Hiding a winform & bringing it back from a DLL - c++

Question

In my DDL, I have the following function defined:
extern "C" __declspec(dllexport) void hideme(HWND h) {
//ShowWindow(h, SW_HIDE);
SendMessage(h, WM_SHOWWINDOW, FALSE, 0);
}
As you can see, I've tried multiple things to get this working...
I have it declared in my winform as followed:
typedef void (*HideMe)(HWND);
In my System::Windows::Forms::Form Load event, I have the following code:
...
HINSTANCE hinst = LoadLibrary(_T("My.dll"));
if (hinst == NULL)
System::Diagnostics::Debug::WriteLine("null hinst");
else
hideme = (HideMe) GetProcAddress(hinst, "hideme");
...
In my System::Windows::Forms::Form Shown event, I have the following code:
...
hideme((HWND)this->Handle.ToPointer());
...
I think the problem must lie in the way I send over the HWND, but I've been searching all day, and can't find an alternative.
I've also tried setting this->Visible = false; but then I can't figure out how to trigger it back to visible from the DLL (abuse sendmessage?).
I'm not a C++ programmer, I normally only program in managed languages, so any help (and patience is appreciated.
Thanks,
Nick.
N.B.
One weird thing I can't explain is that I can't call ShowWindow from the WinForm itself. It will throw up this gem:
Error 2 error LNK2028: unresolved token (0A00001E) "extern "C" int __stdcall ShowWindow(struct HWND__ *,int)" (?ShowWindow##$$J18YGHPAUHWND__##H#Z) referenced in function "private: void __clrcall CheckMSNCpp::frmMain::frmMain_Shown(class System::Object ^,class System::EventArgs ^)" (?frmMain_Shown#frmMain#CheckMSNCpp##$$FA$AAMXP$AAVObject#System##P$AAVEventArgs#4##Z)
Error 3 error LNK2019: unresolved external symbol "extern "C" int __stdcall ShowWindow(struct HWND__ *,int)" (?ShowWindow##$$J18YGHPAUHWND__##H#Z) referenced in function "private: void __clrcall CheckMSNCpp::frmMain::frmMain_Shown(class System::Object ^,class System::EventArgs ^)" (?frmMain_Shown#frmMain#CheckMSNCpp##$$FA$AAMXP$AAVObject#System##P$AAVEventArgs#4##Z)
Error 4 error LNK1120: 2 unresolved externals
[EDIT1]
Hans Passant said:
The linker error message you got on
the ShowWindow() attempt is also a
strong hint why your current code is
not working. You forgot to add error
checking code, GetProcAddress() can
fail and will return a NULL pointer.
It will, the function isn't exported
by the "hideme" name. In a 32-bit
build, it will be exported as
"_hideme", note the underscore. Which
was added to indicate that the
function uses the __cdecl calling
convention. The linker error on
ShowWindow demonstrates C++ name
decoration, you forgot to #include
windows.h and made up your own,
incorrect, declaration for ShowWindow.
This is not the right way to do it,
you should just set the Visible
property to false. Like you tried. Why
you cannot set it back to true is
quite unguessable.
What you said makes no sense to me at all for a few reasons.
1)
The imports for the winform:
#include "stdafx.h"
#include <windows.h>
#include <iostream>
#include <stdio.h>
As you can tell, windows.h is in there.
2) the ShowWindor error is not in the DLL, there I can call it fine, it's in the WinForm code.
3) I never exported ShowWindow, just my own functions.
[/EDIT1]
[EDIT2]
Uwe Keim said:
Why do you need an extra DLL that is
merely just a wrapper to another DLL
function?
I would do it like in e.g. C# by using
PInvoke from your managed C++
application. The signature for
SendMessage would be (in C#):
The PInvoke stuff just doesn't make any sense, like commenters said.
As for the DLL, it needs to be a DLL because i require a global hook to respond to my hotkeys & WH_CBT. In this case, I want the program to start hidden & just show a setting screen when pressing a key combo.
Everything in the program works as expected, the hotkey works like a charm, the only thin I can't get right, is showing the application, triggered from the DLL.
[/EDIT2]
[EDIT3]
Example code at: http://www.nickkusters.com/CPP_PROBLEM_Demo-NOBIN.zip
[/EDIT3]

Answer 1

The linker error message you got on the ShowWindow() attempt is also a strong hint why your current code is not working. You forgot to add error checking code, GetProcAddress() can fail and will return a NULL pointer. It will, the function isn't exported by the "hideme" name. In a 32-bit build, it will be exported as "_hideme", note the underscore. Which was added to indicate that the function uses the __cdecl calling convention. The linker error on ShowWindow demonstrates C++ name decoration, you forgot to #include windows.h and made up your own, incorrect, declaration for ShowWindow.
This is not the right way to do it, you should just set the Visible property to false. Like you tried. Why you cannot set it back to true is quite unguessable.

Answer 2

Why do you need an extra DLL that is merely just a wrapper to another DLL function?
I would do it like in e.g. C# by using PInvoke from your managed C++ application. The signature for SendMessage would be (in C#):
[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(
IntPtr hWnd,
UInt32 Msg,
IntPtr wParam,
IntPtr lParam);