custom control的用法和樣例：
例句
- Designers must specify such mechanisms in detail, and programmers will likely need to implement them as custom controls.
設計者必須詳細地指定這種反饋機制，程序員也可能需要用定製控制項來實現。 - A handy little custom control that removes all the grunt work of allowing the user to move items between two listboxes.
這個小定製控制項去掉了讓用戶在兩個列表框之間進行選擇的大量工作。
