Checking TreeView Control Nodes and SubNodes - StackOverflowException!

Question

I have a Tree View control for a Windows Application that uses the CheckBoxes property.

Sometimes (often) when a Tree Node is either checked or unchecked, I get Stack Overflow Exceptions in my static methods below.

Could someone point out why? Maybe even show me how to do this the right way?

In the After Check Event, I have written the following:

void TreeNode_AfterCheck(object sender, TreeViewEventArgs e) {
  if (0 < e.Node.Nodes.Count) {
    if (e.Node.Checked) {
      e.Node.Expand();
      TreeNodes_SetChecksTo(e.Node, true);
    } else {
      if (!TreeNode_SomethingChecked(e.Node)) {
        e.Node.Collapse(false);
      }
    }
  }
}

Generally, the Exception is thrown when something in a static method fires the After Check Event above and trickles into one of the static methods below:

static void TreeNodes_SetChecksTo(TreeNode node, bool value) {
  if (node != null) {
    if (node.Checked != value) node.Checked = value;
    if (0 < node.Nodes.Count) {
      foreach (TreeNode sub in node.Nodes) {
        TreeNodes_SetChecksTo(sub, value);
      }
    }
  }
}

static bool TreeNode_SomethingChecked(TreeNode node) {
  if (node != null) {
    if (node.Checked) return true;
    if (0 < node.Nodes.Count) {
      foreach (TreeNode sub in node.Nodes) {
        if (TreeNode_SomethingChecked(sub)) {
          return true;
        }
      }
    }
  }
  return false;
}

Answer 1

The tree is iterated recursively every time a (sub-) node gets checked or unchecked and this happens pretty often as you react on every check state change with your AfterCheck callback. Either "forbid" the callback while an "instance" of it is already processing or do it without explicit recursion and let the callback do the recursion implicitly.

Answer 2

Setting IsChecked inside TreeNodes_SetChecksTo is resulting in the AfterCheck event being raised and thus the TreeNode_AfterCheck method being called. I suspect you want to disable/ignore the event whilst processing it:

private bool latch;

void TreeNode_AfterCheck(object sender, TreeViewEventArgs e) {
  if (latch)
      return;

  latch = true;

  try
  {
      if (0 < e.Node.Nodes.Count) {
        if (e.Node.Checked) {
          e.Node.Expand();
          TreeNodes_SetChecksTo(e.Node, true);
        } else {
          if (!TreeNode_SomethingChecked(e.Node)) {
            e.Node.Collapse(false);
          }
        }
      }
  }
  finally
  {
      latch = false;
  }
}

HTH,
Kent

Answer 3

if (node.Checked != value) node.Checked = value;

This is the statement that probably causes it. It fires the AfterCheck event. Your event handler will get called again, while it is already running. You need to protect yourself against that and break the recursion with a private field. Something like this:

private bool updatingChecks;

void TreeNode_AfterCheck(object sender, TreeViewEventArgs e) {
  if (updatingChecks) return;
  updatingChecks = true;
  try {
    // etc..
  }
  finally {
    updatingChecks = false;
  }
}