How to convert integer to ASCII charectors in c#?

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C#

I am using a vb function to code the string to asscii chars for some security purpose.
And this string info is stored to database.
Now i am fetching the same (password) data from a c# mvc application.
But when i cast the same string using c# code it returns a different string of ascii chars.
Its a user login process.I am using same users in both vb and c# applications.The users and their passwords will be saved from vb application.
The codes in vb and c#,

Public Function DecodeId(ByVal mmId) As String
       Dim DVal As String
       Dim I As Integer
       On Error Resume Next
       If mmId.ToString.Length < 10 Then mmId = mmId.ToString.PadRight(10)
       DVal = ""
       For I = 1 To mmId.ToString.Length
           DVal = DVal & Chr(Asc(Mid(mmId, I, 1)) + 58)
       Next
       DecodeId = DVal
   End Function
c#

    public string DecodeId(string mmId)
             {
               	string DVal = string.Empty ;
               	int I = 0;
                            
               	for (I = 0; I <= mmId.Length-1; I++) {
                    char c = (char)(System.Convert.ToInt32(mmId[I]) + 58);
                    //int x = (System.Convert.ToInt32(mmId[I]) + 58);
                   // char nw = (Char)x;
                   // string s = char.ConvertFromUtf32(x).ToString();
                    DVal = DVal + s.ToString();
               	}
               	return DVal;
           
              }

Please reply if there is any difference in ascii values according o.s or prgmng languages etc (the ascii value for 155 is '>' but from my c# code it gives '""').
Please reply
Thanks