variable decimal places in .Net string formatters?

Question

Fixed decimal places is easy

String.Format(\"{0:F1}\", 654.321);

gives

654.3

How do I feed the number of

Answer 1

The string to format doesn't have to be a constant.

int numberOfDecimalPlaces = 2;
string formatString = String.Concat("{0:F", numberOfDecimalPlaces, "}");
String.Format(formatString, 654.321);