this might work:

string GetString<T>(List<T> list) where T : IEntityCore {...}

try this:

string GetString<T>(List<T> list) where T : IEntityCore {...}

that way you get a generic method that takes a list of objects which implement IEntityCore. there's no need to cast and the compiler will make sure that you pass in the right objects. works starting at framework 2.0.