Question

I would like to add new iEnumerable object to original one. Can I do this updating original object in extenstion method like following?

public static void AddItems<TSource>(this IEnumerable<TSource> orginalColl,    
IEnumerable<TSource> collectionToAdd)
{
foreach (var item in collectionToAdd)
{
    orginalColl.ToList<TSource>().Add(item);             
}             
}

I am calling like this: OrgCollecation.AddItems(newCollection).
But this does not seems to work. Any idea?

Answer 1

An IEnumerable[<T>] is not intended for adding. You can concatenate (generating a new sequence), but that doesn’t change the original sequence (/list/array/etc). You can do that with Enumerable.Concat, i.e. return orginalColl.Concat(...). But emphasis: this does not update the original collection.

What you could do would be to cast to IList[<T>] or similar, but that would be abusive (and will only work for some scenarios, not all). It won’t work, for example, for anything that is based on an iterator block (or any other IEnumerable<T> that is not also an IList<T>) – for example it won’t work on someSource.Where(predicte).

If you expect to change the source, then you should be passing in something like IList[<T>]. For example:

public static void AddItems<TSource>(this IList<TSource> orginalColl,
    IEnumerable<TSource> collectionToAdd)
{
    foreach (var item in collectionToAdd)
    {
        orginalColl.Add(item);             
    }             
}

(btw, AddRange would be an alternative name for the above, to match List<T>.AddRange)