How to check whether the window handle is valid?

24 08 2008


Since communication by messages are so easy, windows used to communicate with each other by using messages. For instance, if simple data blocks are to be transfered – WM_COPYDATA can be used. For all those instances the window should be alive. But how to know whether the current window handle is valid or whether it points to a dead window?


You can call the api – IsWindow() by passing window handle. If the handle points to a live window, then the function returns true else false. Have a look at the code snippet.

// The handle to be tested.
HWND hWindow = GetSafeHwnd();

// Check whether the window is still there.
BOOL bWindowAlive = IsWindow( hWindow );


While digging for the api, i found an interesting info. What about window handle re-cycling? For instance you have a window handle and you’re going to check that window by calling IsWindow() function. But in between that the real window is closed and a new window is created. Whether the window handler will be allocated to the new window?

Well, the answer is here. Have a look at it. Its interesting. The oldnewthing. 😉


Targeted Audience – Beginners.

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WM_COPYDATA can be used as simple IPC mechanism.

12 04 2008


Sending and receiving data between processes are one of the greatest headaches that we face. For that many mechanisms are there – such as COM, Sockets, but a bit complicated… There is comparatively simple mechanism for doing the same. i.e. WM_COPYDATA window message.


For IPC communication between two processes, you just need a hidden window created in each and by using the window handle in target process you can send data to it. For sending data via WM_COPYDATA, the following are requirements.

WPARAM – Handle of window in target process.
LPARAM – Pointer to COPYDATASTRUCT which contains data to be exported.

Please see the following code block about how to use it.

void CMyDialog::SendData( HWND hTargetWindow_i,
void* pData_i,
DWORD dwSize_i )
{
// structure holding data information.
COPYDATASTRUCT stCopyData = { 0 };
stCopyData.lpData = pData_i;
stCopyData.dwData = dwSize_i;

// Send the data.
SendMessage( WM_COPYDATA,
(UINT) hTargetWindow_i,
(ULONG) &stCopyData );
}

In the receiving Window, if you are using MFC, you need to handle the WM_COPYDATA message by adding message handle using ON_WM_COPYDATA() and handler function with proto –

afx_msg BOOL OnCopyData( CWnd* pWnd,
COPYDATASTRUCT* pCopyDataStruct );


Targeted Audience _ intermediate.