下午在按照Programming C#的例程写一个TreeView的Event，其中一个是在Check一个Node之后，把它的所有的子结点以及子结点的子结点都Check上，Uncheck时也一样。按照书上的实现方法，我添加了一个TreeView.AfterCheck event handler，并在里面设置了TreeNode.Checked = check。但是运行的时候出错，debug过程中发现，总是在TreeNode.Checked = check之后，又回到TreeView.AfterCheck的event handler。原来是在设置TreeNode.Checked的值之后，又引发了AfterCheck event，如此往复，进入死循环。

google求解，在msdn上发现解答：
http://msdn.microsoft.com/library/default.asp?url=/library/en-s/cpref/html/frlrfsystemwindowsformstreeviewclassafterchecktopic.asp

摘录一段：
Note   Setting the

TreeNode.Checked property from within the BeforeCheck or AfterCheck event causes the event to be raised multiple times and can result in unexpected behavior. For example, you might set the Checked property in the event handler when you are recursively updating the child nodes, so the user does not have to expand and check each one individually. To prevent the event from being raised multiple times, add logic to your event handler that only executes your recursive code if the Action property of the TreeViewEventArgs is not set to TreeViewAction.Unknown.

原来关键就是靠TreeViewEventArgs.Action是否是TreeViewAction.Unknown来判断是否是用户Check了Node。

msdn上还有一段例程代码：

Example

The following example updates all the child tree nodes of a TreeNode when the user changes its checked state. This code assumes you have a Form with a TreeView that has TreeNode objects in its TreeNodeCollection. The TreeNodeCollection should have tree nodes with child nodes.