Very often when questions are tagged as WPF, I find myself putting little snippets of C# into the answer (even if the question itself is specified in purely xaml, occasionally some C# makes a better solution). However, after making my answer I always notice the C# code doesn't highlight, and I have to edit the C# tag into the question. Couldn't it be implied by the WPF tag that both C# code and xaml code should be highlighted, or should I just continue to add the C# tag to questions if I have any C# in my answer?
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No, because many different languages can be used for the code-behind (C#, F#, VB.NET, Python, ...).
This is why the wpf tag has no explicit language hint, so if there is no xaml tag it will highlight both c# and xaml via language detection. If either of those two (or any other tag with an associated language) is used, only the associated language is highlighted.
So you can retag or as noted by Bart use an explicit hint:
See this question for a more in-depth explanation of language hints.