Why is AddRange faster than using a foreach loop?

Question

var fillData = new List();
for (var i = 0; i < 100000; i++)
     fillData.Add(i);

var stopwatch1 = new Stopwatch();
stopwatch1.Start();

var autoFill          


        

Answer 1

If you are using Add, it is resizing the inner array gradually as needed (doubling), from the default starting size of 10 (IIRC). If you use:

var manualFill = new List(fillData.Count);

I expect it'll change radically (no more resizes / data copy).

From reflector, AddRange does this internally, rather than growing in doubling:

ICollection is2 = collection as ICollection;
if (is2 != null)
{
    int count = is2.Count;
    if (count > 0)
    {
        this.EnsureCapacity(this._size + count);
        // ^^^ this the key bit, and prevents slow growth when possible ^^^