C# generates an instance of random strings

/// <summary> 
/// Generate random string
 /// </summary> 
 private class RandomStringGenerator 
 { 
     static readonly Random r = new Random(); 
     const string _chars = "0123456789"; 
     public static string GetRandomString() 
     { 
         char[] buffer = new char[5]; 
         for (int i = 0; i < 5; i++) 
         { 
             buffer[i] = _chars[(_chars.Length)]; 
          } 
          return new string(buffer); 
      } 
 }