Catching unhandled exceptions in ASP.NET UserControls

Question

I'm dynamically loading user controls adding them to the Controls collection of the web form.

I'd like to hide user controls if they cause a unhandled exception while rendering.

So, I tried hooking to the Error event of each UserControl but it seems that this event never fires for the UserControls as it does for Page class.

Did some googling around and it doesn't seem promising. Any ideas here?

Answer 1

mmilic, following on from your response to my previous idea..

No additional logic required! That's the point, your doing nothing to the classes in question, just wrapping them in some instantiation bubble-wrap! :)

OK, I was going to just bullet point but I wanted to see this work for myself, so I cobbled together some very rough code but the concept is there and it seems to work.

APOLOGIES FOR THE LONG POST

The SafeLoader

This will basically be the "bubble" I mentioned.. It will get the controls HTML, catching any errors that occur during Rendering.

public class SafeLoader
{
    public static string LoadControl(Control ctl)
    {
        // In terms of what we could do here, its down
        // to you, I will just return some basic HTML saying
        // I screwed up.
        try
        {
            // Get the Controls HTML (which may throw)
            // And store it in our own writer away from the
            // actual Live page.
            StringWriter writer = new StringWriter();
            HtmlTextWriter htmlWriter = new HtmlTextWriter(writer);
            ctl.RenderControl(htmlWriter);

            return writer.GetStringBuilder().ToString();
        }
        catch (Exception)
        {
            string ctlType = ctl.GetType().Name;
            return "<span style=\"color: red; font-weight:bold; font-size: smaller;\">" + 
                "Rob + Controls = FAIL (" + 
                ctlType + " rendering failed) Sad face :(</span>";
        }
    }
}

And Some Controls..

Ok I just mocked together two controls here, one will throw the other will render junk. Point here, I don't give a crap. These will be replaced with your custom controls..

BadControl

public class BadControl : WebControl
{
    protected override void Render(HtmlTextWriter writer)
    {
        throw new ApplicationException("Rob can't program controls");
    }
}

GoodControl

public class GoodControl : WebControl
{
    protected override void Render(HtmlTextWriter writer)
    {
        writer.Write("<b>Holy crap this control works</b>");
    }
}

The Page

OK, so lets look at the "test" page.. Here I simply instantiate the controls, grab their html and output it, I will follow with thoughts on designer support etc..

Page Code-Behind

    protected void Page_Load(object sender, EventArgs e)
    {
        // Create some controls (BadControl will throw)
        string goodHtml = SafeLoader.LoadControl(new BadControl());
        Response.Write(goodHtml);

        string badHtml = SafeLoader.LoadControl(new GoodControl());
        Response.Write(badHtml);
    }

Thoughts

OK, I know what you are thinking, "these controls are instantiated programatically, what about designer support? I spent freaking hours getting these controls nice for the designer, now you're messing with my mojo".

OK, so I havent really tested this yet (probably will do in a min!) but the idea here is to override the CreateChildControls method for the page, and take the instance of each control added on the form and run it through the SafeLoader. If the code passes, you can add it to the Controls collection as normal, if not, then you can create erroneous literals or something, up to you my friend.

Finally..

Again, sorry for the long post, but I wanted to get the code here so we can discuss this :) I hope this helps demonstrate my idea :)

Update

Tested by chucking a control in on the designer and overriding the CreateChildControls method with this, works fine, may need some clean up to make things better looking, but I'll leave that to you ;)

protected override void CreateChildControls()
{
    // Pass each control through the Loader to check
    // its not lame
    foreach (Control ctl in Controls)
    {
        string s = SafeLoader.LoadControl(ctl);
        // If its bad, smack it downnnn!
        if (s == string.Empty)
        {
            ctl.Visible = false; // Prevent Rendering
            string ctlType = ctl.GetType().Name;
            Response.Write("<b>Problem Occurred Rendering " + 
                ctlType + " '" + ctl.ID + "'.</b>");
        }
    }
}

Enjoy!