C# Show Inline Errors
This extension displays Compiler errors, warnings and other messages in the line they occur. there's an option page where the display of errors and warnings and other methods can be configured.
The options page is hidden under the category "VarAdorner", which is a legacy name I have to work on. You can specify if you want to see anything (including errors) (true/false), warnings and suggestions (true/false). You will have to reopen already opened editor windows for changed settings to take effect.