.NET thinks screens are ordered differently than Windows Control Panel | Display | Screen Resolution does

Question

Weird thing I saw last night on a PC running Windows 7 Embedded Standard 64-bit with 2 monitors connected. The Display Resolution dialog shows the monitors as being display 1 and display 2 (from left to right). When I run a program I wrote that used both screens, the output I expected on display 1 showed up on the rightmost display, and the output I expected on display 2 showed up on the leftmost display.

Spent some time here and on Google, and ended up using some code I found to create a console app that displayed the contents of the System.Windows.Forms.Screen.AllScreens:

// For each screen, add the screen properties to a list box.
foreach (var screen in System.Windows.Forms.Screen.AllScreens)
{
    Console.WriteLine("Device Name: " + screen.DeviceName);
    Console.WriteLine("Bounds: " +
        screen.Bounds.ToString());
    Console.WriteLine("Type: " +
        screen.GetType().ToString());
    Console.WriteLine("Working Area: " +
        screen.WorkingArea.ToString());
    Console.WriteLine("Primary Screen: " +
        screen.Primary.ToString());
}

Ran it on one test machine and got what I expected -- AllScreens order matches the DIsplay Resolution dialog order.

But on the problematic machine, the output of the code above came out reversed. DISPLAY1 was at a position that was the second screen (rightmost), and DISPLAY2 was positioned at the leftmost screen.

That's really weird. Since this WASN'T using my program, it's something in the innards of .NET. The same code works on other similar machines, so I don't think it's a general problem, but is a problem isolated to this machine. I've tried more searching for similar problems but have come up empty, probably due to the search terms (monitor, screen, .NET all match a LOT of items that have nothing to do with this issue).

Anyone seen this before, and ideally, have an idea on how to fix it? Reinstalling the .NET 4 package did not help. Changing locations in the Display Resolution dialog and then changing them back did not help, even with a reboot in between.

Thanks.

Answer 1

The AllScreens property is populated via the EnumDisplayMonitors function.

The documentation for EnumDisplayMonitors does not indicate what order it returns the monitors in, thus one can infer it is an implementation detail which can change from OS version to OS version.