My Visual Studio (2008) Editor has stopped to underline Errors (this nifty wavy red lines). I can't really tell when, but it can be related to the installation of .Net Framework 3.5 SP 1 or the MVC Beta (which I guess is unlikely). Furthermore have I installed and uninstalled both CodeRush and Resharper for evaluation purposes (decided not to keep either one of them).
Does anyone know the problem and how to restore this functionality again?
Have you checked Tools→Options...→Text Editor→C#→Advanced→Underline errors in the editor?
I usually like to reset my settings after messing around with plugins, as they tend to mess with settings: Tools→Import and Export Settings...→Reset all settings.
Found it in Visual Studio 2019 as: Tools > Options > Text Editor > General > Show error squiggles
I know its an old question, and with various solutions, but I have fixed it in different way. I'm working with Unity3D on my C# code using VS2017, when suddenly VS decides to stop underlining error while im typing. However, if I close the file tab and reopen, it suddenly undelines the error.
For example:
class A {
public int x;
s;
}
should obvsiouly give an error for that lonely 's' symbol. But, VS doesn't underline it until I close and reopen this file tab.
Solution: Copied the entire Unity Project folder (which is like a regular VS Solution folder basically) and worked with the new folder, which issue was gone there.
For visual studio 2017 act according to HeeJae's comments in:
i.e:
Hi. you are probably hitting a known issue. can you try this?
1.Update to latest release If that doesn’t solve it
2.Go to Tools\Options\Projects and Solutions\General and uncheck “Allow parallel project initialization”.
3.Close VS.
4.Delete the “.vs” directory beside their solution file.
5.Reopen VS.
..
thank you
You can re-enable the "Allow parallel project initialization" option after the issue was solved.
I tried to upgrade VS, reset VS settings, clear VS cache and everything people do conventionally but none of them solved this issue! At the end the mentioned solution worked for me magically.
Good luck
Unloading and loading same project again from the solution does the trick. Just right click on the project and click "Unload Project". Once unloaded, again right click the same project and click "Reload Project". Error highlighting will return.
I had the same issue with 2017. There was a 'disable intelisense' option, make sure that is set to false.
For everyone wondering in 2021..
search for "C_Cpp.errorSquiggles" in the settings.
Make sure to have it active for the user, as well as the workspace. No need to restart Visual Studio.
For me (VS 2019) , after trying the other answers also, setting the scope of analysis from "Current document" to Open document" brought back the missing error markers 
Just go to settings and search for errors and Image in Error Squiggles. You can see the Error squiggles (Modified: Workspace - Right now you can't see it because I modified it). Just click on modified and you will see the disabled option. If by mistake you disabled it, just enable it and you can see the red line errors again in your code.
In latest edition, check for .vscode folder in same project folder. There will be a setting.json file in that. Delete the key value pair of "C_Cpp.errorSquiggles": "Disabled". Restart the vs code.
