C# List.ToArray performance is bad?

Question

I\'m using .Net 3.5 (C#) and I\'ve heard the performance of C# List.ToArray is \"bad\", since it memory copies for all elements to form a new array. Is

Answer 1

For any kind of List/ICollection where it knows the length, it can allocate an array of exactly the right size from the start.

T[] destinationArray = new T[this._size];
Array.Copy(this._items, 0, destinationArray, 0, this._size);
return destinationArray;

If your source type is IEnumerable (not a List/Collection) then the source is:

items = new TElement[4];
..
if (no more space) {
    TElement[] newItems = new TElement[checked(count * 2)];
    Array.Copy(items, 0, newItems, 0, count);
    items = newItems;

It starts at size 4 and grows exponentially, doubling each time it runs out of space. Each time it doubles, it has to reallocate memory and copy the data over.

If we know the source-data size, we can avoid this slight overhead. However in most cases eg array size <=1024, it will execute so quickly, that we don't even need to think about this implementation detail.

References: Enumerable.cs, List.cs (F12ing into them), Joe's answer