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           [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        internal struct MyStruct
        {
            public int Number;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
            public string Message;
        }  
        // .....
           myStruct.Number = 1234;
            myStruct.Message = "hello";

            int myStructSize = Marshal.SizeOf(myStruct);
            IntPtr pMyStruct = Marshal.AllocHGlobal(myStructSize);
            try
            {
                Marshal.StructureToPtr(myStruct, pMyStruct, true);

                COPYDATASTRUCT cds = new COPYDATASTRUCT();
                cds.cbData = myStructSize;
                cds.lpData = pMyStruct;

                NativeMethod.SendMessage(hTargetWnd, WM_COPYDATA, NativeMethod.GetForegroundWindow(), ref cds);
            }                    

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type
  TMyStruct = packed record
    Number: integer;
    Msg: string[255];
  end;
  PMyStruct = ^TMyStruct;

procedure TfrmHelper.WMCOPYDATA(var msg: TWMCopyData);
var
  MyRecord: PMyStruct;
  s: String[255];
  Number: integer;
begin
  s := PMyStruct(msg.CopyDataStruct.lpData)^.Msg;
  Number := PMyStruct(msg.CopyDataStruct.lpData)^.Number;
  ShowMessage('Msg ' + s + ' Number: ' + IntToStr(Number));
end;