Program.cs

using System.Runtime.InteropServices;
using System.Windows.Forms;
using DisplayDefaults;

// WinExe has no console of its own; borrow the terminal's. Output lands after the shell
// prompt returns, which is ugly but readable.
[DllImport("kernel32.dll")]
static extern bool AttachConsole(int processId);

if (args.Contains("--uninstall"))
{
    AttachConsole(-1);
    Console.WriteLine("Removed:");
    Console.WriteLine(TrayApp.Uninstall());
    Console.WriteLine();
    Console.WriteLine("Exit the tray app if it is running, then delete the exe.");
    return;
}

if (args.Contains("--selftest"))
{
    AttachConsole(-1);

    // Not Debug.Assert: that compiles out in Release, so the published exe would print
    // "selftest ok" having checked nothing. CI builds Release.
    static void Check(bool ok, string what)
    {
        if (ok) return;
        Console.Error.WriteLine("FAIL: " + what);
        Environment.Exit(1);
    }

    var seen = new HashSet<string>(new[] { "MON-A" }, StringComparer.OrdinalIgnoreCase);
    Check(NewDisplayWatcher.Unseen(new[] { "MON-A", "MON-B" }, seen).Single() == "MON-B", "unseen monitor is detected");
    Check(NewDisplayWatcher.Unseen(new[] { "MON-A" }, seen).Count == 0, "known monitor does not retrigger");
    Check(NewDisplayWatcher.Unseen(new[] { "mon-a" }, seen).Count == 0, "identity is case-insensitive");
    Check(NewDisplayWatcher.Unseen(Array.Empty<string>(), seen).Count == 0, "no monitors is not a new monitor");

    // Primary 1920x1080 at the origin, new monitor 2560x1440. Left/Above must use the NEW
    // monitor's size and Right/Below the primary's, or the two rectangles overlap; the
    // deliberately different dimensions catch a swap.
    Check(DisplayTopology.Offset(Placement.Right, 1920, 1080, 2560, 1440) == (1920, 0), "right of primary");
    Check(DisplayTopology.Offset(Placement.Left, 1920, 1080, 2560, 1440) == (-2560, 0), "left by the new monitor's width");
    Check(DisplayTopology.Offset(Placement.Above, 1920, 1080, 2560, 1440) == (0, -1440), "above by the new monitor's height");
    Check(DisplayTopology.Offset(Placement.Below, 1920, 1080, 2560, 1440) == (0, 1080), "below the primary");
    Check(DisplayTopology.Offset(Placement.Default, 1920, 1080, 2560, 1440) == (0, 0), "default moves nothing");

    Console.WriteLine("selftest ok");
    Console.WriteLine("attached monitors:");
    foreach (var id in DisplayTopology.AttachedMonitorIds()) Console.WriteLine("  " + id);
    return;
}

// Autostart plus a manual launch would otherwise give two tray icons and two watchers.
// Local\ scopes it to the logon session, which is the right scope for a per-user tray app.
// installer.iss declares the same name as AppMutex — change one and change the other.
using var single = new Mutex(initiallyOwned: true, @"Local\DisplayDefaults", out bool isFirstInstance);
if (!isFirstInstance)
{
    // Silently exiting here looks like a broken launch, so say why.
    MessageBox.Show(
        "DisplayDefaults is already running. Look for the monitor icon in the notification area.",
        "DisplayDefaults", MessageBoxButtons.OK, MessageBoxIcon.Information);
    return;
}

ApplicationConfiguration.Initialize();
Application.Run(new TrayApp());