c# - Question regarding return types with collections

In my BL (will be a public API), I am using ICollection as the return types in my Find methods, like:

public static ICollection FindCustomers()
{
   Collection customers = DAL.GetCustomers();


   return customers;
}

Note the use of ICollection instead of Collection<>.

Now in my GUI, I need to cast the results back to Collection, like:

Collection customers = (Collection)BL.FindCustomers();

This is because I need to use some Collection<> specific methods on my returned list, which I cannot do with ICollection<>.

Is this the correct usage? Or should I simply change the return type from Collection<> instead to ICollection<> in order to avoid this casting?

Secondly, I did not use IEnumerable because it is more generic than ICollection and does not even have simple properties like Count. And I really don’t see a point in generalizing the return types here. Am I missing something important?

Answer

The whole point of using ICollection is to be more generic and hide more information, which is a good thing.

But if you need to convert it back it has become pointless and you might as well return the more functional Collection< > instead.