WPF Program throwing unexplainable Invocation Exception

Question

The problem is: When I remove the first message box line, my program doesn't run and throws "Exception has been thrown by the target of an invocation" on the if statement line. However, when I leave the messagebox there, it runs fine. Can someone explain to me why this is happening and what I can do to fix it? I'm fairly new to WPF by the way, any help would be appreciated.

public BrowserMode() {

       InitializeComponent();

       MessageBox.Show("Entering Browser Mode");
       if (webBrowser1.Source.Scheme == "http")
       {
           //cancel navigation
           //this.NavigationService.Navigating += new NavigatingCancelEventHandler(Cancel_Navigation);

           qd = new QuestionData();

           // code where stuff happens
           var url = webBrowser1.Source;
           HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);

           // from h.RequestUri = "webcam://submit?question_id=45"
           var parseUrl = request.RequestUri; //the uri that responded to the request.
           MessageBox.Show("The requested URI is: " + parseUrl);

Answer 1

This sort of work is not suited for a constructor and should be moved out until after the WebBrowser is fully loaded. You have two options:

1. Hook Control.Loaded and perform this behavior there.

   public BrowserMode()
   {
       InitializeComponent();
   
       this.Loaded += BroswerMode_Loaded;
   }
   
   void BrowserMode_Loaded(object sender, EventArgs e)
   {
       if (webBrowser1.Source != null
        && webBrowser1.Source.Scheme == "http")
       {
           qd = new QuestionData();
           // ...
       }
   }
   

2. Hook WebBrowser.Navigating and perform this behavior there.

   public BrowserMode()
   {
       InitializeComponent();
   
       this.webBrowser1.Navigating += WebBrowser_Navigating;
   }    
   
   void WebBrowser_Navigating(object sender, NavigatingCancelEventArgs e)
   {
       if (e.Uri.Scheme == "http")
       {
           qd = new QuestionData();
           // ...
       }
   }