“System.FormatException: Input string was not in a correct format.”

When using .Net’s StringBuilder, you might see this error. The likely cause is that you have a curly brace in the format string that you want to appear in the final string.  That will mislead .Net into thinking it’s a placeholder (the expected use of curly braces), and escaping with doesn’t seem to help.

Break up the format string so that the “literal” curly braces appear only in Append’s.