What does purple underlines mean in visual studio editor?

Question

I am facing some purple (or violet?) underlines in Visual Studio 2015 today, something I have never seen before.

I held the cursor over the text but nothing happened(It happens sometimes). I googled it and got nothing.

What does it mean? How do difference types of underlines represent different kinds of warnings in VS?

Here is some hint: I later found myself editing the source while the program is still running. When I stop the program, the underline disappeared. So I presume it has something to do with the current state of debugging.

Answer 1

In this case, you see the purple edit because you're modifying code while it's running. see https://msdn.microsoft.com/en-us/library/ba77s56w.aspx:

When you make an unauthorized edit, the change is marked with a purple wavy underline and a task is displayed in the Task List. You must undo an unauthorized edit if you want to continue to use Edit and Continue. Certain unauthorized edits may be permitted if done outside Edit and Continue. If you want to retain the results of such an unauthorized edit, you must stop debugging and restart your application.

A purple underline could also show up when there's an error in code that won't be run because of some circumstance, like an #IFDEF. see https://msdn.microsoft.com/en-us/library/dn957937.aspx:

A purple squiggle appears in a shared project when there is an error in code that is not currently active, for example when you are working in the Windows context but enter something that would be an error in an Android context. A red squiggle indicates a compiler error or warning in active code that you need to deal with.