Refactor some codes.

Author: clown

Applications/Rss/Reader/Sources/Models/Settings/SettingFolder.cs

@@ -162,7 +162,7 @@ public SettingFolder(Assembly assembly, string root, IO io) :
         /// </summary>
         ///
         /* ----------------------------------------------------------------- */
-        public string UserAgent => _userAgent ?? (_userAgent = GetUserAgent());
+        public string UserAgent => _userAgent ??= GetUserAgent();
 
         #endregion