Catching Unhandled Exceptions in ASP.NET UserControls

When working with ASP.NET UserControls, one common challenge developers face is managing unhandled exceptions during rendering. This can lead to undesirable outcomes such as broken user interfaces or disrupted user experience.

In this post, we will delve into how you can effectively catch these exceptions using a safe-loading technique, allowing you to gracefully handle scenarios where controls may fail to render.

The Problem: Unhandled Exceptions in UserControls

When dynamically loading user controls in ASP.NET, you might encounter situations where one or more controls cause unhandled exceptions during their rendering process. Since the Error event does not fire for UserControls like it does for the Page class, it’s essential to find an alternative way to catch and manage these failures.

The goal here is to prevent such exceptions from crashing the overall page and instead hide the problematic controls while providing a fallback mechanism.

The Solution: Implementing a Safe Loader

The solution involves creating a wrapper class called SafeLoader that safely loads your controls. This class essentially creates a “bubble” that captures the rendering process, allowing you to catch errors and respond accordingly. Here’s how you can set this up:

Step 1: Create the SafeLoader Class

The SafeLoader class will have just one method, LoadControl, which attempts to render a control and captures any exceptions that may occur.

public class SafeLoader
{
    public static string LoadControl(Control ctl)
    {
        try
        {
            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>";
        }
    }
}

Step 2: Implement Bad Control and Good Control

To illustrate this method effectively, you can create two simple UserControls: one that throws an exception (BadControl) and one that renders correctly (GoodControl).

BadControl Class

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

GoodControl Class

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

Step 3: Incorporate the SafeLoader in the Page

In your ASP.NET page, you will override the Page_Load event to create instances of your controls and utilize the SafeLoader to load their HTML.

protected void Page_Load(object sender, EventArgs e)
{
    string goodHtml = SafeLoader.LoadControl(new BadControl());
    Response.Write(goodHtml);

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

Step 4: Handle Designer Support

To maintain designer support while using dynamic loading, you can override the CreateChildControls method. This ensures that every control added to the page checks via the SafeLoader.

protected override void CreateChildControls()
{
    foreach (Control ctl in Controls)
    {
        string s = SafeLoader.LoadControl(ctl);
        if (s == string.Empty)
        {
            ctl.Visible = false; // Prevent Rendering
            string ctlType = ctl.GetType().Name;
            Response.Write("<b>Problem Occurred Rendering " + 
                ctlType + " '" + ctl.ID + "'.</b>");
        }
    }
}

Conclusion

Handling unhandled exceptions in ASP.NET UserControls is crucial for building robust web applications. By implementing the SafeLoader class, you can effectively manage rendering errors without compromising user experience. This method ensures that deleting buggy controls is as simple as wrapping them in a secure instantiation process, thus preventing the entire page from failing.

Feel free to test this method in your applications, and enhance it further to suit your specific needs. With robust error handling like this, you can significantly improve the reliability of your ASP.NET projects.