Inter-Process Communication with C#

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Introduction

This app shows how to send and receive messages between two apps using WM_COPYDATA.

There are two samples. One is testmessage app and one is testMessage2 app. The testmessage will check and open app 2 if it is not running and if so, will start another instance with a different window header.

• The 'Send' button will send the text from the textbox to the testMessage2 app.
• The received data will show up in the textbox.

Using the Code

The exchange of data is performed by finding the other application (using FindWindow) and sending a WM_COPYDATA message to that window:

 public static bool SendArgs(IntPtr targetHWnd, string args)
        {
            Win32.CopyDataStruct cds = new Win32.CopyDataStruct();
            try
            {
                cds.cbData = (args.Length + 1) * 2;
                cds.lpData = Win32.LocalAlloc(0x40, cds.cbData);
                Marshal.Copy(args.ToCharArray(), 0, cds.lpData, args.Length);
                cds.dwData = (IntPtr)1;
                Win32.SendMessage(targetHWnd, Win32.WM_COPYDATA, IntPtr.Zero, ref cds);
            }
            finally
            {
                cds.Dispose();
            }

            return true;
        } 
protected override void WndProc(ref Message m){
            switch(m.Msg){
                case Win32.WM_COPYDATA:
                    Win32.CopyDataStruct st =
            	  (Win32.CopyDataStruct)Marshal.PtrToStructure(m.LParam,             
		  typeof(Win32.CopyDataStruct));
                    string strData = Marshal.PtrToStringUni(st.lpData);
                    txtmessagereceive.Text = strData;
                    break;

                default:
                    // let the base class deal with it
                    base.WndProc(ref m);
                    break;
            }
        }

History

• 12^th July, 2007: Initial post