Properly handle WM_PASTE in subclass procedure

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C++

Win32

RELEVANT INFORMATION:

I have a subclass procedure that needs to validate the content of the clipboard before it gets pasted.

I have managed to get the content of the clipboard successfully, at least I think so.

QUESTION:

I do not know how to construct the following if statement ( the following is pseudo code ):

if( clipboard content is OK )
    defaul handler;
else
    discard message;

MY EFFORTS TO SOLVE THIS:

So far this is what I have in mind:

LRESULT CALLBACK Decimalni( HWND hwnd, 
    UINT message, WPARAM wParam, LPARAM lParam, 
    UINT_PTR uIdSubclass, DWORD_PTR dwRefData )

{
    switch (message)
    {
    case WM_PASTE:
        {
            bool IsTextValid = true; // indicates validity of text
            
            if( OpenClipboard(hwnd) ) // open clipboard
            {
                HANDLE hClipboardData;
                
                // get clipboard data
                if( hClipboardData = GetClipboardData(CF_UNICODETEXT) )
                {
                    // Call GlobalLock so that to retrieve a pointer
                    // to the data associated with the handle returned
                    // from GetClipboardData.
                    
                    wchar_t *pchData = (wchar_t*)GlobalLock(hClipboardData);
                    
                    // copy clipboard data so we can free clipboard
                    
                    wchar_t result[10]; // I just need first 9 characters
                    memset( result, L'0', sizeof(result) );
                    
                    // copy clipboard data WITH TRUNCATION!!!
                    wcsncpy_s( result, 10, pchData, _TRUNCATE );
                    
                    // Unlock the global memory.
                    GlobalUnlock(hClipboardData);
                    
                    /*** parse the text for validity ****/
                    // code for parsing text 
                    // update IsTextValid to indicate success or fail
                    /*** end of parsing *******/
                
                }
                
                // Finally, when finished I simply close the Clipboard
                // which has the effect of unlocking it so that other
                // applications can examine or modify its contents.
                
                CloseClipboard();
            }
            
            // here should be the problematic if statement
            if( IsTextValid )
                return ::DefSubclassProc( hwnd, message, wParam, lParam);
            else
                return FALSE;
        }
        break;
    case WM_CHAR:
        {
        // filter out some invalid keys
        }
        break;
    case WM_NCDESTROY:
        ::RemoveWindowSubclass( hwnd, Decimalni, 0 ); // remove subclassing
        break;
    }
    return ::DefSubclassProc( hwnd, message, wParam, lParam);
}

Is my idea correct or is there another way to form my if statement?

Thank you.

Best regards.