title: Random is not working good
body: <p>I generate a string with 62 options ^ 6 letters = 56,800,235,584</p>
  
  <p>But when running the code, it repeats the same string less then every 200,200 times</p>
  
  <p>What is the problem here?</p>
  
  <p>BTW:
  This code is based on the answer <a href="https://stackoverflow.com/questions/1122483/random-string-generator-returning-same-string">here</a></p>
  
  <pre><code>class Program
  {
      static void Main(string[] args)
      {
          var d = new Dictionary&lt;string, bool&gt;();
  
          for (int i = 0; ; i++)
          {
              var s = GenerateString(6);
              try
              {
                  d.Add(s, false);
              }
              catch (Exception ex)
              {
                  Console.WriteLine(String.Format("{0} - {1} - {2}", i, s, ex.Message));
                  i = 0;
              }
          }
  
          Console.ReadKey();
      }
  
  
      static Random _rnd = new Random();
      public static string GenerateString(int len)
      {
          const string bigLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
          const string smallLetters = "abcdefghijklmnopqrstuvwxyz";
          const string numbers = "1234567890";
  
          var validChars = bigLetters + smallLetters + numbers;
  
          var result = new StringBuilder();
  
          for (int i = 0; i &lt; len; i++)
          {
              result.Append(validChars[_rnd.Next(validChars.Length)]);
          }
  
          return result.ToString();
      }
  }
  </code></pre>