Today I really fell over while trying to answer a question about a (fairly stupid) C# expression. It went something like the following:

Consider the following code:

int i = 0;
while (i < 10)
{
  Console.WriteLine(i++);
}
while (i >= 10);


This code:

1. Will not compile
2. Will print numbers from 0..9
3. Will print numbers from 0..9 and then 9 forever
4. Will print numbers from 0 to ∞

Something like this. At first I thought this wouldn’t compile, but in fact my SnippetCompiler ate it just fine. I thought I was going insane. Then I assumed that this construct is, in fact, allowed (maybe a bug in the C# language spec?) and it acts as a double-ended do, cycling the precondition while before cycling the postcondition while, thus yielding the answer 4.

And then it finally hit me: a while loop doesn’t need a comma after it. What this meant is that you could write the code as follows:

int i = 0;
while (i < 10)
{
  Console.WriteLine(i++);
}
// empty while loop!
while (i >= 10);


It was a trick question, of course, but it goes to show that vigilance against blatant trickery is often more useful than scientific analysis. Another lesson learned.