Assigning a Nullable.Value is expensive...

Whilst enhancing some existing code today to support Nullable within some highly performant serialization code I noticed a doubling in the serialization times. My tests indicated that the coding resulted in a doubling of the time to perform the serialization on average, it went from 2 milliseconds to 4 per Type to serialize. This is with 25% of the fields being defined as Nullable. Not significant in the overall scheme of things within a total transaction time on the server, but certainly enough to peek my interest to investigate further.

I wrote a little console test app in an attempt to isolate the problem in case I was doing something stupid - which is quite possible ;-) I am using the latest Visual 2005 Beta 2.

The code basically boxes a DateTime/Nullable and then casts each back to the DateTime primitive using a Stopwatch to determine the total ticks. The following is the test code:

private static void PerfTestNullables()
{
       PerfTestNullable();
       PerfTestNullable();
       PerfTestNullable();
}
 
private static void PerfTestNullable()
{
       DateTime dateTime1 = DateTime.Now;
       Nullable dateTime2 = DateTime.Now;
       Nullable dateTime3 = null;
 
       object obj = dateTime1;
       DateTime dt1;
       Nullable dt2;
 
       // Test normal DateTime cast.
       Stopwatch sw = new Stopwatch();
       sw.Start();
       for (int i = 0; i < 1000000; i++)
              dt1 = (DateTime)dateTime1;
 
       sw.Stop();
       Console.WriteLine(sw.ElapsedTicks);
 
       // Test generics cast, HasValue and assignment
       sw = new Stopwatch();
       sw.Start();
       for (int i = 0; i < 1000000; i++)
       {
              dt2 = (Nullable)dateTime2;
              if (dt2.HasValue)
                     dt1 = dt2.Value;
       }
 
       sw.Stop();
       Console.WriteLine(sw.ElapsedTicks);
 
       // Test generics cast and no value (no assignment)
       sw = new Stopwatch();
       sw.Start();
       for (int i = 0; i < 1000000; i++)
       {
              dt2 = (Nullable)dateTime3;
              if (dt2.HasValue)
                     dt1 = dt2.Value;
       }
 
       sw.Stop();
       Console.WriteLine(sw.ElapsedTicks);
}

The results are as follows - I was certainly surprised by the difference - it is significant.

18212   // Test normal DateTime cast.
110856  // Test generics cast, HasValue and assignment
37330   // Test generics cast and no value (no assignment)
 
18047   // Test normal DateTime cast.
107263  // Test generics cast, HasValue and assignment
44612   // Test generics cast and no value (no assignment)
 
24410   // Test normal DateTime cast.
108138  // Test generics cast, HasValue and assignment
51104   // Test generics cast and no value (no assignment)

The following MSDN bug has been created.