Double question marks ('??') vs if when assigning same var

Question

Referring to the following SE answer.

When writing

A = A ?? B; 

it is the same as

if( null != A )     A = A; else     A = B; 

Does that mean that

if( null == A ) A = B; 

would be preferred, performance wise?

Or can I assume that the compiler optimizes the code when the same object is in the ?? notation?

Answer 1

Don't worry about the performance, it will be negligible.

If you are curious about it, write some code to test the performance using Stopwatch and see. I suspect you'll need to do a few million iterations to start seeing differences though.

You can also never assume about the implementation of things, they are liable to change in future - invalidating your assumptions.

My assumption is the performance difference is likely very, very small. I'd go for the null coalescing operator for readability personally, it is nice and condense and conveys the point well enough. I sometimes do it for lazy-load checking:

_lazyItem = _lazyItem ?? new LazyItem(); 

Answer 2

Although performance for ?? is negligible, the side effect sometimes may not be negligible. Consider the following program.

using System; using System.Diagnostics; using System.Threading;  namespace TestProject {     class Program     {         private string str = "xxxxxxxxxxxxxxx";         public string Str         {             get             {                 return str;             }             set             {                 if (str != value)                 {                     str = value;                 }                 // Do some work which take 1 second                 Thread.Sleep(1000);             }         }          static void Main(string[] args)         {             var p = new Program();              var iterations = 10;              var sw = new Stopwatch();             for (int i = 0; i < iterations; i++)             {                 if (i == 1) sw.Start();                 if (p.Str == null)                 {                     p.Str = "yyyy";                 }             }             sw.Stop();             var first = sw.Elapsed;              sw.Reset();             for (int i = 0; i < iterations; i++)             {                 if (i == 1) sw.Start();                 p.Str = p.Str ?? "yyyy";             }             sw.Stop();             var second = sw.Elapsed;              Console.WriteLine(first);             Console.WriteLine(second);              Console.Write("Ratio: ");             Console.WriteLine(second.TotalMilliseconds / first.TotalMilliseconds);             Console.ReadLine();         }      } } 

Run result on my PC.

00:00:00.0000015 00:00:08.9995480 Ratio: 5999698.66666667 

Because there is an extra assignment using ??, and the performance of the assignment sometimes might not guaranteed. This might lead to a performance issue.

I would rather use if( null == A ) A = B; instead of A = A ?? B;.