Didn\'t find how to do that. What I found was more or less on the lines of this (http://blog.stevex.net/string-formatting-in-csharp/):
There really isn’t any formatt
What you want is not "natively" supported by C# string formatting, as the String.ToString methods of the string object just return the string itself.
When you call
string.Format("{0:xxx}",someobject);
if someobject implements the IFormattable interface, the overload ToString(string format,IFormatProvider formatProvider) method gets called, with "xxx" as format parameter.
So, at most, this is not a flaw in the design of .NET string formatting, but just a lack of functionality in the string class.
If you really need this, you can use any of the suggested workarounds, or create your own class implementing IFormattable interface.