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

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.

Instead of Select you can stick to the Cast LINQ-operator:

List<int> first = new List<int>() {1, 2, 3};
List<int?> second = first.Cast<int?>().ToList();

If you want to know what's the faster solution, you should do a little benchmark by using the three different ways:

List<int> list = Enumerable.Range( 0, 10000 ).ToList( );
Stopwatch sw = Stopwatch.StartNew( );

for ( int i = 0; i < 100000; i++ ) {
   List<int?> newList = new List<int?>( );
   foreach( int integer in list )
      newList.Add( ( int? ) integer );
}

sw.Stop( );
TimeSpan timespan = sw.Elapsed;
Console.WriteLine( String.Format( "Foreach: {0:00}:{1:00}:{2:00}", timespan.Minutes, timespan.Seconds, timespan.Milliseconds / 10 ) );
sw.Restart( );

for ( int i = 0; i < 100000; i++ ){
   List<int?> newList = list.Select( x => ( int? ) x ).ToList( );
}

sw.Stop( );
timespan = sw.Elapsed;
Console.WriteLine( String.Format( "LINQ-Select: {0:00}:{1:00}:{2:00}", timespan.Minutes, timespan.Seconds, timespan.Milliseconds / 10 ) );
sw.Restart( );

for ( int i = 0; i < 100000; i++ ){
   List<int?> newList = list.Cast<int?>( ).ToList( );
}

sw.Stop();
timespan = sw.Elapsed;
Console.WriteLine( String.Format( "LINQ-Cast: {0:00}:{1:00}:{2:00}", timespan.Minutes, timespan.Seconds, timespan.Milliseconds / 10 ) );

Results:

As we could expect the best way is the first solution (foreach) which means loop through the elements, cast and add them to a new list.