How Do I Dynamically Switch the DefaultButton at Run-Time?

Update!! 01-15-2006
I just tried this concept in a larger application using validator controls, and then things seems to break. Somehow, the validator controls and switching the DefaultButton dynamically don't play nice together. I haven't yet found a work-around, but if you know a way to fix this, please let me know.

Introduction

It's not uncommon that your page contains multiple text boxes and buttons that should each fire different events at the server. For example, consider the two boxes at the top of my web site. The first Go button should fire a search action with the search term entered in the first text box. The second button should take you directly to an article specified by a Quick Doc ID in the second text box.

Most users will fill in one of the two text boxes, simply hit enter and then expect the proper action to be carried out. But in my current site (built with .NET 1.1) this is not the case. (Try it and you'll see what I mean). If you enter a search term and hit enter, you go to the search page. However, if you enter a Quick Doc ID and hit enter, nothing happens.

The new DefaultButton from ASP.NET 2.0 is a welcome addition in that it allows you to specify the button that must be "clicked" when you hit enter. However, in the scenario described above, this is not enough. If I am typing a search term, I want the first Go button to be the DefaultButton and if I am entering a Quick Doc ID I want the second button to be the DefaultButton.

Attributes to the Rescue

Fortunately, switching what button is considered the default button at run-time isn't too difficult. If you look at the source of a page that has the DefaultButton set up, you'll see something similar to the following in the <form> tag:

<form name="form1" method="post" action="SomePage.aspx" 
     onkeypress="javascript:return WebForm_FireDefaultButton(event, 
     'Button1')" id="form1"
>

As you can see, all this code does is call a method called WebForm_FireDefaultButton and pass it up the event that triggered the initial action and the unique ID of a control. You can use this knowledge to change the DefaultButton from JavaScript at run-time. The simplest way to use this, is to call this method in the onclick event of a text box. You can add these onclick handlers through code using the controls Attributes collection. For example, in my new ASP.NET 2.0 version of this web site, I added the following code in the Page_Load event of the user control that has the search and the Quick Doc ID buttons:

txtQuickSearch.Attributes.Add("onclick",
   "document.forms[0].onkeypress = "
   + "new Function(\"return WebForm_FireDefaultButton(event, '" 
   + btnQuickSearch.UniqueID + "');\");");

txtQuickDocId.Attributes.Add("onclick",
   "document.forms[0].onkeypress = "
   + "new Function(\"return WebForm_FireDefaultButton(event, '" 
   + btnQuickDocId.UniqueID + "');\");");  

Whenever you click the search text box, onclick is fired and WebForm_FireDefaultButton is called. This method is then passed the unique ID of the appropriate button. The same applies to the Quick Doc ID button: when you click it, WebForm_FireDefaultButton is called and the DefaultButton is switched again.

The only thing you need to watch out for is that you need to set the DefaultButton at least once in your page using "normal" ways (as an attribute on the <form> tag for example, or through code as shown in my article about DefaultButtons in master pages). If you don't do this, ASP.NET won't inject the <script> tag that retrieves the required JavaScript code (including WebForm_FireDefaultButton) from the server.