How can I add an IEnumerable<T> to an existing ICollection<T>

Question

Given an existing ICollection<T> instance (e.g. dest) what is the most efficient and readable way to add items from an IEnumerable<T>?

In my use case, I have some kind of utility method Collect(IEnumerable items) which returns a new ICollection with the elements from items, so I am doing it in the following way:

public static ICollection<T> Collect<T>(IEnumerable<T> items) where T:ICollection<T>
{
    ...
    ICollection<T> dest = Activator.CreateInstance<T>();
    items.Aggregate(dest, (acc, item) => { acc.Add(item); return acc; });
    ...
    return dest;
}

Question: Is there any “better” way (more efficient or readable) of doing it?

UPDATE: I think the use of Aggregate() is quite fluent and not so inefficient as invoking ToList().ForEach(). But it does not look very readable. Since nobody else agrees with the use of Aggregate() I would like to read your reasons to NOT use Aggregate() for this purpose.

Answer 1

Most efficient:

foreach(T item in itens) dest.Add(item)

Most readable (BUT inefficient because it is creating a throwaway list):

items.ToList().ForEach(dest.Add);

Less readable, but Not so inefficient:

items.Aggregate(dest, (acc, item) => { acc.Add(item); return acc; });

Answer 2

Just use Enumerable.Concat:

IEnumerable<YourType> result = dest.Concat(items);

If you want a List<T> as result use ToList:

List<YourType> result = dest.Concat(items).ToList();
// perhaps:
dest = result;

If dest is actually already a list and you want to modify it use AddRange:

dest.AddRange(items);

---

Update:

if you have to add items to a ICollection<T> method argument you could use this extension:

public static void AddRange<T>(this ICollection<T> collection, IEnumerable<T> seq)
{
    List<T> list = collection as List<T>;
    if (list != null)
        list.AddRange(seq);
    else
    {
        foreach (T item in seq)
            collection.Add(item);
    }
}

// ...

public static void Foo<T>(ICollection<T> dest)
{
    IEnumerable<T> items = ... 
    dest.AddRange(items);
}