Force length on integer in C#

Today, I had to do file renaming on 1.000 files in a directory so they could be sorted alphabetically. I will not go into details, but it was a lot trickier than anticipated because the original filenames where very different.

Basically, I wanted all filenames to be 4 digits long, so that 1.psd became 0001.psd and 754.psd became 0754.psd

In order to do that, I needed a method that would prefix any given number with the right number of zeros. So I came up with this:

private static string ForceLength(int number, int length)
{
   string s = number.ToString();
   string returnString = "";

   for (int i = s.Length; i < length; i++)
   {
      returnString += "0";
   }

   return returnString + s;
}

If you pass 54 as the number parameter and 4 as the length parameter, the method returns "0054".

I’m not sure this is the best way to go, but it works. What do you think?