WPF known issues: Application will FailFast when not find the Arial font from system

• .NET Core Version: All
• Windows version: All
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

Problem description:

When we indirectly or directly call the FirstFontFamily method of FontFamily.cs, if we call FindFirstFontFamilyAndFace and the font is not found, then we will look for the system’s Arial font.

https://github.com/dotnet/wpf/blob/6cd4ee063a4d1f764a8e5f27f02df971dbc4a900/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/FontFamily.cs#L350-L362

The LookupFontFamily method return NULL when the user deletes the Arial font. And then we will enter the Invariant.Assert(family != null) to make the Application to FailFast.

https://github.com/dotnet/wpf/blob/6cd4ee063a4d1f764a8e5f27f02df971dbc4a900/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/FontFamily.cs#L361

https://github.com/dotnet/wpf/blob/6cd4ee063a4d1f764a8e5f27f02df971dbc4a900/src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/Invariant.cs#L85-L91

https://github.com/dotnet/wpf/blob/6cd4ee063a4d1f764a8e5f27f02df971dbc4a900/src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/Invariant.cs#L199-L213

I don’t think this is a good design.

Actual behavior:

Application will FailFast and I can not do anything and do not know why

Expected behavior:

We can handle it or receive the event

Minimal repro:

[walterlv]

Some users like to activate the hidden Administrator account which is very dangerous. These users may delete the fonts easily by accident without being aware of the consequence. I know that this behavior is extremely not recommended but our product has to support most of the computers and users all over the world. That’s why we bring up the issue here.

[miloush]

While I agree there are other ways to handle this, how likely is this an issue in practice?

Windows does not let users delete the font:

Moreover, imagine you get an opportunity to handle this scenario. How do you handle it? With a fallback to different family? If so, you can already handle it be specifying your desired fallback when asking for a font family.