Xtykutl

A coworker asked me today how to add a range to a collection. He has a class that inherits from Collection<T>. 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<T>-style AddRange would be far more elegant.

It's easy enough to write an extension method:

public static class CollectionHelpers

{

    public static void AddRange<T>(this ICollection<T> destination,

                                   IEnumerable<T> 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?

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

Try casting to List in the extension method before running the loop. That way you can take advantage of the performance of List.AddRange.

public static void AddRange<T>(this ICollection<T> destination,

                               IEnumerable<T> source)

{

    List<T> list = destination as List<T>;



    if (list != null)

    {

        list.AddRange(source);

    }

    else

    {

        foreach (T item in source)

        {

            destination.Add(item);

        }

    }

}

Since .NET4.5 if you want one-liner you can use System.Collections.Generic ForEach.

source.ForEach(o => destination.Add(o));

or even shorter as

source.ForEach(destination.Add);

Performance-wise it's same as for each loop (syntactic sugar).

Also don't try assigning it like

var x = source.ForEach(destination.Add) 

cause ForEach is void.

Remember that each Add will check the capacity of the collection and resize it whenever necessary (slower). With AddRange, the collection will be set the capacity and then added the items (faster). This extension method will be extremely slow, but will work.