If you have ever uninstalled or disabled ReSharper and then realised that Visual Studio is not the same anymore (the intellisense and error hilighting does not work) then this is for you:
Click "Tools" then "Options" to bring up the options window.
If you are primarily using C# then select "Text Editor" then "C#", then:
Select "General" and re-enable "Auto list members" and "Parameter information".
Now select "Advanced" and re-enable "Underline errors in the editor" and "Show live semantic errors".
That should do the trick … visual studio is back to its old self and behave the way you expect it to.