event handler for WebBrowser control onpropertychange events - sender and e objects are null

Question

In C#, I am running the WebBrowser (WB) control in a server-side thread and want to monitor (listen) for "onpropertychange" events. I can successfully attach a .NET delegate signature method that is executed when a property changes, but the sender and e objects are both null in every call to qEventHndlr, therefore, I don't know which property changed to fire the event. el is an HTMLElement that is iterated in a foreach loop to attach the eventhandler to each element to monitor (listen).

el.AttachEventHandler("onpropertychange", qEventHndlr);     // in the foreach loop
public void qEventHndlr(Object sender, EventArgs e) {…}     // the event handler

The documentation indicates that the EventArgs class; "This class contains no event data; it is used by events that do not pass state information to an event handler when an event is raised. If the event handler requires state information, the application must derive a class from this class to hold the data." It also seems (I'm not certain and haven't tested) that sender is only populated for a subset of WB events and not onpropertychange.

So, I then thought that if I can't get identifier information in the send or e objects, I could use an anonymous method in the AttachEventHandler and then pass a unique runtime programmable string parameter to a method embedded in the event handler method call.

el.AttachEventHandler("onpropertychange", delegate(Object sender, EventArgs e) { anonoMeth(elID); });
public void anonoMeth(string specificProperty) {..}

The compiler accepted the syntax, however, even though the elID string changes in the foreach loop, only the first iteration value is used so that when anonoMeth(string specificProperty) is called with each onpropertychange event, specificProperty has the same value because the same elID was attached to the respective element.

I haven't tried Extension Methods yet and wanted to get this posted to see if anyone has encountered similar challenges (and hopefully has a solution). I prefer not to resort to C++ unless I absolutely have to.

Please don't prefix your titles with "C#" and such. That's what the tags are for. — John Saunders, Feb 07 '12 at 22:41

score 4 · Accepted Answer · answered Feb 07 '12 at 23:00

You need to cast your sender to an object of the appropriate type (an HTMLElement in your case) in your event handler.

Try this example on for size:

namespace WebBrowserEventTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            webBrowserTest.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(webBrowserTest_DocumentCompleted);
            webBrowserTest.Navigate("http://mondotees.com");
        }

        private void webBrowserTest_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            foreach(HtmlElement el in webBrowserTest.Document.GetElementsByTagName("div"))
            {
                el.AttachEventHandler("onpropertychange", delegate { testEventHandler(el, EventArgs.Empty); });
            }

            foreach (HtmlElement el in webBrowserTest.Document.GetElementsByTagName("div"))
            {
                el.Name = "test";
            }
        }

        public void testEventHandler(object sender, EventArgs e)
        {
            var he = (HtmlElement)sender;
        }
    }
}

Thanks LiquidPony for the guidance. After looking at your code, I realized that I didn't need to implement this part of the project as a webform and changed to a windows form (as you had done). I modified your code a bit to get at the attribute of interest within the element and was able to pass the correct string as an object to the "testEventHandler". I was getting a null sender object because I wasn't setting the correct value in the method parameters. Your code opened my mind to a couple of different solutions that I had not previously considered. Thx again. — robWDC, Feb 08 '12 at 23:13
Glad to be of service! If you feel it's appropriate, vote the answer up or mark it as accepted so that others can identify the solution as a useful resource. — LiquidPony, Feb 08 '12 at 23:37

event handler for WebBrowser control onpropertychange events - sender and e objects are null

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