What does this icon in the margin of the text editor mean?

Question

There is an icon in the margin of the text editor that has an up arrow and a "I" with a blue background. What does this mean?

I am using VS2019 16.10.0. and it is .netfx Winform project.

Answer 1

This icon shows the inheritance chain/margin and was added in version 16.10 of Visual Studio. From the 16.10 release notes for Visual Studio:

There is now a visual representation for navigating and inspecting the inheritance chain. This option is off by default so you will need to turn it on in Tools > Options > Text Editor > C# > Advanced and select Show inheritance margin. Enabling inheritance margin will add new icons to the margins representing your code’s implementations and overrides. Clicking on the inheritance margin icon will display inheritance options that you can select to navigate to.

Sure enough, if you navigate to the option in the tools menu, it is at the bottom:

I would check your settings for this - it should be off by default, meaning somehow it was turned on for you. The icon should show the inheritance chain if you click on it. If it isn't, perhaps turn the option off and back on, or restart Visual Studio (it's experimental apparently, so it could be bugged).