Why doesn't String.Format throw a FormatException if you pass too many parameters?

Raymond Chen

Welcome to CLR Week 2009. As always, we start with a warm-up.

The String.Format method doesn’t throw a FormatException if you pass too many parameters, but it does if you pass too few. Why the asymmetry?

Well, this is the type of asymmetry you see in the world a lot. You need a ticket for each person that attends a concert. If you have too few tickets, they won’t let you in. If you have too many, well, that’s a bit wasteful, but you can still get in; the extras are ignored. If you create an array with 10 elements and use only the first five, nobody is going to raise an ArrayBiggerThanNecessary exception. Similarly, the String.Format message doesn’t mind if you pass too many parameters; it just ignores the extras. There’s nothing harmful about it, just a bit wasteful.

Besides, you probably don’t want this to be an error:

if (verbose) {
  format = "{0} is not {1} (because of {2})";
} else {
  format = "{0} not {1}";
}
String.Format(format, "Zero", "One", "Two");

Think of the format string as a SELECT clause from the dataset provided by the remaining parameters. If your table has fields ID and NAME and you select just the ID, there’s nothing wrong with that. But if you ask for DATE, then you have an error.

Raymond Chen

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