Custom Numeric Format Strings

You can create a custom numeric format string, which consists of one or more custom numeric specifiers, to define how to format numeric data. A custom numeric format string is any format string that is not a standard numeric format string.
Custom numeric format strings are supported by some overloads of the ToString method of all numeric types. For example, you can supply a numeric format string to the ToString(String) and ToString(String, IFormatProvider) methods of the Int32 type. Custom numeric format strings are also supported by the .NET Framework composite formatting feature, which is used by some Write and WriteLine methods of the Console and StreamWriter classes, the String.Format method, and the StringBuilder.AppendFormat method.