Bad Use of Null Coalescing Operator?

Question

``` myFoo = myFoo ?? new Foo(); ``` instead of ``` if (myFoo == null) myFoo = new Foo(); ``` Am I correct in thinking that the first line of code will always perform an assignment? Also, is this a bad use of the null-coalescing operator?

Accepted Answer

I compared the CIL of the generated code (making sure to do a Release build - with Optimize Code checked in the Project Properties, which corresponds to the `/optimize` switch on `csc.exe`). This is what I got (using VS 2008 - note that `Foo.MaybeFoo()` is a method that sometimes returns `null`, sometimes a `Foo`)

`GetFooWithIf`:

```
  IL_0000:  call       class Application3.Foo Application3.Foo::MaybeFoo()
  IL_0005:  stloc.0
  IL_0006:  ldloc.0
  IL_0007:  brtrue.s   IL_000f
  IL_0009:  newobj     instance void Application3.Foo::.ctor()
  IL_000e:  stloc.0
  IL_000f:  ldloc.0
  IL_0010:  ret
```

`GetFooWithCoalescingOperator`:

```
  IL_0000:  call       class Application3.Foo Application3.Foo::MaybeFoo()
  IL_0005:  stloc.0
  IL_0006:  ldloc.0
  IL_0007:  dup
  IL_0008:  brtrue.s   IL_0010
  IL_000a:  pop
  IL_000b:  newobj     instance void Application3.Foo::.ctor()
  IL_0010:  stloc.0
  IL_0011:  ldloc.0
  IL_0012:  ret
```

Thus, the same except for an extra top-of-stack-duplication and pop. If this can be made to make a measurable performance difference, I will purchase a hat specifically for the purpose of eating it; therefore, go with the one that you feel offers better readability.

(edit) oh, and the JITter might be clever enough to get rid of even that difference!

Bad Use of Null Coalescing Operator?

Solution

Problem