Are 2 IDWriteFontFace instances equals?

Question

I need to determine whether two IDWriteFontFace instances represent the same font file (and index for .ttc font). Would using this solution be a reliable approach?

From my testing, even if two IDWriteFontFace instances reference the same font file, their IUnknown values can differ depending on how they were created.

For example:

• If I create an IDWriteFontFace using IDWriteGdiInterop::CreateFontFaceFromHdc, calling this method twice (even with different HDCs but the same LOGFONT) results in the same IUnknown value.
• However, when comparing an IDWriteFontFace obtained via IDWriteFontSet::GetMatchingFonts with one created using IDWriteGdiInterop::CreateFontFaceFromHdc, the IUnknown values sometimes match and sometimes don’t, even when they reference the same file.

Given this inconsistency, is this solution only reliable when both IDWriteFontFace instances are created using the same method? Or even, is it reliable?

Answer 1

What you can do rather reliably is to compare the following:

• GetFiles() -> GetLoader(). If you get same loader instance, you can then compare reference keys;
• GetIndex() have to match;

If you loader is always IDWriteLocalFontFileLoader, you can then get a path directly, and compare that.

Equals() will consider simulations too, meaning faces using same {fontfile, index} could still be unequal.