can work is mapped inheritance but it would force your Post and Product to inherit from mapped base entity and this inheritance will be reflected in the database. In such case you can have separate table for base entity and this table can have relation to Comment (instead of derived tables).

When working with relations always think about the way how would you done it in the database. Your classes mapped by EF must follow similar rules.