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

The dissassembly from reflector for the List AddRange method has the following code

ICollection is2 = collection as ICollection;
if (is2 != null)
{
    int count = is2.Count;
    if (count > 0)
    {
        this.EnsureCapacity(this._size + count);
        if (index < this._size)
        {
            Array.Copy(this._items, index, this._items, index + count, this._size - index);
        }
        if (this == is2)
        {
            Array.Copy(this._items, 0, this._items, index, index);
            Array.Copy(this._items, (int) (index + count), this._items, (int) (index * 2), (int) (this._size - index));
        }
        else
        {
            T[] array = new T[count];
            is2.CopyTo(array, 0);
            array.CopyTo(this._items, index);
        }
        this._size += count;
    }
}

As you can see there are some optimizations like EnsureCapacity() call and using Array.Copy().