wchar* question

[Sam Hu]

Given below code(Win32 SDK):

int /*LRESULT*/ wndProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
        {
                switch(msg)
                {
                                                        case WM_LBUTTONDOWN:
                                {
                                        wchar* szFileName=cast(wchar*)(new 
wchar[1024]);// ***questioned line
                                                                                
HINSTANCE hInstance=GetModuleHandleW(null);
                                        
GetModuleFileNameW(hInstance,szFileName,1024);
                                        MessageBoxW(null,cast(const 
wchar*)szFileName, "Full Path:",
                                        MB_ICONINFORMATION);
                                }
                                
                                break;
}

My program runs OK and can print the exe path as expected.But if I change 
wchar* szFileName=cast(wchar*)(new wchar[1024]);

to 
wchar* szFileName;

The program runs also but prints blank.Why?

Thanks for your help in advance.
Regards,
Sam