Misleading error message in .NET Core WPF

When you compile a WPF (on .NET Core 3) app with the following XAML, I get the error message

> Unknown build error, 'Could not find assembly ‘Extension, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null’. Either explicitly load this assembly using a method such as LoadFromAssemblyPath() or use a MetadataAssemblyResolver that returns a valid assembly.

```

```

It’s normal that I get a build error (the XAML is wrong; in particular, the binding should read “{RelativeSource FindAncestor, …”), but this build error is not helping me to figure out what’s wrong. In fact, my first thought was not that I did something wrong but that Visual Studio’s build chain somehow got broken.

Note that you get a better error message if you add a space in `RelativeSource ,` (notice the extra space). In this case, I get the error:

> ’ , AncestorType={x:Type Window}}’ is not valid. Markup extensions require only spaces between the markup extension name and the first parameter. Cannot have comma or equals sign before the first parameter.

That’s a good error description.

Also note that the code in the example above produces a better error description on WPF for the full .NET Framework. In this case, I get the error

> The character “,” is unexpected at this position.

which is an excellent description of the problem.