How to Convert IList<SomeObject> to IList<ISomeInterface> where SomeObject implements ISomeInterface using covariance in C# 4.0

Question

How to Convert IList to IList where SomeObject implements ISomeInterface using covariance in C# 4.0

I have something similar to following

IList<Items> GetItems;

IList<IItems> items = GetItems() as IList<IItems>; 

but items is null;

the answer here was for pre 4.0:

Converting an array of type T to an array of type I where T implements I in C#

Answer 1

why not simply use

IList<Items> GetItems;
IList<IItems> items = GetItems().Cast<IItems>().ToList(); 

Answer 2

For this to work as you are thinking then I believe IList would have to be declared as covariant, not the items in the list. And IList does not support covariance. The only .NET interfaces that were updated to support covariance are:

• IEnumerable (T is covariant)
• IEnumerator (T is covariant)
• IQueryable (T is covariant)
• IGrouping (TKey and TElement are covariant)
• IComparer (T is contravariant)
• IEqualityComparer (T is contravariant)
• IComparable (T is contravariant)

This from http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx

Answer 3

Technically, the following will work, but I don't think this is exactly what you're looking for...

IList<IItems> items = GetItems().ToArray() as IList<IItems>;