把VB函数转换为C#中的问题

VB code:
Function passEncode(byval strPass,byval strKey)
Dim priI,passLen,keyLen,numPos,chrT,aryPass()
passLen=Len(strPass)
keyLen=Len(strKey)
ReDim aryPass(passLen-1)
for priI=1 to passLen
aryPass(priI-1)=Mid(strPass,priI,1)
next
for priI=1 to passLen - 1
numPos=Asc(Mid(strKey,(priI mod keyLen)+1,1)) Mod (passLen-priI) + priI
chrT=aryPass(priI-1)
aryPass(priI-1)=aryPass(numPos)
aryPass(numPos)=chrT
next
passEncode=join(aryPass,"")
End Function

这是给已经MD5加密过的密码进行再加密的函数，我用C#写出来是下面这样，可是有错误，请大侠们帮帮忙看看该怎么改？
C# code:
public static string pwdEncode(string pwd, string key)
{
int passLen = pwd.Length;
int keyLen = key.Length;
string[] aryPass = new string[passLen + 1];
for (int i = 1; i < passLen + 1; i++)
{
aryPass[i - 1] = pwd.Substring(i, 1);
}
for (int i = 1; i < passLen; i++)
{
int numPos = Convert.ToInt32(pwd.Substring((i % keyLen) + 1, 1)[0]) % (passLen - i) + i;
string chrT = aryPass[i - 1];
aryPass[i - 1] = aryPass[numPos];
aryPass[numP