Covariance in generic interfaces

Question

I wanted to create an observableCollection that is sortable so i started creating a class that inherit observable with some methods to sort it, then i wanted that class to p

Answer 1

You need DerivedClass to be a ISortable:

public class DerivedClass : BaseClass, ISortable
{
    public new ISortableCollection ParentCollection
    {
        get { throw new NotImplementedException(); }
        set { throw new NotImplementedException(); }
    }
}

Co- and contravariance on T cannot work here because you are deriving from IList which is invariant.

Even removing IList and removing the getter I can't get it to work right now with variance. Not exactly a strength of mine. This is a part of the type system that is better left alone if you can help it.

If the type system makes your head explode consider a dynamic solution:

((dynamic))item).ParentCollection = this;