T in class? AddRange ICollection?

Question

I try to do static class, add to icollection but i got some issues i cant seem to overcome. that is how i get so i can pass a ICollection in the method? cause T is that say it can not be resolved.

and then i wonder is there a way to do AddRange on icollection?

i was thinking of something like this but maby i am way out of my mind with it?

public static ICollection<T> add(this IEnumerable<T> list)
    {
        ICollection<T> collection = null;

        return collection.AddRange(list);            
    }

Answer 1

No, ICollection<T> doesn't have an AddRange method - and even if it did, you'd be trying to dereference null which will throw a NullReferenceException. You haven't specified a collection to add the list to... what exactly are you trying to do?

You could create (say) a new List<T> - and that has the benefit of already having a constructor which can take an IEnumerable<T>:

public static ICollection<T> Add<T>(this IEnumerable<T> list)
{
    return new List<T>(list);            
}

However, at that point you've really just reimplemented Enumerable.ToList() and given it a different return type...

If you want to add everything to an existing collection, you might want something like this:

public static ICollection<T> AddTo<T>(this IEnumerable<T> list,
                                      ICollection<T> collection)
{
    foreach (T item in list)
    {
        collection.Add(item);
    }
    return collection;
}

Answer 2

If I understand correctly you want to add a IEnumerable<T> to an empty collection.

Wouldn't it be easier to just do:

ICollection<MyObject> collection = new List<MyObject>(GetIEnumerableOfMyObject());

Or even:

ICollection<MyObject> collection = GetIEnumerableOfMyObject().ToList();

Answer 3

The other ways seem to assume that your ICollection is empty and/or your ICollection is a type of List. However, if you want AddRange, then you can Extend the ICollection class as follows:

public static void AddRange<T>(this ICollection<T> ic, IEnumerable<T> ie)
{
    foreach (T obj in ie)
    {
        ic.Add(obj);
    }
}

Note, however, that since List impliments ICollection, this may cause ambiguity when dealing directly with List objects (though I haven't tested yet if the compiler will be able to resolve it--my gut reaction is that it should, though, since AddRange is a member of List and the compiler will go through member functions first before looking at extensions, but if I'm wrong I'm sure someone will correct me).

Answer 4

Depending on the collection type of your source list an alternative approach is to use List(T).ForEach, as in:

List<string> source = ...
ICollection<string> dest = ...

source.Foreach(dest.Add);

However, the readability of this is easy to dispute.