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

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