Question

I am trying to get to grips with more complicated inheritance structures and generics and I am trying to create some architecture for a current project which is following this suit. My problem currently is I am getting this error:

Type argument ‘Foo’ does not inherit from or implement the constraint type ‘ListBase’

  public class ItemBase {}
  public class ListBase<T> where T : ItemBase
  {
    public virtual List<T> ListExample {get; set; }
  }

These are my base classes, although they probably aren’t named appropriately I have just tried to show a simple example of what I am trying to achieve.

  public class FooItem : ItemBase { }
  public class Foo : ListBase<FooItem>
  {
    public override List<FooItem> ListExample { get; set;}
  }

So I can then extend the initial base class for the lists and do more with it, but I want a generic way of handling all of these classes.

  public class ListHandler<T> where T : ListBase<ItemBase> { }

When I try to pass Foo as T to the ListHandler I get the error mentioned, I thought that inevitably because Foo is a List<ItemBase> and FooItem is of type ItemBase I would be able to do this var handler = new ListHandler<Foo>();.

Could anybody explain why I can’t do this or what I am doing wrong?

Answer 1

A ListBase<ItemBase> is not the same as a ListBase<FooItem>.
In particular, you can add any kind of ItemBase to a ListBase<ItemBase>.

You need to accept two generic parameters:

public class ListHandler<TList, TItem> where T : ListBase<TItem> where TItem : ItemBase { }