Program.cs

using System.Diagnostics;
using System.Runtime.InteropServices;

namespace ImageCropper;

internal static class Program
{
    [STAThread]
    private static void Main(string[] args)
    {   // GUID erhält man durch Eingabe in Developer PowerShell: [Guid]::NewGuid()
        var mutexName = @"Local\ImageCropperRunningMutex"; // Local Mutex to allow multiple users on same machine
        using Mutex singleMutex = new(true, mutexName, out var isNewInstance);
        if (isNewInstance)
        {
            ApplicationConfiguration.Initialize();
            var bildPfad = args.Length > 0 ? args[0] : null;
            Application.Run(new Main(bildPfad));
        }
        else
        {
            using var otherProcess = Process.GetProcessesByName(Process.GetCurrentProcess().ProcessName).FirstOrDefault(static p => p.Id != Environment.ProcessId);

            if (otherProcess?.MainWindowHandle is { } handle && handle != IntPtr.Zero)
            {
                Utils.ShowWindow(handle, Utils.SW_RESTORE);
                Utils.SetForegroundWindow(handle);

                if (args.Length > 0)
                {
                    var argument = args[0];
                    var lpData = Marshal.StringToHGlobalUni(argument);
                    try
                    {
                        var cds = new Utils.COPYDATASTRUCT
                        {
                            dwData = 1,
                            cbData = (argument.Length + 1) * 2, // UTF-16: 2 Bytes pro Zeichen + Null-Terminator
                            lpData = lpData
                        };

                        Utils.SendMessage(handle, Utils.WM_COPYDATA, IntPtr.Zero, ref cds);
                    }
                    finally
                    {
                        Marshal.FreeHGlobal(lpData);
                    }
                }
            }
        }
        GC.KeepAlive(singleMutex); // Ensure the mutex is not released prematurely  
    }
}