1. private string FromBase64String(string base64String)
    2. {
    3.     if (string.IsNullOrWhiteSpace(base64String))
    4.         return base64String;
    5.     base64String.Replace('_', '/').Replace('-', '+');
    6.     switch (base64String.Length % 4)
    7.     {
    8.         case 2: base64String += "=="; break;
    9.         case 3: base64String += "="; break;
    10.     }
    11.     byte[] byteArray = Convert.FromBase64String(base64String);
    12.     return Encoding.Default.GetString(byteArray);
    13. }