While you are editing a variable name in Visual Studio 2015, Visual Studio 2017 or Visual Studio 2019, a dashed outline is displayed around it:
It's a new feature of Visual Studio 2015 that works for C# and Visual Basic languages. The same outline is displayed when you change a method, class or namespace name.
If you find this outline distracting, install the free Plain Rename extension that completely removes it.
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