Unable to get structure into managed data space using GetLParam

[Robert Manookian]

We have a C++ windows application that sends windows messages to a running
VB application to perform various activities. On of these activities needs
more information than can fit into a 32 bit integer. I created a structure
to hold the information needed. However, when I get the information in the
VB.Net code using the GetLParam method it returns incorrect results. How
does one pass a structure from unmanaged code to managed code via a windows
message? Here's the code:

C++ Code:
#include "stdafx.h"
#include "resource.h"
#include <Shellapi.h>
#include "objbase.h"

#pragma pack(4)

struct DataviewMessage {
long TaskID;
long SettingKey;
};

void PostBusinessDesktopDataviewMessage(long lTaskID, long lSettingKey);

int APIENTRY WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPSTR lpCmdLine,
int nCmdShow)
{
PostBusinessDesktopDataviewMessage((long)1, (long)1);
return TRUE;
}

void PostBusinessDesktopDataviewMessage(long lTaskID, long lSettingKey)
{
HWND hTargetWnd = ::FindWindow(NULL,
kBsinessDesktopHelperWindowCaption);

if (hTargetWnd != NULL) {
DataviewMessage *DvMsg = (DataviewMessage
*)malloc(sizeof(DataviewMessage));
DvMsg->TaskID = lTaskID;
DvMsg->SettingKey = lSettingKey;
::SendMessage(hTargetWnd, WM_COMMAND, 1026, (long)DvMsg);
free(DvMsg);
}
}


VB Code:
Imports System.Runtime.InteropServices

<StructLayout(LayoutKind.Explicit)> _
Public Structure DataviewShortcut
<FieldOffset(0)> Public TaskID As Integer
<FieldOffset(4)> Public SettingKey As Integer
End Structure

Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
Try
If (m.WParam.ToInt32 = 1026) Then
If (m.LParam.ToInt32 <> 0) Then
Dim DataviewSetting As Object
DataviewSetting = m.GetLParam(GetType(DataviewShortcut))
Console.WriteLine(CType(DataviewSetting,
DataviewShortcut).TaskID & ", " & CType(DataviewSetting,
DataviewShortcut).SettingKey)
End If
End If
Catch oException As Exception
MessageBox.Show(oException.Message, Me.Text,
MessageBoxButtons.OK, MessageBoxIcon.Error)
End Try
MyBase.WndProc(m)
End Sub

 

[Mattias Sjögren]

Robert,

Are you sure you're looking at the right message, and not some other
message that happens to pass 1026 as wParam? I'd include a check to
verify that the message you're looking at is indeed WM_COMMAND in the
VB.NET code.



Mattias

 

[Robert Manookian]

You are indeed correct that I should include this check. However, the data
that GetLParam returns is still incorrect after I implemented the check.