AorB workout example - try to get a well-defined function to work on, and get some practices on bit manipulations.

AorBworkout1.cs

using System
;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System.Threading.Tasks;

namespace AtoB
{
    class Program
    {
        static void Main(string[] args)
        {
            string fromValue = "80";
            string orValue = "70";
            string output = "";   // should be "00"

            //int no = updateHexaDecimal_Remove1s(fromValue, orValue, ref output);

            int no2 = updateHexaDecimal_Remove1s("F1", "70", ref output); // should be no2 = 2, output = "70"
        }                               

        /*
         * HexaDecimal Char update
         * For example, A = 1000
         * A | B = C, 
         * C = 0111
         * Then, 4th bit 1 in A should be set to 1. 
         * number is 1, A' = 0000
         * 
         * Another example, A = 1100, 
         * A | B = C, 
         * C = 0111, 
         * then, 4th bit 1 in A should be set to 0
         * function return is 1, A' = 0100
         * 
         * 1. count how many bits to update (only bit with value 1)
         * 2. return A's new value A'
         * 3. Work on bits from left to right, lowest significant bit first
         * 4. assume the value > 0, and value < 10^ 50000
         * 
         * Work on HexaDecimal string, 
         * For example, D8, work on '8' first, 4 bits 1000, from right to left; 
         * and then, work on 'D', 1101 - 13
         * 
         * edge case: 
         * from = "8D", 
         * orValue = "E", 
         * orValue is short in length. 
         * 
         * This function design is questionable - need to work on later, make it as simple as possible
         * 
         */
        public static int updateHexaDecimal_Remove1s(string from, string orValue, ref string output)
        {
            int count = 0;

            StringBuilder sb_Out = new StringBuilder();

            for (int i = from.Length - 1; i >= 0; i--)
            {
                int nFrom    = hexaDecimalCharToInt(from[i]);

                int nOrValue = (i < orValue.Length) ? hexaDecimalCharToInt(orValue[i]) : 0;                                

                string s1 = "";
                count += getBinaryStrInReverseOrder(nFrom, nOrValue, ref s1);

                sb_Out.Append(convertBinaryStringToHexaDecimal(Reverse(s1)));
            }

            output = Reverse(sb_Out.ToString()); // bug001 - need to reverse the binary string 
            return count;
        }
       
        /*
         *   Make it well defined function - 
         *   work on more! 
         *  
         */
        public static int getBinaryStrInReverseOrder(int value, int targetValue, ref string output)
        {
            if (value == 0)
            {
                output = "0"; 
                return 0;
            }

            StringBuilder sb = new StringBuilder();

            int count = 0; 
            while (value > 0)
            {
                int bitNo = value & 1;
                int bitTarget = targetValue & 1;

                if ((bitNo == 1 && bitTarget == 0))
                {
                    count++;
                    bitNo = 0;
                }

                char c = (char)(bitNo + '0');
                sb.Append(c);

                // right shift 1 bit 
                value       = value >> 1;
                targetValue = targetValue >> 1;
            }

            output =  sb.ToString();

            return count; 
        }

        /*
         * input is string of binary number, at most 4 bits, 
         * for example, "1100" -> 12 -> 'C' 
         */
        public static char convertBinaryStringToHexaDecimal(string s)
        {
            int sum = 0;

            foreach (char c in s)
            {
                int value = (c - '0');
                sum = value + sum * 2;
            }

            if (sum >= 0 && sum <= 9)
                return (char)(sum + '0');
            else
            {
                string charStr = "ABCDEF";
                char[] charArr = charStr.ToCharArray();

                return charArr[sum - 10];
            }
        }

        /*
         * Convert HexaDecimal char to integer 
         */
        public static int hexaDecimalCharToInt(char c)
        {
            int number = c - '0';
            if (number >= 0 && number <= 9)
                return number;
            else
            {
                string charStr = "ABCDEF";
                char[] charArr = charStr.ToCharArray();
                int value = Array.IndexOf(charArr, c);
                return 10 + value;
            }
        }

        /*
         * http://stackoverflow.com/questions/228038/best-way-to-reverse-a-string
         */
        public static string Reverse(string s)
        {
            char[] charArray = s.ToCharArray();
            Array.Reverse(charArray);
            return new string(charArray);
        }
    }
}