Fastest way to convert List<int> to List<int?>

Question

What is the fastest way of taking a list of primitives and converting it to a nullable list of primitives? For example: List<int> to List<int?>.

The easy solution, creating a new list and adding every item with a foreach loop, takes too much time.

Answer 1

There is no way faster than creating a new list:

var newList = list.Select( i => (int?)i ).ToList(); 

However using LINQ is slower than using a bare loop.

The fastest way is to use a List<int?> with pre-allocated capacity:

List<int?> newList = new List<int?>(list.Count); // Allocate enough memory for all items foreach (var i in list)     newList.Add(i); 

If you are seeking for in-place type change of list items, it's not possible.