In KeyDown, a ShowDialog makes SuppressKeyPress not work

Question

In my application I have added keyboard shortcuts (an event handler) to a TextBox. One is Ctrl+H, which shows a Find & Replace popup.

In the KeyDown event handler of my form I check for the Ctrl+H keypress:

case Keys.H:
    ShowFindReplaceDialog(true); // This line makes the SuppressKeyPress not work
    e.SuppressKeyPress = true;
    break;

Now, Ctrl+H is a standard keyboard shortcut that is equivalent to pressing backspace, so I need to suppress that.

The problem is that showing a popup causes the suppression not to work. So the popup is shown, and after it closes I see that the backspace (Ctrl+H) key still comes through.

How can this be made to work?

N.B. For completeness' sake: you run into this same issue with a MessageBox, it's all ShowDialog underneath.

Answer 1

Yes, that's because you call ShowDialog(). That's a blocking call so the e.SuppressKeyPress statement doesn't get executed until after the dialog is closed. And ShowDialog pumps a message loop, DoEvents style, so the keystroke message gets dispatched as normal and triggers the KeyPress event.

The most straight-forward workaround is to delay displaying the dialog until the message handling is complete. Elegantly done with Control.BeginInvoke(), like this:

case Keys.H:
    this.BeginInvoke(new Action(() => ShowFindReplaceDialog(true)));
    e.SuppressKeyPress = true;
    break;