Windows Forms: Set form Visibility Based on Focus enabled of external window

Question

I would like to create a Windows Forms application that when run is only visible once another external window (notepad.exe) is enabled/focused. Any hints, I don't know where to begin.

If my form is running I would like it to popup when Notepad is enabled and disappear when Notepad loses focus.

Answer 1

You could try looking at the Windows automation APIs for C#. With these you should be able to look through all open windows and find Notepad. I haven't looked at the API's a bunch but best base scenario is that there would be a activate/lostfocus event for the window you could handle. Worst case, you could just poll every 100 ms or so and look to see if the Notepad window has focus.

Answer 2

You can do this simply with API calls and a timer. Add this line to your form's using statements:

using System.Runtime.InteropServices;

Next, add these declarations to your form:

[DllImport("user32.dll", SetLastError = true)]
private static extern IntPtr FindWindow(string lpClassName, 
    string lpWindowName); 

[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

Finally, put a Timer on your form, and set its Enabled property to true. In its Tick event, put this code:

IntPtr hWndNotepad = FindWindow(null, "Whatever.txt - Notepad");
IntPtr hWndForegroundWindow = GetForegroundWindow();
if (this.Handle != hWndForegroundWindow)
{
    this.Visible = (hWndNotepad == hWndForegroundWindow);
}

I haven't tested this code, but it should work. The code is looking for a specific file to be open in Notepad; a different file would result in different text in the titlebar, so this code wouldn't work. I think that if you change the FindWindow call to FindWindow("notepad", null) it would work with any open instance of Notepad (it might be "notepad.exe" - not sure).

Update: if you want your form to be visible if any instance of Notepad is open, you can instead put this code into your Timer's Tick event:

IntPtr hWndForegroundWindow = GetForegroundWindow();
bool NotepadIsForeground = false;
Process[] procs = Process.GetProcessesByName("notepad");
foreach (Process proc in procs)
{
    if (proc.MainWindowHandle == hWndForegroundWindow)
    {
        NotepadIsForeground = true;
        break;
    }
}
if (this.Handle != hWndForegroundWindow)
{
    this.Visible = NotepadIsForeground;
}

And you'll need this in your using directives:

using System.Diagnostics;

Also not tested, but I'm doing so well today, so why bother?