Format A String

You can use Insert:

"949123U0456789".Insert(11, ".").Insert(8, "-").Insert(6, "-").Insert(3, ".")

Note that I am inserting from the end, to avoid the addition of the inserted string from affecting the index.

The format string you have used is meant for numeric types - a string is not a numeric types (and when passing in a string to string.Format, it will simply be returned, as it already is a string).

There are other approaches, as can be seen in Format string with dashes:

Using regex:

Regex.Replace("949123U0456789",
              @"^(.{3})(.{3})(.{2})(.{3})(.{3})$",
              "$1.$2-$3-$4.$5");

Or with Substring.

Regex can solve the problem of formatting when there is a mix of letters and numbers.

        using System.Text.RegularExpressions;

        string pattern = @"([a-z0-9A-Z]{3})([a-z0-9A-Z]{3})
                    ([a-z0-9A-Z]{2})([a-z0-9A-Z]{3})([a-z0-9A-Z]{3})"
        string oldFormat = "949123U0456789";
        string newFormat = Regex.Replace(oldFormat, pattern, 
                    "$1.$2-$3-$4.$5");

I love regular expressions :D, they have the added benefit of allowing you to check the syntax of the code, if it has to follow a certain convention (i.e. perform validation).

In order to perform validation, you can use:

        Regex.IsMatch(oldFormat, 
                    @"([a-z0-9A-Z]{3})([a-z0-9A-Z]{3})
                    ([a-z0-9A-Z]{2})([a-z0-9A-Z]{3})
                    ([a-z0-9A-Z]{3})")

string s="949123U0456789";

Regex r=new Regex(@"(\w{3})(\w{3})(\w{2})(\w{3})(\w{3})");
Console.WriteLine(r.Replace(s,"$1.$2-$3-$4.$5"));