c# - AddRange to a Collection

A coworker asked me today how to add a range to a
collection. He has a class that inherits from
Collection. There's a get-only property of that type
that already contains some items. He wants to add the items in another collection to the
property collection. How can he do so in a C#3-friendly fashion? (Note the
constraint about the get-only property, which prevents solutions like doing Union and
reassigning.)

Sure, a foreach with
Property. Add will work. But a List-style AddRange
would be far more elegant.

It's easy enough to
write an extension
method:

public static
            class CollectionHelpers
{
 public static void AddRange(this
            ICollection destination,
 IEnumerable source)

            {
 foreach (T item in source)
 {

            destination.Add(item);
 }


            }
}

But I
have the feeling I'm reinventing the wheel. I didn't find anything similar in
System.Linq or rel="noreferrer">morelinq.

Bad
design? Just Call Add? Missing the obvious?