Technical Blog - Tool tips

27 May 2009

I wanted to be able to customise the tool tip that shows when the mouse is hovered over a listview cell. Primarily, I wanted to be able to make it look like a balloon, but any other customisations would be a bonus.

As always, the simplest approach is the best, so I created a ToolTip and installed it on the ObjectListView:

protected override void OnCreateControl() {
    this.toolTip = new ToolTip();
    this.toolTip.BackColor = Color.Red;
    this.toolTip.SetToolTip(this, "using normal tooltip");
}

This works and gives tooltips with a red background. You can even make the hover tooltip show a balloon like this:

protected override void OnCreateControl() {
    this.toolTip = new ToolTip();
    this.toolTip.IsBalloon = true;
    this.toolTip.SetToolTip(this, "using normal tooltip");
}

Winner! Less than an hour and I can customise the tool tip.

Reality bites back

But then I noticed one small problem – it didn’t actually work. If I did this:

protected override void OnCreateControl() {
    this.toolTip = new ToolTip();
    this.toolTip.SetToolTip(this, "using normal tooltip");
    this.toolTip.IsBalloon = true;
}

No tool tip appeared at all. In fact, changing one of several properties (IsBalloon, ShowAlways, StripAmpersands, UseAnimation, UseFading) after the tool tip was installed AND the listview was created caused the tooltip to disappear.

The problem is that when these properties are changed on a ToolTip, the underlying handle is destroyed and then recreated. However, when the underlying control handle is recreated, it is not reinstalled into the ListView (or any other control that uses a ToolTip).

We could get around this by subclassing ToolTip, and whenever any of those properties are changed, we could call SetToolTip() to force the new handle to be installed.

This almost works except that I wanted to be able to change these settings during a TTN_GETDISPINFO callback (this is the notification message that is sent when a tooltip control wants to know what text it should show). I want to do this so that each cell can decide how it wants its tooltip to appear. If something in a cell is important, it should be able to be in red with an icon and title.

But it seems that this is just impossible. Any trick I tried during that callback simply resulted in the tooltip disappearing for good.

Not getting a Handle on things

It should be possible to get around all these problems and just change the IsBalloon setting by changing the WindowStyle. To do that, all we need is the handle to the tooltip control. But ToolTip doesn’t make that available: it doesn’t have a Handle property. It also doesn’t expose a WndProc method that can be overridden. At this point, I began to be seriously displeased with the ToolTip class.

Another ToolTipControl

To get around all these problems, I wrote ToolTipControl, a NativeWindow which wraps a tooltip control. It is not a replacement for a ToolTip, but that does all the tool tip shown by a ListView or its header to be customised in just about any way you would like.

The Vista from here is not so good

But it’s still not perfect. It works flawlessly on XP, but under Vista, it has some limits. Most significantly, changing to balloon style during an event does not work reliably. It works mostly, but occassionally, the tool tip doesn’t recalculate its display rectangle correctly. It displays the balloon tip using the same bounds as if it wasn’t a balloon. If you set IsBalloon outside of an event, it works fine.

One final problem is that setting the ForeColor and BackColor simply doesn’t work. I suspect that these colours are controlled by the system theme, but I’m not sure about that. It’s certainly not documented anywhere.