c# - Cannot implicitly convert List<T> to Collection<T>

Question

This is a compiler error (slightly changed for readability).

This one always puzzled me. FxCop tells that this is a bad thing to return List and classes that arederived from Collection<T> should be preferrable as return types.

Also, FxCop says that it is OK to use List<T> for internal data storagemanipulation. Ok, I get it, but what I don't get is that compiler complains about trying to implicitly convert List<T> to Collection<T>. Isn't List<T> more interface-charged and functional? Why prohibit implicit conversion?

And another question that stems from above: is new List<int>(some collection<int>) constructor expensive?

Thank you,

Valentin Vasiliev

Answer 1

Why not just do the following:

Collection<string> collection = new Collection<string>(theList);

as Collection(IList input) takes a List as part of construction.