C# – Treeview prevent Node collapse

This is a side effect of the way native windows work in Windows. Many of the window options are specified by style flags in the CreateWindowEx() call. Basic stuff, like what the border looks like on a form. What kind of View a ListView should have. Whether a TreeView should display checkboxes.

These styles are exposed as properties on a control. But there's a problem. Changing such a property requires the window to be recreated from scratch so that new style flags can be specified in the CreateWindowEx() call. That has side effects, the window gets completely recreated so it loses all previous state.

Windows Forms does a pretty good job of making this look smooth, restoring the previous state after recreating the window. But it there are leaks here and there. And a few outright bugs. One leak in TreeView is the exact state of which nodes are collapsed and which are not. Keeping track of this is just not practical.

Diagnose this by putting the Handle property of the TreeView in a watch window. If you see it change, you've found the property. The list of style flags is available here, you can probably map their names to their corresponding property.

Perhaps it is clear that there is no great workaround for this problem, other than avoiding changing that property. Trouble like this is what begat WPF.

1). Add an ImageList Control to your WinForm.

2). Populate the ImageList with the pictures/icons you wish to change/display in response to what the user does at run-time with the TreeView, such as expanding, or collapsing nodes.

3). Assign the 'ImageList Control to the 'ImageList property of the 'TreeView

At this point you may want to make an initial pass over the TreeView, assuming it is populated, assigning the Node.ImageIndex property to point to the Image ... in the ImageList ... you want to use for the Node depending on whether it has children, or whatever.

4). If a user expands a Node, for example, you can use the BeforeExpand Event of the TreeView to change the Node's picture : like so : in this case we use the index of the Picture in the ImageList :

    private void treeView1_BeforeExpand(object sender, TreeViewCancelEventArgs e)
    {
        e.Node.ImageIndex = 3;
    }

5) You can also set the Node's image by using the ImageKey property which is the string name of the Image

6) There lots of other possible Node Picture variations to use : check out : SelectedImageIndex and SelectedImageKey : You can change Node pictures in the BeforeSelect, AfterSelect and BeforeExpand, events also, depending on the effect you are after.