A number of readers have the mysterious fifth sense which gives them the ability to deduce that the GetBars method from yesterday's post contains a yield return and is therefore an iterator. Remember, as the standard states (in section 10.14.4):
[...] execution of the code in the iterator block occurs when the enumerator object's MoveNext method is invoked.
Since the test program does not invoke MoveNext, the check for null is never executed, and therefore the exception is never thrown.
Since most of the interesting new sequence operators available in C# 3.0 are implemented with iterators it probably will be increasingly important for developers to understand a bit more about how iterators work behind the scenes. I may do some blog posts on that over the next little while.
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About Eric Lippert
Eric Lippert is a senior developer on the Microsoft C# compiler team. Before that he worked on the framework of Visual Studio Tools For Office. Before that, he worked on the compilers, runtimes and tools for VBScript, JScript, Windows Script Host and other Microsoft Scripting technologies. He lives in Seattle and spends his free time editing books about programming languages, playing the piano, and trying to keep his tiny sailboat upright in Puget Sound.
