Why am I getting “The type parameter must be invariantly valid…” error?

Question

I\'ll attempt to shorten this code example:

public interface IThing
{
    //...  Stuff
}

public class Thing1 : IThing
{  
}

public class Thing2 : IThing
{          


        

Answer 1

You cannot make a covariant ICollection<T>, since it allows you to put Ts into it.

You can make a covariant read-only collection, a contravariant write-only collection, or an invariant read-write collection.
You can't do both, or it wouldn't be typesafe.

Answer 2

To expand on SLaks answer:
To make your code compile, change the return type of ViewAll from ICollection<T> to IEnumerable<T>.