string ext = ".grf";
string title = "Graphicprogram";
string extdescription = "The graphicprogram";
FileRegistrationHelper.SetFileAssociation(@"D:\CAD\CAD.exe", ext, title + "." + extdescription);



using Microsoft.Win32;
using System.IO;


    public class FileRegistrationHelper {

        public static void SetFileAssociation(string fullPath, string extension, string progID) {
            // Create extension subkey
            SetValue(Registry.ClassesRoot, extension, progID);

            // Create progid subkey
            string assemblyFullPath = System.Reflection.Assembly.GetExecutingAssembly().Location.Replace("/", @"");
            StringBuilder sbShellEntry = new StringBuilder();
            sbShellEntry.AppendFormat("\"{0}\" \"%1\"", fullPath);
            SetValue(Registry.ClassesRoot, progID + @"shellopencommand", sbShellEntry.ToString());
            StringBuilder sbDefaultIconEntry = new StringBuilder();
            sbDefaultIconEntry.AppendFormat("\"{0}\",0", fullPath);
            SetValue(Registry.ClassesRoot, progID + @"DefaultIcon", sbDefaultIconEntry.ToString());

            // Create application subkey
            SetValue(Registry.ClassesRoot, @"Applications" + Path.GetFileName(fullPath), "", "NoOpenWith");
        }

        private static void SetValue(RegistryKey root, string subKey, object keyValue) {
            SetValue(root, subKey, keyValue, null);
        }
        private static void SetValue(RegistryKey root, string subKey, object keyValue, string valueName) {
            bool hasSubKey = ((subKey != null) && (subKey.Length > 0));
            RegistryKey key = root;

            try {
                if (hasSubKey) key = root.CreateSubKey(subKey);
                key.SetValue(valueName, keyValue);
            }
            finally {
                if (hasSubKey && (key != null)) key.Close();
            }
        }

    }