Inside AutoEventWireup

Friday, February 17, 2006

The following information applies to ASP.NET 2.0

Basics

The default value for AutoEventWireup is true for a C# web form, and false for a VB.NET web form. The IDE adds the default values to the @ Page directive for a new web form. The difference in defaults is partly because VB.NET has a mechanism for defining an event handler and subscribing to an event in one graceful motion (the Handles keyword).

Protected Sub Page_Load(ByVal sender As Object, _
                        ByVal e As System.EventArgs) _
                        Handles Me.Load
    ' ...

End Sub

An easy way to generate the above code is to use the drop down controls that sit just above the editor. Note: C# doesn’t make the dropdown list of events available when editing a code-beside file, but the dropdown is available when writing in-line code.

There is no equivalent to the Handles keyword in C# (anonymous event handlers are arguably close, but just not the same). When AutoEventWireup is true, all we need to do is follow the method naming convention of Page_EventToHandle. The ASP.NET runtime will automatically find and fire the method for the appropriate event.

protected void Page_Load(object sender, EventArgs e)
{

}

Once Again, In Reverse

If we switch to AutoEventWireup=”true” for a VB.NET web form, we can use the magic Page_EventName approach. The only change to the earlier VB.NET code would be to drop the Handles clause, and the events fire correctly.

If we switch to AutoEventWireup=”false” for a C# web form, there is a little extra work to do. Somewhere we need to explicitly wire up events. Here is one approach.

public partial class _Default : Page
{
   public _Default() // ctor
   {
      Load += new EventHandler(Page_Load);
      PreInit += new EventHandler(Page_PreInit);         
   }

    protected void Page_Load(object sender, EventArgs e)
    {
       // ...
    }

   protected void Page_PreInit(object sender, EventArgs e)
   {
      // ...
   }
}

Here are the methods the runtime will look for when AutoEventWireup is true.

Page_PreInit
Page_Init
Page_InitComplete
Page_PreLoad
Page_Load
Page_LoadComplete
Page_DataBind
Page_SaveStateComplete
Page_PreRender
Page_PreRenderComplete
Page_Unload
Page_Error
Page_AbortTransaction
Page_CommitTransaction

Trivia

When AutoEventWireup is true, the runtime has to look for each of the page event handlers using code like the following.

bool ignoreCase = true;
bool throwOnFailure = false;
Delegate d = null;

d = Delegate.CreateDelegate(
                      typeof(EventHandler), this,
                      "Page_Load", ignoreCase,
                      throwOnFailure
                    );

Notice the member search is case-insensitive, which isn’t surprising since VB.NET is case-insensitive. What is surprising (to me) is that the following works, too.

protected void Page_Load()
{
   // ...
}

If the runtime can’t make an EventHandler delegate, it will then try to make a delegate to a method with an empty parameter list. Yes, that's up to 28 calls to CreateDelegate per web form with AutoEventWireup="true".

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