我比较了生成代码的CIL(确保执行发布版本-在项目属性中选中优化代码"，这与/optimize相对应)开启csc.exe).这就是我得到的(使用VS 2008-注意Foo.MaybeFoo()是一种有时会返回null有时是Foo的方法)

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

因此，除了额外的堆栈顶部复制和弹出操作外，其他操作均相同.如果可以通过这种方式来显着改善性能，我将专门购买帽子以供食用.因此，请选择您认为具有更好可读性的代码.

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.

(编辑)哦，准时器可能足够聪明，甚至可以消除这种差异！

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