The Add(Char) method adds the input character value to the first available position in the formatted string after the position that was last assigned, which is represented by the LastAssignedPosition property. This method will fail for any of the following reasons:
The input value, input, is not printable, or it does not match its corresponding mask element.
There are zero available edit positions in the formatted string, or there are no available edit positions after the last assigned position.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)