concept default value in category .net

This is an excerpt from Manning's book C# in Depth. Login to get full access to this book.

You always have to specify a constant value when you declare an optional parameter. It doesn’t have to be null; that just happens to be the appropriate default in this situation. The requirement that the default value is a constant applies to any type of parameter, although for reference types other than strings you are limited to null as the only constant value available.