What happens if you call RevertToSelf when not impersonation?

A customer wanted to know what happens if you call Revert­To­Self from a thread that is not impersonating. "Does the call succeed or fail? This particular scenario is not explicitly discussed in the documentation. We have a bunch of places in our code that say if (impersonating) RevertToSelf(); and we were wondering whether the if test was really necessary."

The answer to the question is that calling Revert­To­Self when the thread is not impersonating will return success without doing anything (because the thread is already not impersonating).

However, that doesn't mean that you can blindly remove all your if tests. You don't want to over-revert either. Consider:

// Error checking elided for expository purposes.
void DoSomething()
{
 bool impersonating = false;

 if (!ThreadIsAlreadyImpersonating() &&
     ImpersonationIsNeeded()) {
   StartImpersonating();
   impersonating = true;
 }

 DoWork();

 if (impersonating) {
  RevertToSelf();
 }
}

If you remove the if (impersonating) and unconditionally revert, then you have a security defect if the thread was already impersonating, because your modified code will unconditionally revert and prematurely end the existing impersonation.

So yes, it's okay to call Revert­To­Self when the thread is not impersonating, but that doesn't relieve you of the responsiblity of knowing when to revert.