Program logic to calculate hexacode checksum?

Question

I have a hexadecimal code for which I have to perform a checksum logic which will give me a checksum that will be added to the end of the Hexacode message and passed to the receiving TCP server and after that, the message I received from the server needs to be checked by performing checksum logic

Eg: I have a hexacode of 07040A13 for which checksum value is coming 58 which is added and passed as 07040A1358. In receiving I am getting 25000A01F0FFFFFFFF for this the checksum value is 64 and whole message is 25000A01F0FFFFFFFF64 The sum of this value should be 0 and the logic for checksum is (80 + Message)NOT+1 = Checksum value And (80 + Message)Not +1 + Checksum = 0

I want this logic to be performed in C# code but I do not know where and how to start.

I have got one working solution but it is in javascript:

function calculate_checksum8(N) 
{

     // convert input value to upper case
     strN = new String(N);
     strN = strN.toUpperCase();

     strHex = new String("0123456789ABCDEF");
     result = 0;
     fctr = 16;
     for (i=0; i<strN.length; i++) 
     {
       if (strN.charAt(i) == " ") continue;

        v = strHex.indexOf(strN.charAt(i));
        if (v < 0) 
        {
          result = -1;
          break;
        }
        result += v * fctr;

        if (fctr == 16) 
         fctr = 1;
         else            
         fctr = 16;
       }

        if (result < 0) 
        {
         strResult = new String("Non-hex character entered");
        }
        else if (fctr == 1) 
        {
          strResult = new String("Odd number of characters entered. e.g. correct value = aa aa");
        }
         else 
        {
         // Calculate 2's complement
         result = (~(result & 0xff) + 1) & 0xFF;
         // Convert result to string
          strResult = strHex.charAt(Math.floor(result/16)) + strHex.charAt(result%16);
         }
         return strResult;
}

Answer 1

public string CreateCheckSum(string hex)
        {
            string withchk = "80" + hex;
            string strres = "";
            string strHex = "0123456789ABCDEF";
            int res = 0;
            int fctr = 16;
            for (int i = 0; i < withchk.Length; i++)
            {
                if (withchk[i] == ' ')
                    continue;
                var v = strHex.IndexOf(withchk[i]);
                if (v < 0)
                {
                    res = -1;
                    break;
                }
                res += v * fctr;
                if (fctr == 16)
                    fctr = 1;
                else
                    fctr = 16;
            }
            if (res < 0)
                return "Invalid";
            else if (fctr == 1)
                return "Invalid";
            else
            {
                res = (~(res & 0xff) + 1) & 0xFF;
                strres = strHex[(int)Math.Floor(res / 16.0)].ToString() + strHex[res % 16].ToString();
            }
            return strres;
           
        }