A窗体： 

        const int WM_COPYDATA = 0x004A;



        private struct COPYDATASTRUCT

        {

            public IntPtr dwData;

            public int cbData;

            [MarshalAs(UnmanagedType.LPStr)]

            public string lpData;

        }



            //发送内容Content

            string FormName = "B";

            IntPtr handle = FindWindow(null, windowsName);



            if (handle != IntPtr.Zero)

            {

                byte[] s = Encoding.Default.GetBytes(Content);

                int len = s.Length;

                COPYDATASTRUCT cds;

                cds.dwData = (IntPtr)100;

                cds.lpData = Content;

                cds.cbData = len + 1;

                SendMessage(handle, WM_COPYDATA, 0, ref cds);

            }

B窗体： 

         protected override void DefWndProc(ref Message m)

        {

            switch (m.Msg)

            {

                case WM_COPYDATA:

                    COPYDATASTRUCT mystr = new COPYDATASTRUCT();

                    Type mytype = mystr.GetType();

                    mystr = (COPYDATASTRUCT)m.GetLParam(mytype);

                    string receiveMsg = mystr.lpData;



                    MessageBox.Show(receiveMsg);