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copied!<p>For other people wanting to achieve this, I'm posting below my implementation, using Matt Davis' solution.</p> <p>In Program.cs</p> <pre><code>static class Program { #region Dll Imports public const int HWND_BROADCAST = 0xFFFF; [DllImport("user32.dll")] public static extern bool SetForegroundWindow(IntPtr hWnd); [DllImport("user32")] public static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam); [DllImport("user32")] public static extern int RegisterWindowMessage(string message); #endregion Dll Imports public static readonly int WM_ACTIVATEAPP = RegisterWindowMessage("WM_ACTIVATEAPP"); [STAThread] static void Main() { bool createdNew = true; //by creating a mutex, the next application instance will detect it //and the code will flow through the "else" branch using (Mutex mutex = new Mutex(true, "MyMutexName", out createdNew))//make sure it's an unique identifier (a GUID would be better) { if (createdNew) { Application.EnableVisualStyles(); Application.SetCompatibleTextRenderingDefault(false); Application.Run(new MainForm()); } else { //we tried to create a mutex, but there's already one (createdNew = false - another app created it before) //so there's another instance of this application running Process currentProcess = Process.GetCurrentProcess(); //get the process that has the same name as the current one but a different ID foreach (Process process in Process.GetProcessesByName(currentProcess.ProcessName)) { if (process.Id != currentProcess.Id) { IntPtr handle = process.MainWindowHandle; //if the handle is non-zero then the main window is visible (but maybe somewhere in the background, that's the reason the user started a new instance) //so just bring the window to front if (handle != IntPtr.Zero) SetForegroundWindow(handle); else //tough luck, can't activate the window, it's not visible and we can't get its handle //so instead notify the process that it has to show it's window PostMessage((IntPtr)HWND_BROADCAST, WM_ACTIVATEAPP, IntPtr.Zero, IntPtr.Zero);//this message will be sent to MainForm break; } } } } } } </code></pre> <p>In MainForm.cs</p> <pre><code>protected override void WndProc(ref Message m) { base.WndProc(ref m); //someone (another process) said that we should show the window (WM_ACTIVATEAPP) if (m.Msg == Program.WM_ACTIVATEAPP) this.Show(); } </code></pre>

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