string taking to much time in reading a hex file

See more:

C#

ASP.NET

hi
the below code is working fine , the problem is that its taking more than 15 min to read data, since the hex file has more 20 thousand line. is there any fast way to do.

2) i have tried using string builder but i am getting lots of error. plz give code example how to implement string builder rather than string.

public void LoadImageSize()
        {
            openFileDialog1.Title = "Select a Hex file";
            openFileDialog1.DefaultExt = "*.hex";
            openFileDialog1.Filter = "HEX Files|*.hex";
            if (openFileDialog1.ShowDialog() != System.Windows.Forms.DialogResult.Cancel &&
               openFileDialog1.FileName.Length > 0)
            {
                richTextBox3.Text = System.IO.Path.GetFullPath(openFileDialog1.FileName);

            }
            string PathofFile = System.IO.Path.GetFullPath(openFileDialog1.FileName);
            StreamReader Sr = new StreamReader(PathofFile);
            string FileReader = Sr.ReadToEnd();
            int CountLines = File.ReadLines(PathofFile).Count();
            for (int i = 1;   i < CountLines-2; i++)
            {
                string[] lines = File.ReadAllLines(PathofFile);
                string line = lines[i];
                 using (StringReader CharReader = new StringReader(line))
                {
                    int offset = 9;
                    int length = line.Length - 11;
                    string Hexdata = line.Substring(offset, length);
                    Int64 lengthHexData = Hexdata.Length;
                    byte[] Hexintobyte = StringToByteArray(Hexdata);
                    Int64 ImageSize = Hexintobyte.Length;
                    TotalImageSize += ImageSize;

               }


             }
            Int64 SizeofDataBlock = TotalImageSize;

            Sr.Close();
            }

        //Methode for converting into 16 Byte Start
        public static byte[] StringToByteArray(String hex)
        {
            int NumberChars = hex.Length;
            byte[] bytes = new byte[NumberChars / 2];
            for (int j = 0; j < NumberChars; j += 2)
                bytes[j / 2] = Convert.ToByte(hex.Substring(j, 2), 16);
            return bytes;
        }