Are these two lines the same, '? … :' vs '??'?

Question

Is there a difference between these two lines?

MyName = (s.MyName == null) ? string.Empty : s.MyName

or

MyName = s.MyName ?? st         


        

Answer 1

UPDATE: I wrote a blog post that discusses this topic in more depth. http://www.codeducky.org/properties-fields-and-methods-oh-my/

---

Generally they will return the same result. However, there are a few cases where you will experience noticeable differences when MyName is a property because the MyName getter will be executed twice in the first example and only once in the second example.

For example, you may experience performance differences from executing MyName twice:

string MyName
{
    get 
    {
        Thread.Sleep(10000);
        return "HELLO";
    }
}

Or you may get different results from executing MyName twice if MyName is stateful:

private bool _MyNameHasBeenRead = false;

string MyName
{
    get 
    {
        if(_MyNameHasBeenRead)
                throw new Exception("Can't read MyName twice");
        _MyNameHasBeenRead = true;
        Thread.Sleep(10000);
        return "HELLO";
    }
}

Or you may get different results from executing MyName twice if MyName can be changed on a different thread:

void ChangeMyNameAsync()
{
    //MyName set to null in another thread which makes it 
    //possible for the first example to return null
    Task.Run(() => this.MyName = null);
}

string MyName { get; set; }  

Here's how the actual code is compiled. First the piece with the ternary expression:

IL_0007:  ldloc.0     // s
IL_0008:  callvirt    s.get_MyName       <-- first call
IL_000D:  brfalse.s   IL_0017
IL_000F:  ldloc.0     // s
IL_0010:  callvirt    s.get_MyName       <-- second call
IL_0015:  br.s        IL_001C
IL_0017:  ldsfld      System.String.Empty
IL_001C:  call        set_MyName

and here is the piece with the null-coalescing operator:

IL_0007:  ldloc.0     // s
IL_0008:  callvirt    s.get_MyName       <-- only call
IL_000D:  dup         
IL_000E:  brtrue.s    IL_0016
IL_0010:  pop         
IL_0011:  ldsfld      System.String.Empty
IL_0016:  call        s.set_MyName

As you can see the compiled code for the ternary operator will make two calls to get the property value, whereas the null-coalescing operator will only do 1.