Catastrophic Failure attaching to IISExpress

Are you running more than one site within the same application pool? I was having the same problem and believe that separating the app sites into different application pools fixed the issue.

Additionally I had issue when the wrong start-up project was selected in Visual studio. Make sure the correct start-up project is selected before attaching, though I can't see why this should matter.

Also I created a controller for the debugger to be launched from the application, which no only makes it much easier, also appears to have less issues.

    #if DEBUG
    public virtual ActionResult Attach()
    {
        System.Diagnostics.Debugger.Launch();
        return new EmptyResult();
    }
    #endif