winrar

private static string GetPath(string extension)
        {
            try
            {
                var openSubKey = Registry.ClassesRoot.OpenSubKey(extension);
                if (openSubKey != null)
                {
                    var appName = (string)openSubKey.GetValue(null);
                    var registryKey = Registry.ClassesRoot.OpenSubKey(appName + @"\shell\open\command");
                    if (registryKey != null)
                    {
                        var openWith =
                            (string)
                            registryKey.GetValue(null);
                        var appPath = openWith.Substring(1, openWith.Length - 1);
                        var max = appPath.LastIndexOf(".exe", StringComparison.Ordinal);
                        if (max < 1)
                            max = appPath.Length;
                        appPath = appPath.Substring(0, max + 4);
                        return new FileInfo(appPath).FullName;
                    }
                }
            }
            catch
            {
                return "";
            }
            return null;
        }

var unpacker = GetPath(".rar")