public static string RandomPassword(int PasswordLength)
    {
     string _allowedChars="abcdefghijkmnopqrstuvwx
                     yzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789=+/#$";
      Random randNum = new Random();
      char[] chars = new char[PasswordLength];
      int allowedCharCount = _allowedChars.Length;

      for (int i = 0; i < PasswordLength; i++)
      {
          chars[i] = _allowedChars[(int)((_allowedChars.Length) 
                       * randNum.NextDouble())];
      }
      return new string(chars);
    }

 protected void Button1_Click(object sender, EventArgs e)
    {
        if (TextBox1.Text != "")
        {
           string myInt = TextBox1.Text.ToString();
           Label1.Text = "<b>Your generated password is: 
                   </b> " + RandomPassword(int.Parse(myInt));
        }
    }

 protected void Page_Load(object sender, EventArgs e)
 {
  if (IsPostBack)
   {
   Label1.Text = "Please enter a password length (e.g.6)";
   }
 }


				
		

Click here to check the Live Example of random generate password.