Visual Studio not displaying compile time errors in editor
Solution 1:
You need to turn on the underline errors in the editor and show live semantic errors options in Visual Studio.
These options can be found here:
Tools > Options > Text Editor > C# > Advanced > Editor Help
Edit: You will need to install SP1 for this functionality to work.
Solution 2:
Select Tool -> Options, then Text Editor. Under the language you are using (ie C#), go to the Advanced and make sure the Underline errors in the editor and Show live semantic errors are checked