Willem's...

Thanks! This is much more elegant than the method I was using. Mine was working but the form would flicker when it closed. Also, I need the system tray icon visible all the time, so I commented out the line

//niApp.Visible = Visible ? false : true;

This is not an elegant solution. Keep it simple and use the events on the form - no reason to override the form's OnClosing() or anything else.

Also, simplify use of booleans, so instead of:
niApp.Visible = Visible ? false : true;
use the obvious and proper:
niApp.Visible = !Visible;

So, instead of statements like:
if (boolean1 == true) boolean2 = false;
if (boolean1 == true) doSomething();
rather use:
boolean2 = !boolean1;
if (boolean1) doSomething();

Also, avoid defining cryptic constants like WM_QUERYENDSESSION in your example above. Not only are values like this exposed by the C# system classes but you should avoid overriding the WndProc() method. There is a much easier way (the proper way) to determine whether Windows is shutting down or not. Using WndProc() to do this is way over the top and reminds me of unfriendly C++ code of five years ago. You are supposed to use the FormClosingEventArgs provided by the form's FormClosing() event. Everything you need to know about the form's closing down is provided to you by C# in the relevant place: the FormClosing() method. You are re-inventing the wheel here and also creating difficult to read code that is unnecessarily convoluted. All this can be done in so many fewer lines of clearer code.

See these principles at work in this cleaner FormClosing() event:

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
// Only close app if a "close" menu item chosen or
// Windows is shutting down.
bool leaveApp = (exitApplication || (e.CloseReason == CloseReason.WindowsShutDown));
e.Cancel = !leaveApp;
Visible = leaveApp; // If not leaving hide form instead.
}

private void exitToolStripMenuItem_Click(object sender, EventArgs e)
{
exitApplication = true; // Really do want to close app and not merely hide it.
Application.Exit();
}

private void restoreToolStripMenuItem_Click(object sender, EventArgs e)
{
Visible = true; // Restore form to sight.
}

Here, exitApplication can be set by the tray icon context menu or even a main menu "exit" to enable the behaviour that the user can close the app by choosing an "exit" menu item from the tray icon popup menu or the app's main menu, but the app will not close if the close "X" button is clicked or the system menu's "close" item is clicked. The app also closes if Windows is shutting down.

Hi Stefan, you're absolutely right...in most cases it is inappropriate to use constructs and overrides that are applicable to the C++/MFC/win32 world, especially if .NET/C#/VB functional equivalent constructs can be used. Additionally, your code is definitely more readable.

One comment though for the readers - the 'FormClosing' event is only available in .NET 2.0+, and is not applicable to this particular bit of code which was originally written during the days of .NET 1.1.

Wow, I did not realize .NET 1.1 did not include a FormClosing() event!

My minor quibbles on boolean use aside I must say your example here got me up and running with my first notify tray app within a couple of minutes - well done. Some other examples I trawled for via google were not so clear and simple in demonstrating the bare bones elements needed, so thanks for your effort in putting this example together.

Thanks for the example Willem.

Just to let you know, the .NET 2.0 FormClosing event is more or less the same as the .NET 1.1 Closing event, which will allow you to stop the window from closing by modifying the event arguments, as demonstrated by Stefan.

Here's the MSDN page for the event if you're curious:
http://msdn2.microsoft.com/en-us/library/system.windows.forms.form.closing(VS.71).aspx

Good one
Thanks