Catching server errors with WatiN: redux

Stumbled upon this post about how to catch server errors for your WatiN tests.  The approach outlined provides a decent mechanism for detecting server errors by sub-classing the WatiN IE object.  While I do appreciate the ability to subclass, it bothers me a bit that I have to write the logic in my subclass to detect server errors.  After poking around a bit, I think there's a more generic approach that can be achieved by tapping into the NavigateError event of the native browser:

public class MyIE : IE
{
    private InternetExplorerClass ieInstance;
    private NavigateError error;

    public MyIE()
    {
        ieInstance = (InternetExplorerClass) InternetExplorer;
        ieInstance.BeforeNavigate += BeforeNavigate;
        ieInstance.NavigateError += NavigateError;
    }

    public override void WaitForComplete()
    {
        base.WaitForComplete();
        if (error != null)
        {
            throw new ServerErrorException(Text);
        }
    }

    void BeforeNavigate(string URL, int Flags, string TargetFrameName, ref object PostData, string Headers, ref bool Cancel)
    {
        error = null;
    }

    void NavigateError(object pDisp, ref object URL, ref object Frame, ref object StatusCode, ref bool Cancel)
    {
        error = new NavigateError(URL,StatusCode);
    }

    private class NavigateError
    {
        public NavigateError(object url, object statusCode)
        {
            _url = url;
            _statusCode = statusCode;
        }

        private object _url;
        private object _statusCode;
    }
}
public class ServerErrorException : Exception 
{
    public ServerErrorException(string message) : base(String.Format("A server error occurred: {0}",message))
    { } 
}

Few caveats:

• Constructor of MyIE needs to be updated to reflect the other constructor overloads.
• Need to ensure that URL of NavigateError is the same URL of BeforeNavigate
• Test library needs to reference the Interop.SHDocVw wrapper for Internet Explorer
• Only tested with IE7

While I wouldn't consider COM Interop to be a "clean" solution, it is more bit more portable between solutions.  And if it was this easy, why isn't it part of WatiN anyway?