YouOnlyRunOnce.exe C:\Tips.txt

• DllImport - for accessing Win32 API functions
• System.Threading.Mutex - A synchronization mechanism built into Windows
• SendMessage - for passing Windows messages to your application
• WM_COPYDATA - the Windows message we will use in this example
• COPYDATASTRUCT - a Win32 defined structure for sharing data between processes

    bool mutexCreated;
    System.Threading.Mutex mutex = new System.Threading.Mutex(true,"9F2A6412-6339-43bf-94d6-D73520a74227", out mutexCreated);

• true - indicating that the currente thread should immediately take ownership of the mutex when it is created
• a GUID name for the mutex. The mutex name can be any unique string name but I recommend using a GUID. There are too many developers making too many applications and naming them from too few real dictionary words to count on your application name always being unique. Imagine the frustration of a user (customer) who cannot run your application because they have some other totally unrelated application running on their PC. Using a GUID ensures that only instances of your own application will prevent a second instance from running.
• out mutexCreated - the last parameter is a boolean value that the constructor sets to indicate whether the mutex was successfully created. If true, then our application instance is holding the mutex and can execute. If false, then another application instance is holding the mutex and our instance must shut down.

    if (mutexCreated)
    {
        Application.Run(new Form1());
        GC.KeepAlive(mutex);
    }
    else
    {
        Form1.SendMessageToRunning();
    }

    // Tag this window with a GUID property and a value of this window's handle
    if (Win32.SetProp(
        this.Handle, 
        "9F2A6412-6339-43bf-94d6-D73520a74227", 
        (int)this.Handle) 
        == 0)
    {
        throw new ApplicationException("Could not set AppID.");
    }

        /// <summary>
        /// Sends the command line string to the previous instance for processing
        /// </summary>
        private static void SendMessageToRunning()
        {
            Form1.GetRunningInstance();
            IntPtr ptr = Marshal.StringToHGlobalAnsi(Environment.CommandLine);
            Win32.COPYDATASTRUCT cds = new Win32.COPYDATASTRUCT();
            cds.dwData = IntPtr.Zero;
            cds.cbData = Environment.CommandLine.Length;
            cds.lpData = ptr;
            
            long result = Win32.SendMessage(Form1.prevPtr,WM_COPYDATA,0,ref cds);

            // Required to free unmanaged memory otherwise a memory leak occurs
            Marshal.FreeHGlobal(ptr);

        protected override void WndProc(ref Message m) 
        {
            // Listen for operating system messages.
            switch (m.Msg)
            {
                    // Listen for WM_COPYDATA
                case WM_COPYDATA:
                    string strCmdLine = string.Empty;
                    Win32.COPYDATASTRUCT cds = new Win32.COPYDATASTRUCT();
                    cds = (Win32.COPYDATASTRUCT) Marshal.PtrToStructure(m.LParam, typeof(Win32.COPYDATASTRUCT));
                    if (cds.cbData > 0)
                    {
                        strCmdLine = Marshal.PtrToStringAnsi(cds.lpData).Substring(0,cds.cbData);

                        // This section attempts to extract the filename to open from the 
                        // command line arguments.  This is tricky because of the multitude
                        // of ways the OS can handle quotes and spaces in commandlines.
                        // While this worked for me for the purposes of this demonstration
                        // it may not work for your commands.  Parsing the command line
                        // is beyond the scope of this demonstration so if this doesn't work
                        // for you, you will have to work out your own means of parsing the
                        // command line.
                        // Of course, if you have a better way, I'd love to hear from you so I can
                        // update both this article and my skills.
                        if (strCmdLine.StartsWith("\""))
                        {
                            strCmdLine = strCmdLine.Substring(strCmdLine.IndexOf("\"",1));
                        }
                        else if (strCmdLine.IndexOf("\"") > 0)
                        {
                            int firstIndex = strCmdLine.IndexOf("\"");
                            int lastIndex = strCmdLine.LastIndexOf("\"");
                            strCmdLine = strCmdLine.Substring(firstIndex+1,lastIndex-firstIndex-1);
                        }
                        else if (strCmdLine.LastIndexOf(" ") > 0)
                        {
                            strCmdLine = strCmdLine.Substring(strCmdLine.LastIndexOf(" ")+1).Trim();
                        }
                        else 
                        {
                            strCmdLine = string.Empty;
                        }

                        if (strCmdLine.Trim().Length > 0)
                        {
                            // Display the file passed to the second instance
                            this.DisplayFileContent(strCmdLine);
                        }
                        // bring this instance to the front
                        this.Activate();
                    }
                    break;                
            }
            // make sure to call the base.WndProc or no other
            // messages will get handled
            base.WndProc(ref m);
        }