namespace FreeTextBoxControls {
/// <summary>
/// An HTML editor using MSHTML written for ASP.NET.
/// </summary>
[
ToolboxData("<{0}:FreeTextBox runat=server></{0}:FreeTextBox>"),
ValidationPropertyAttribute("Text"),
Designer(typeof(FreeTextBoxControls.Design.FreeTextBoxDesigner)),
DescriptionAttribute("一个基于 MSHTML 的 ASP.NET 可视化编辑器。")
]
public class FreeTextBox : Control, IPostBackDataHandler, INamingContainer, IPostBackEventHandler
{
// Events
/// <summary>
/// The event fired when a Save button is pressed.
/// </summary>
[
DescriptionAttribute("当“保存”按钮按下时发生。")
]
public event EventHandler SaveClick;

/// <summary>
/// Event for Save button
/// </summary>
[
DescriptionAttribute("引发 SaveClick 事件。")
]
protected virtual void OnSaveClick(EventArgs e) {
if (SaveClick != null) {
SaveClick(this, e);
}
}

上面代码C#类中的中括号是什么意思