DirectWrite strikes again

I was debugging an odd-looking Columns odd-looking crash from a Windows 8.1 device. The crash was odd because it looked like it should be impossible with the code in question – it was caused by a pure virtual function call when trying to call a IDWriteFont3 method.

The reason it was happening became obvious when examining the DirectWrite headers. Compare the start of the declaration of the IDWriteFont2 interface:

interface DWRITE_DECLARE_INTERFACE("29748ed6-8c9c-4a6a-be0b-d912e8538944") IDWriteFont2 : public IDWriteFont1

…with the start of the declaration of the IDWriteFont3 interface:

interface DWRITE_DECLARE_INTERFACE("29748ED6-8C9C-4A6A-BE0B-D912E8538944") IDWriteFont3 : public IDWriteFont2

Notice anything odd? I did – they both have the same UUID as their IID. That means, you can successfully query a COM object for the IDWriteFont3 interface, when it in fact only supports IDWriteFont2. And that’s exactly what happens on Windows 8.1.

Sheesh.