C# Definition:

[StructLayout(LayoutKind.Sequential)]
struct COPYDATASTRUCT
{
     public IntPtr dwData;
     public int cbData;
     public IntPtr lpData;
}

const int WM_COPYDATA = 0x004A;

VB.NET Definition:

<StructLayout(LayoutKind.Sequential)> _
Structure COPYDATASTRUCT
     Public dwData As IntPtr
     Public cdData As Integer
     Public lpData As IntPtr
End Structure

User-Defined Field Types:

None.

public static IntPtr IntPtrAlloc<T>(T param)
{
     IntPtr retval = Marshal.AllocHGlobal(Marshal.SizeOf(param));
     Marshal.StructureToPtr(param, retval, false);
     return (retval);
}

    public static void IntPtrFree(IntPtr preAllocated)
    {
    if (IntPtr.Zero == preAllocated) throw (new Exception("Go Home"));
    Marshal.FreeHGlobal(preAllocated); preAllocated = IntPtr.Zero;
    }

    [StructLayout(LayoutKind.Sequential)]
    struct COPYDATASTRUCT
    {
    public IntPtr dwData;
    public int cbData;
    public IntPtr lpData;
    }

    public SendMessage()
    {
    IntPtr buffer = IntPtrAlloc(txStruct);
    COPYDATASTRUCT copyData = new COPYDATASTRUCT();
    copyData.dwData = IntPtr.Zero;
    copyData.lpData = buffer;
    copyData.cbData = Marshal.SizeOf(txStruct);
    IntPtr copyDataBuff= IntPtrAlloc(copyData);
    SendMessage(hWnd, WM_COPYDATA, IntPtr.Zero, copyDataBuff);
    IntPtrFree(copyDataBuff); copyDataBuff = IntPtr.Zero;
    IntPtrFree(buffer); buffer = IntPtr.Zero;
    }

    protected override void WndProc(ref Message m)
    {
    switch (m.Msg)
    {
        case (int)WM.COPYDATA:
        var msg = Marshal.PtrToStructure<COPYDATASTRUCT>(m.LParam);
        var pData = Marshal.PtrToStructure<GEORECT>(msg.lpData);
        if (pData.Falconview != FalconShu.FALCONVIEW)
            break;
        if (MessageType.FV_RUBBERBAND_GEORECT_MSG == pData.Type
            && this.messageId == pData.MessageId)
        {
            this.Simulation.WeatherArea.N = new LatLong(this.ctrlLatitudeN.Latitude = pData.NW_Latitude, LL.Latitude);
            this.Simulation.WeatherArea.W = new LatLong(this.ctrlLongitudeW.Longitude = pData.NW_Longitude, LL.Longitude);
            this.Simulation.WeatherArea.S = new LatLong(this.ctrlLatitudeS.Latitude = pData.SE_Latitude, LL.Latitude);
            this.Simulation.WeatherArea.E = new LatLong(this.ctrlLongitudeE.Longitude = pData.SE_Longitude, LL.Longitude);
            CheckForReadiness();
            Externals.SetForegroundWindow(this.Handle);
            return;
        }
        else if (MessageType.FV_RUBBERBAND_GEORECT_CANCELED == pData.Type)
        {
            // Message overwritten by another attempt to get it
            return;
        }

        break;
    }

    base.WndProc(ref m);
    }