Hi!

I have a generic abstract base class MyCollection<T> which represents a collection
of T's with some common utility methods.
I then have a number of non-generic subclasses, e.g.

class FooCollection : MyCollection<Foo>
{   /* utility methods only for foo collections */ }

class BarCollection : MyCollection<Bar>
{   /* utility methods only for Bar collections */ }

Now, I'd like to provide a subset method in MyCollection - e.g. for getting
the first 10 items
(In real life selecting the subset is more complex and based on the structure
of MyCollection as well as properties of the T base type)

FooCollection allFoos = ...
FooCollection tenFoos = allFoos.GetSubset(10);

But I'd like to return a FooCollection - not a MyCollection<Foo>. The best
I have been able to come up is this method in MyCollection:

DERIVEDCOLLECTION GetSubset<DERIVEDCOLLECTION>(int count) where DERIVEDCOLLECTION:
MyCollection<T>, new()
{
 DERIVEDCOLLECTION result = new DERIVEDCOLLECTION();
 for(int i = 0; i < count; i++) // example logic only!
    result.Add(this[i]);
 return result;
}

This is not very elegant, since it requires the user to specify the target
class:
FooCollection tenFoos = allFoos.GetSubset<FooCollection>(10);

An alternative would be to return an IEnumerable<T>, but that would require
the user to write

FooCollection allFoos = ...
FooCollection tenFoos = new FooCollection();
tenFoos.AddAll(allFoos.GetSubset(10))

Or, finally, I could implement new GetSubset in all derived collections -
but since there are a number of such functions, that's not too useful either.

Does anybody have any suggestions?