title: generate 13 digit unique random number in c#
body: <p>Modify the follwing code to generate 13 digit unique random number in c#</p>
  
  <pre><code>public static string GenerateUniqueRandomNumbers()
      {
          Random generator = new Random();
          String r = generator.Next(0, 1000000).ToString("D6");
          if (r.Distinct().Count() == 1)
          {
              r = GenerateUniqueRandomNumbers();
          }
          return r;
      }
  </code></pre>