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 AddRange extension does not iterate through each item, but applies each item as a whole. Both foreach and .AddRange have a purpose. Addrange will win the contest of speed for your current situation.

Answer 2

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<int>(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<T> is2 = collection as ICollection<T>;
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 ^^^

Answer 3

When using AddRange the Collection can increase the size of the array once and then copy the values into it.

Using a foreach statement the collection needs to increase size of the collection more than once.

Increasing thr size means copying the complete array which takes time.

Answer 4

Try out initialize intiial list capacity before manually adding items:

var manualFill = new List<int>(fillData.Count); 

Answer 5

It is because the Foreach loop will add all the values that the loop is getting one a time and
the AddRange() method will gather all the values it is getting as a "chunk" and add that chunk at once to the specified location.

Simply understanding, it is just like you have a list of 10 items to bring from the market, which would be faster bringing all that one by one or all at single time.

Answer 6

This is like asking the waiter to bring you one beer ten times and asking him to bring you 10 beers at once.

What do you think is faster :)