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Using WM_COPYDATA for interprocess communication (VFP9)

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Before you begin:
An application sends the WM_COPYDATA message to pass data to another application running on the same PC.

The screen copy above shows a message sent from VFP to C# application. The MessageBox is invoked by C# form upon receiving the message.

See also:
  • Peer-to-peer LAN messenger built with Mailslot API functions
  • Using mailslots to send messages on the network
  • Using named pipes for interprocess communication
  • Using the NetMessageBufferSend to send messages on the network
  • How to create non-blocking Winsock server
  • using System;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    //compile with /unsafe
    namespace CopyDataInterop
        public partial class Form1 : Form
            public const int WM_COPYDATA = 0x004a;
            public struct COPYDATASTRUCT
                public int dwData;
                public int cbData;
                public IntPtr lpData;
            public Form1()
                this.Text = "HWND " + this.Handle.ToString();
            unsafe protected override void WndProc(ref Message message)
                if (message.Msg == WM_COPYDATA)
                    COPYDATASTRUCT data = (COPYDATASTRUCT)
                    string str = new string((char*)(data.lpData), 
                        0, data.cbData/2);
                    MessageBox.Show(str, "Data received", 
                        MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                base.WndProc(ref message);
            unsafe private void SendString()
                string s = this.textBox2.Text;
                IntPtr lpData = Marshal.StringToHGlobalUni(s);
                COPYDATASTRUCT data = new COPYDATASTRUCT();
                data.dwData = 0;
                data.cbData = s.Length * 2;
                data.lpData = lpData;
                IntPtr lpStruct = Marshal.AllocHGlobal(
                Marshal.StructureToPtr(data, lpStruct, false);
                int hTarget = Convert.ToInt32(textBox1.Text);
                SendMessage(hTarget, WM_COPYDATA, 
                    this.Handle, lpStruct);
            private void button1_Click(object sender, EventArgs e)
            private static extern bool SendMessage(int hWnd,
                int wMsg, IntPtr wParam, IntPtr lParam);

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    2220 bytes  
    Created: 2008-03-28 13:32:01  
    Modified: 2008-04-03 11:52:28  
    Visits in 7 days: 74  
    Listed functions:
    Printer friendly API declarations
    My comment:
    The forms belonging to different VFP sessions (different processes) can exchange data in the same way. The window handle stays global through the Windows session.

    The C# and VFP versions of this code sample are functionally identical. That allows exchanging data between .NET and VFP forms (applications).

    In real life situation, prior to exchanging data the forms must have exchanged their window handles. That can be arranged by each form broadcasting a specific message that the other forms would recognize and respond. The FindWindow API is another option.

    * * *
    Comparing to other ways of interprocess communication (named pipes, mailslots, Winsock) the WM_COPYDATA has a certain edge: no need to implement any listening part. The OS does it for you.

    * * *
    if you need to pass more than 32767 characters of information to a child process, you`ll have to use something other than the command line.
    * * *
  • Interprocess Communications chapter on MSDN.
  • Foxpro Advisor Exchange Data Between Applications with Visual FoxPro 9 by Christof Wollenhaupt (recommended).
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