When NOT to use yield (return) [duplicate]

Answer 1

There are a lot of excellent answers here. I would add this one: Don't use yield return for small or empty collections where you already know the values:

IEnumerable GetSuperUserRights() {
    if(SuperUsersAllowed) {
        yield return UserRight.Add;
        yield return UserRight.Edit;
        yield return UserRight.Remove;
    }
}

In these cases the creation of the Enumerator object is more expensive, and more verbose, than just generating a data structure.

IEnumerable GetSuperUserRights() {
    return SuperUsersAllowed
           ? new[] {UserRight.Add, UserRight.Edit, UserRight.Remove}
           : Enumerable.Empty();
}

Update

Here's the results of my benchmark:

These results show how long it took (in milliseconds) to perform the operation 1,000,000 times. Smaller numbers are better.

In revisiting this, the performance difference isn't significant enough to worry about, so you should go with whatever is the easiest to read and maintain.

Update 2

I'm pretty sure the above results were achieved with compiler optimization disabled. Running in Release mode with a modern compiler, it appears performance is practically indistinguishable between the two. Go with whatever is most readable to you.