String kontonummer = "123456";
String blz = "50010060";
string bban = blz+ kontonummer.PadLeft(10, '0');
decimal zahl;
decimal rest;
decimal pruefziffern;
String iBan;
// nun wird der Zaehler bestimmt
// bban + DE + 00   =  bban + 131400    A=10 B=11 C=12 D=13
if (decimal.TryParse(bban + "131400", out zahl)) {
   rest = zahl % 97;
   pruefziffern = 98 - rest;
   if (pruefziffern >= 0) {
      iBan = "DE " + pruefziffern.ToString("00") +
              " " + bban.Substring(0, 4) + " " + bban.Substring(4, 4) + " " + bban.Substring(8, 4) + " "
              + bban.Substring(12, 4) + " " + bban.Substring(16);
   }
   else
     iBan = "DE " + bban ;
}
else {
  iBan = "DE " + bban ;
}