VS2010 does not show unhandled exception message in a WinForms Application on a 64-bit version of Windows

Question

When I create a new project, I get a strange behavior for unhandled exceptions. This is how I can reproduce the problem:

1) create a new Windows Forms Application (C

Answer 1

In my experience, I only see this issue when I'm running with a debugger attached. The application behaves the same when run standalone: the exception is not swallowed.

With the introduction of KB976038, you can make this work as you'd expect again. I never installed the hotfix, so I'm assuming it came as part of Win7 SP1.

This was mentioned in this post:

• The case of the disappearing OnLoad exception – user-mode callback exceptions in x64

Here's some code that will enable the hotfix:

public static class Kernel32
{
    public const uint PROCESS_CALLBACK_FILTER_ENABLED = 0x1;

    [DllImport("Kernel32.dll")]
    public static extern bool SetProcessUserModeExceptionPolicy(UInt32 dwFlags);

    [DllImport("Kernel32.dll")]
    public static extern bool GetProcessUserModeExceptionPolicy(out UInt32 lpFlags);


    public static void DisableUMCallbackFilter() {
        uint flags;
        GetProcessUserModeExceptionPolicy(out flags);

        flags &= ~PROCESS_CALLBACK_FILTER_ENABLED;
        SetProcessUserModeExceptionPolicy(flags);
    }
}

Call it at the beginning of your application:

    [STAThread]
    static void Main()
    {
        Kernel32.DisableUMCallbackFilter();

        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }

I've confirmed (with the the simple example shown below) that this works, just as you'd expect.

protected override void OnLoad(EventArgs e) {
    throw new Exception("BOOM");   // This will now get caught.
}

So, what I don't understand, is why it was previously impossible for the debugger to handle crossing kernel-mode stack frames, but with this hotfix, they somehow figured it out.