List<T> Concatenation for 'X' amount of lists

Question

I have for example 5 List all of the same type. Can I simply do

List<T> newset = List1.Concat(List2).Concat(List3).Concat(List4).....

Answer 1

You can do this (although you need .ToList() at the end).

However, it would be (slightly) more efficient to generate a single list, and use AddRange to add in each list. Just initialize the list with the total size of all of your lists, then call AddRange repeatedly.

You might want to consider doing something like:

public List<T> ConcatMultiple<T>(this List<T> list, params[] ICollection<T> others)
{
    List<T> results = new List<T>(list.Count + others.Sum(i => i.Count));
    results.AddRange(list);
    foreach(var l in others)
        results.AddRange(l);
    return results;
}

Then calling via:

List<MyClass> newset = List1.ConcatMultiple(List2, List3, List4);

Answer 2

Yes, you can do that.

List<Thing> newSet = List1.Concat(List2).Concat(List3).Concat(List4).ToList();