Understanding ASP.NET User Control’s DefaultEvent

When developing user controls in ASP.NET, developers often face a specific challenge: how to set the DefaultEvent for a user control. This is particularly relevant when you want to streamline your workflow, enabling double-click actions to automatically generate the event handlers you need. In this blog post, we’ll dive into this issue and explore potential solutions and insights.

The Problem Statement

Imagine you’ve created a user control in ASP.NET and defined an event that you would like to be recognized as the default event when double-clicking the control in the design view. In this scenario, you have a user control named AddressBox with an event called OKClicked. You expect that when you double-click the control in an ASPX page, Visual Studio would generate a handler for OKClicked, but instead, it generates a handler for the Load event. Why is this happening?

Current Implementation

Here’s a glance at the implementation of your user control’s code:

[System.ComponentModel.DefaultEvent("OKClicked")]
public partial class AddressBox : System.Web.UI.UserControl
{
    public event EventHandler OKClicked;
}

Current Results

After double-clicking the control, the generated code is:

protected void AddressBox1_Load(object sender, EventArgs e)
{
}

As you can see, the generated event handler for Load is not what you expected, and it raises the question: Is it possible to define a DefaultEvent for a UserControl?

Investigating the DefaultEvent Behavior

Inherited DefaultEvent

In understanding why Visual Studio defaults to generating the Load event, it’s crucial to note that the DefaultEventAttribute can be inherited. The UserControl class in ASP.NET has its default event set to Load. As such, your custom user control, AddressBox, inherits this behavior.

• Reflection Insight: Using tools like Reflector, you can see that the UserControl class’s default event is indeed the Load event. Therefore, even though you have specified OKClicked as the default, it seems Visual Studio is prioritizing the inherited event.

Potential Workaround

While there may not be a straightforward way to change this behavior due to class inheritance, you can do the following:

• Check Other Properties: Sometimes additional attributes or settings might influence behavior. Review any inherited attributes that could affect the DefaultEvent processing.
• Consider Custom Control: If setting a DefaultEvent is critical for your use case, consider creating a custom control instead, where you can fully control the events and their behavior.

Conclusion

In summary, the default behavior when double-clicking an ASP.NET user control is primarily dictated by the inheritance from the base UserControl class. While you can specify a DefaultEvent for your user control, Visual Studio may still default to the inherited events such as Load. Understanding this behavior can help you tailor your approach accordingly.

Bonus: Underscore Handling in Code

You also inquired about placing underscores in the code. In C#, you typically do not need to escape underscores. You can use them directly in variable and method names. For example:

public void Do_Something()
{
}

Feel free to reach out if you have additional questions about user controls in ASP.NET!