If another instance of the app is allready running, it just opens it

Steam Desktop Authenticator/MainForm.cs

@@ -27,6 +27,16 @@ public partial class MainForm : Form
         // Forms
         private TradePopupForm popupFrm = new TradePopupForm();
 
+        protected override void WndProc(ref System.Windows.Forms.Message message)
+        {
+            if (message.Msg == SingleInstance.WM_SHOWFIRSTINSTANCE)
+            {
+                this.Show();
+                this.WindowState = FormWindowState.Normal;
+            }
+            base.WndProc(ref message);
+        }
+
         public MainForm()
         {
             InitializeComponent();
@@ -724,5 +734,10 @@ private void MainForm_KeyDown(object sender, KeyEventArgs e)
                 CopyLoginToken();
             }
         }
+
+        private void listAccounts_SelectedIndexChanged(object sender, EventArgs e)
+        {
+
+        }
     }
 }

Steam Desktop Authenticator/Program.cs

@@ -1,13 +1,12 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Security;
-using System.Threading.Tasks;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Threading;
 using System.Windows.Forms;
-using System.Diagnostics;
 using CommandLine;
 using CommandLine.Text;
 
+
 namespace Steam_Desktop_Authenticator
 {
 
@@ -32,47 +31,92 @@ public string GetUsage()
         }
     }
 
-    static class Program
+    static public class WinApi
+    {
+        [DllImport("user32")]
+        public static extern int RegisterWindowMessage(string message);
+
+        public static int RegisterWindowMessage(string format, params object[] args)
+        {
+            string message = String.Format(format, args);
+            return RegisterWindowMessage(message);
+        }
+
+        public const int HWND_BROADCAST = 0xffff;
+        public const int SW_SHOWNORMAL = 1;
+
+        [DllImport("user32")]
+        public static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam);
+    }
+
+    static public class SingleInstance
     {
-        public static Process PriorProcess()
-        // Returns a System.Diagnostics.Process pointing to
-        // a pre-existing process with the same name as the
-        // current one, if any; or null if the current process
-        // is unique.
+        public static readonly int WM_SHOWFIRSTINSTANCE = WinApi.RegisterWindowMessage("WM_SHOWFIRSTINSTANCE|{0}", ProgramInfo.AssemblyGuid);
+        static Mutex mutex;
+        static public bool Start()
+        {
+            bool onlyInstance = false;
+            string mutexName = String.Format("Local\\{0}", ProgramInfo.AssemblyGuid);
+
+            // if you want your app to be limited to a single instance
+            // across ALL SESSIONS (multiple users & terminal services), then use the following line instead:
+            // string mutexName = String.Format("Global\\{0}", ProgramInfo.AssemblyGuid);
+
+            mutex = new Mutex(true, mutexName, out onlyInstance);
+            return onlyInstance;
+        }
+        static public void ShowFirstInstance()
+        {
+            WinApi.PostMessage(
+                (IntPtr)WinApi.HWND_BROADCAST,
+                WM_SHOWFIRSTINSTANCE,
+                IntPtr.Zero,
+                IntPtr.Zero);
+        }
+        static public void Stop()
         {
-            try
+            mutex.ReleaseMutex();
+        }
+    }
+
+    static public class ProgramInfo
+    {
+        static public string AssemblyGuid
+        {
+            get
             {
-                Process curr = Process.GetCurrentProcess();
-                Process[] procs = Process.GetProcessesByName(curr.ProcessName);
-                foreach (Process p in procs)
+                object[] attributes = Assembly.GetEntryAssembly().GetCustomAttributes(typeof(System.Runtime.InteropServices.GuidAttribute), false);
+                if (attributes.Length == 0)
                 {
-                    if ((p.Id != curr.Id) &&
-                        (p.MainModule.FileName == curr.MainModule.FileName))
-                        return p;
+                    return String.Empty;
                 }
-                return null;
-            }
-            catch (Exception)
-            {
-                return null;
+                return ((System.Runtime.InteropServices.GuidAttribute)attributes[0]).Value;
             }
         }
+    }
 
+    static class Program
+    {
         /// <summary>
         /// The main entry point for the application.
         /// </summary>
         [STAThread]
         static void Main(string[] args)
         {
             // run the program only once
-            if (PriorProcess() != null)
+            if (!SingleInstance.Start())
             {
-                MessageBox.Show("Another instance of the app is already running.");
+                SingleInstance.ShowFirstInstance();
                 return;
             }
 
-            // Parse command line arguments
-            var options = new Options();
+            Application.EnableVisualStyles();
+            Application.SetCompatibleTextRenderingDefault(false);
+
+
+
+        // Parse command line arguments
+        var options = new Options();
             Parser.Default.ParseArguments(args, options);
 
             Application.EnableVisualStyles();