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VC++ Example Capture Print Screen to Clipboard including dropdown menu, SetWindowsHookEx and UnhookWindowsHookEx, with RegisterWindowMessage

Capture screen image to clipboard including dropdown menu, combobox lists etc


 By Y. Huang


There are many sharewares that capture screen and copy image to clipboard or save as a file. Some of them charge more than $15 and can't capture dropdown menu/list. The problem is when user "active Capture Screen" program, all other windows become inactive and close their file menu or dropdown list. This simple program can capture any screen image including dropdown menu or combobox list.

Basic idea

Since clicking on "Print Screen" will send an entire window screen to the clipboard, we can read "entire window screen" from clipboard and then let users copy any portion of screen to clipboard. To implement this we need

  1. Hook keyboard: when user clicks "Print Screen", application will be notified.
  2. Read image from clipboard.
  3. Copy image and paste to clipboard.

[DLL] Hook keyboard

To hook system wide keyboard event, we need a DLL. The demo source code HScr2Clpbrd_DLL.cpp is pretty much similar to Joseph[2] demo program. The big difference is that Joseph[2] has a pretty neat header file that I don't use it. In case it's first time you create a simple DLL file, you may follow this: VC++ -> File Menu -> New -> Win32 Dynamic-link Library -> A simply DLL project -> finish. you will get a DLLMain() method.

BOOL APIENTRY DllMain( HANDLE hModule,DWORD  ul_reason_for_call, 
LPVOID lpReserved)
BOOL APIENTRY DllMain( HINSTANCE hInstance,DWORD  ul_reason_for_call,
LPVOID lpReserved)
And implement your InstallKBHook() and UnInstallKBHook() that simply use API: SetWindowsHookEx() and UnhookWindowsHookEx(), to hook/unhook keyboard event.
hookKeyBoard = SetWindowsHookEx(WH_KEYBOARD,(HOOKPROC)KeyboardHook,hInst,0);
BOOL bUnHook = UnhookWindowsHookEx(hookKeyBoard);
You may want to pay a little attention to the declaration of
__declspec(dllexport) BOOL InstallKBHook(HWND hWnd);
__declspec(dllexport) BOOL UnInstallKBHook(HWND hWnd);
Because we will export those methods to the application, we declare them as __declspec(dllexport); and, on the other hand, application need to import them (See HScr2ClpDlg.cpp)
__declspec(dllimport) BOOL InstallKBHook(HWND hWnd);
__declspec(dllimport) BOOL UnInstallKBHook(HWND hWnd);

If you like to know more about API Hook, please refer Joseph[2], Ivo[3], and Kyle[4] for more detail.

[DLL] Post message to application

So far, we can InstallKBHook() and UnInstallKBHook() but we need to notify our application that "PrintScreen" has been clicked. We need to register our window message

UWM_ClickPrnScrn = ::RegisterWindowMessage(
Once we have our Window Message, we can Post Message to notify our application that user clicks "PrintScreen"
if( (nCode >=0) && (nCode == HC_ACTION) && (wParam == VK_SNAPSHOT))
It's pretty simple to hook system wide keyboard. Isn't it? Please refer Joseph[1] for more, wide and deep, detail

[Apps] Read image from clipboard

When DLL post message, application need to receive the message. Since it's our, user defined, window message, we need to manually Map it.

1. in header file, HScr2ClpDlg.h, declare this method

afx_msg BOOL OnClickPrintScreen(WPARAM wParam, LPARAM lParam);

2. in cpp file,
HScr2ClpDlg.cpp, message map add this ON_REGISTERED_MESSAGE(UWM_ClickPrnScrn, OnClickPrintScreen)
	ON_REGISTERED_MESSAGE(UWM_ClickPrnScrn, OnClickPrintScreen)
In the definition of OnClickPrintScreen(), we first verify using IsClipboardFormatAvailable(CF_BITMAP) that the clipboard contains BITMAP data and then open (using OpenClipboard()) clipboard. If so far is fine, attach a CBitmap to clipboard BITMAP Handle
CBitmap cBm;
hBm = (HBITMAP)GetClipboardData(CF_BITMAP);
Since we have a CBitmap now, we can Copy it to our application's panel and let user "select" any image she/he needs
CClientDC dcScreen(this);
CDC dcMem;
CBitmap* pOldBitmap = dcMem.SelectObject(&cBm);
dcScreen.BitBlt(0,0,bBm.bmWidth,bBm.bmHeight ,&dcMem,0,0,SRCCOPY);

[Apps] Copy image and paste to clipboard

We copy user's "selection" from client area and paste to clipboard

BOOL CHScr2ClpDlg::CopyRect2Clipboard()
	CRect rect(m_ptFirst,m_ptLast);
	if(rect.IsRectEmpty() || rect.IsRectNull())
		return FALSE;	

	//CDC *pDc = GetWindowDC();//include Non-Client area.
	CClientDC dcScrn(this);
	CDC memDc;
		return FALSE;	
	CBitmap bitmap;
	if( !bitmap.CreateCompatibleBitmap(&dcScrn,rect.Width(),
		return FALSE;
	CBitmap* pOldBitmap = memDc.SelectObject(&bitmap);
	memDc.BitBlt(0,0,rect.Width(),rect.Height(),&dcScrn,rect.left ,
            ,SRCCOPY );		
	return TRUE;


This tiny program is very simple because it doesn't have any complicated coding. You can find many articles showing you how to Copy images to the clipboard, Hook keyboard, or, Capture mouse movement... from or other web sites. 

However, there is one cool/neat "mailto button" on the demo program panel; it's, IMHO, the most difficult coding in this tiny utility. The idea of this program is "put people's idea together and have fun"; and, of course, I need to capture screen for my work. I may enhance it that provide a "Preview" panel which has "Copy", "Save as file", and "Discard" options for user when I have time. If anyone can post your enhancement, I would love to update the article, and thank you.

Known bug

If you run program and leave it on screen, don't minimize it to tucancode.netbar, and click "Print Screen", apps will set itself as MostTop Window as expected. But "minimize" and "close" buttons will not work until you set it to non-top Z-order. In other words, you need to click another window to make apps "inactive" and click apps itself to make it "active"; therefore, "minimize" and "close" buttons will work as usual. On the other hand, if you "minimize" apps to tucancode.netbar and "click "Print Screen", everything just works fine.




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