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 morelinq.

Bad design? Just Call Add? Missing the obvious?

Answer

No, this seems perfectly reasonable. There is a List.AddRange() method that basically does just this, but requires your collection to be a concrete List.