Question

Can anybody tell me why the following code doesn’t work? I don’t get any compiler errors.

short value = 10;
SetProp(hCtl, "value", (short*) value);

Answer 1

The third parameter is typed as a HANDLE, so IMO to meet the explicit contract of the function you should save the property as a HANDLE by allocating a HGLOBAL memory block. However, as noted in the comments below, MSDN states that any value can be specified, and indeed when I try it on Windows 7 using…

SetProp(hWnd, _T("TestProp"), (HANDLE)(10)); // or (HANDLE)(short*)(10)
...
(short)GetProp(hWnd, _T("TestProp"));

… I get back 10 from GetProp. I suspect somewhere between your SetProp and GetProp one of two things happens: (1) the value of hWnd is different — you’re checking a different window or (2) a timing issue — the property hasn’t been set yet or had been removed.

---

If you wanted to use an HGLOBAL instead to follow the specific types of the function signature, you can follow this example in MSDN.

Even though a HANDLE is just a pointer, it’s a specific data type that is allocated by calls into the Windows API. Lots of things have handles: icons, cursors, files, … Unless the documentation explicitly states otherwise, to use a blob of data such as a short when the function calls for a HANDLE, you need a memory handle (an HGLOBAL).

The sample code linked above copies data as a string, but you can instead set it as another data type:

// TODO: Add error handling
hMem = GlobalAlloc(GPTR, sizeof(short));
lpMem = GlobalLock(hMem);
if (lpMem != NULL)
{
    *((short*)lpMem) = 10;
    GlobalUnlock(hMem);
}

To read it back, when you GetProp to get the HANDLE you must lock it to read the memory:

// TODO: Add error handling
short val;
hMem = (HGLOBAL)GetProp(hwnd, ...); 
if (hMem)
{
    lpMem = GlobalLock(hMem);
    if (lpMem)
    {
        val = *((short*)lpMem);
    }
}