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Thanks!

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

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Tagging @dotnet/compat for awareness of the breaking change.

@stephentoub was the breaking change you were identifying here the change to permit a null string in the virtual methods? @dotnet/area-system-xml can you help submit the document?

was the breaking change you were identifying here the change to permit a null string in the virtual methods?

Yes. It's minor, but anyone who was deriving from these types and overriding these methods and who had nullable reference types enabled will now get a compiler warning due to having overrides which say they don't accept nullable when the base virtual now says it does.

(In previous releases, we've had a single breaking change notification that mopped up all of the annotation changes in the release. We might want to do something similar here, if there are others. cc: @jeffhandley)