To interface or not to interface

[Kagamin]

Recently I've hit a problem with collections in C#. Given classes
class A {}
class B {}
And two collections Collection<A> and Collection<B> it's possible to concat them into an array A[]. The efficient way to do it is to use CopyTo(T[],int) method, but it accepts only array of exact collection item's type, so I had to change the Collection<B> type to Collection<A>.