C / C++ / MFC

//<br />
// RtlpNumberOf is a function that takes a reference to an array of N Ts.<br />
//<br />
// typedef T array_of_T[N];<br />
// typedef array_of_T &reference_to_array_of_T;<br />
//<br />
// RtlpNumberOf returns a pointer to an array of N chars.<br />
// We could return a reference instead of a pointer but older compilers do not accept that.<br />
//<br />
// typedef char array_of_char[N];<br />
// typedef array_of_char *pointer_to_array_of_char;<br />
//<br />
// sizeof(array_of_char) == N<br />
// sizeof(*pointer_to_array_of_char) == N<br />
//<br />
// pointer_to_array_of_char RtlpNumberOf(reference_to_array_of_T);<br />
//<br />
// We never even call RtlpNumberOf, we just take the size of dereferencing its return type.<br />
// We do not even implement RtlpNumberOf, we just decare it.<br />
//<br />
// Attempts to pass pointers instead of arrays to this macro result in compile time errors.<br />
// That is the point.<br />
//<br />
extern "C++" // templates cannot be declared to have 'C' linkage<br />
template <typename t,="" size_t="" n=""><br />
char (*RtlpNumberOf( UNALIGNED T (&)[N] ))[N];<br />
<br />
#define RTL_NUMBER_OF_V2(A) (sizeof(*RtlpNumberOf(A)))</typename>

MY_DLL_API UINT MyCustomActionLaunch(MSIHANDLE hMsiModule)
{
	
    MessageBoxW (NULL, L"MyCustomAction launch", L"MyCustomAction", MB_OK);	
    WCHAR msg [500] = {0};
    WCHAR vbuff [1024] = {0};
    DWORD vlen = 1024;
    UINT gp = MsiGetPropertyW(hMsiModule, L"ProgramFilesFolder", vbuff, &vlen);
	
	if(gp == ERROR_SUCCESS )
	{
		 if (wcslen(vbuff) <= 0)
		 {
			<big> MessageBoxW(NULL , L"Zero Bytes",L"MyCustomAction" , MB_OK);</big>		 }
		
		MessageBoxW(NULL, vbuff, L"MyCustomAction", MB_OK);    		

	}	else
	{
		MessageBoxW (NULL, L"Failure", L"MyCustomAction", MB_OK);
    
	}

	return -1;
}