Here’s a yummy and odd thing. I hooked up some global error handling in my big ASP.NET app. I put it in two places.
The first place was a handler for the Error event in the base class for all my pages. I had a call to log4net to log the error and a Redirect that would show the user a friendly explanation of what went wrong.
The second place was the Application Error handler in Global.Asax.cs. Same drill. This handler is for errors that happen outside the scope of my base class (like ASHX files, etc).
Remember that the call to Redirect (Transfer in one case) was commented out for development. It was commented out for months and months. I uncommented it today.
Suddenly I started seeing this ominous dialog when running Visual Studio.NET. NOTE: I said RUNNING VS.NET, not running the app. I get this as soon as the CSProj gets loaded! Eek!
This stumped me, as this dialog is the kind of thing you see when you’re not the DebuggerUsers Group, yada yada yada. I googled some, and all the references to this dialog refer to permissions and such, and this was clearly not a permission thing as nothing had changed in that ‘hood.
Since I had log4net (glorious, BTW) hooked up, I setup the FileAppender to log all my copious Debug statements to a file. I launched VS.NET and loaded the project. Turned out some of my pages were being flat-out executed either as VS.NET asked for them from IIS or as they were loaded into the Designer (I’ve complained about this designer thing before). In my Init() of a deep base class a I was checking Request.UserLanguages for the user’s preferred language. Yeah, I know, I didn’t check to see if  was there. It always has been! Well, apparently it’s not there when VS.NET calls with HTTP to IIS.
Long story short, a NullReferenceException occurred which was now CAUGHT by my Exception handler and I tried to call Response.Transfer which boogered up VS.NET who decided that Debugging was never going to work. Preventing that exception stopped this strange VS.NET startup error.
Happy New Year!