Why does GetLogPen fail with a custom pen?

Question

The calls to GetLogPen() and GetExtLogPen() both fail. They return zero...

CBrush Brush;
Brush.CreateSolidBrush( COLOR_MINORSELECTION );
Brush.GetLogBrush( &lBrush );
DWORD Style[] = { 3, 1 };
CPen CustomPen;
CustomPen.CreatePen( PS_USERSTYLE, 1, &lBrush, 2, Style );
CPen *pOldPen = pDC->SelectObject( &CustomPen );
LOGPEN LogPen;
if( CustomPen->GetLogPen( &LogPen ) == 0 )
{
    EXTLOGPEN ExtLogPen;
    if( CustomPen->GetExtLogPen( &ExtLogPen ) == 0 )
        return;
}

The failure appears to be because of the PS_USERSTYLE being used for the pen style. If I do this with a PS_SOLID pen, I get the LogPen structure filled in as I expect.

Any thoughts?

Answer 1

This is a bug in the CPen::GetExtLogPen implementation:

int CPen::GetExtLogPen(EXTLOGPEN* pLogPen) {
    return ::GetObject(m_hObject, sizeof(EXTLOGPEN), pLogPen);
}

The implementation ignores the trailing variable-sized array of DWORDs in the EXTLOGPEN structure. This structure is defined as:

typedef struct tagEXTLOGPEN {
    DWORD     elpPenStyle;
    DWORD     elpWidth;
    UINT      elpBrushStyle;
    COLORREF  elpColor;
    ULONG_PTR elpHatch;
    DWORD     elpNumEntries;
    DWORD     elpStyleEntry[1];
} EXTLOGPEN, *PEXTLOGPEN;

The call to CPen::GetExtLogPen succeeds, if the elpStyleEntry array is at most one element long. This is true for all pens except for those pens that have the PS_USERSTYLE pen style. When using the PS_USERSTYLE pen style, the elpStyleEntry entry will have at least 2 entries.

The workaround in the case of pens with the PS_USERSTYLE pen style is to use the Windows API GetObject call, and circumvent the MFC implementation:

// Query for required buffer size
int sizeRequired = ::GetObject(CustomPen.m_hObject, 0, nullptr);
// Allocate buffer (may not be properly aligned)
std::vector<byte> buffer(sizeRequired);
// Retrieve the entire EXTLOGPEN structure
int ret = ::GetObject(CustomPen.m_hObject, static_cast<int>(buffer.size()), buffer.data());
assert(ret == static_cast<int>(buffer.size());
// Cast to const ref for convenient access
const EXTLOGPEN& elp = *reinterpret_cast<const EXTLOGPEN*>(buffer.data());

Unfortunately, the solution posted by Mark Ransom doesn't solve the issue, because the CPen::GetExtLogPen passes sizeof(EXTLOGPEN) to the GetObject call, not the true size of its argument.

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Note: A bug report has been submitted to Microsoft Connect.