Why can't “return” and “yield return” be used in the same method?

Question

Why can\'t we use both return and yield return in the same method?

For example, we can have GetIntegers1 and GetIntegers2 below, but not GetIntegers3.

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Answer 1

The compiler re-writes any methods with a yield statement (return or break). It currently cannot handle methods that may or may not yield.

I'd recommend having a read of chapter 6 of Jon Skeet's C# in Depth, of which chapter 6 is available for free - it covers iterator blocks quite nicely.

I see no reason why this would not be possible in future versions of the c# compiler however. Other .Net languages do support something similar in the form of a 'yield from' operator (See F# yield!). If such an operator existed in c# it would allow you to write your code in the form:

public IEnumerable GetIntegers()
{
  if ( someCondition )
  {
    yield! return new[] {4, 5, 6};
  }
  else
  {
    yield return 1;
    yield return 2;
    yield return 3;
  }
}