How Do I Set the DefaultFocus or DefaultButton in a Page Based on a Master Page in ASP.NET 2.0?

Your first attempt may be to add the following code in the Page_Load event of the Content page:

  Page.Form.DefaultFocus = "TextBox1"
  Page.Form.DefaultButton = "Button1"

However, as soon as you run this code, you'll get the following error:

Server Error in '/MyApplication' Application.

The DefaultButton of 'form1' must be the ID of a control of type IButtonControl.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: The DefaultButton of 'form1' must be the ID of a control of type IButtonControl.

If you comment out the second line that sets the DefaultButton, the page loads in the browser, but the focus is not set. If you look at the resulting HTML in the browser, you may start to understand the problem. At the end of the page, you'll see this:

<script type="text/javascript">
<!--
  WebForm_AutoFocus('TextBox1');// -->
</script>

TextBox1 looks good at first, but if you search for it in the code, you won't find a control with an Id of TextBox1. You do, however, find this:

<input name="ctl00$ContentPlaceHolder1$TextBox1" type="text" 
id="ctl00_ContentPlaceHolder1_TextBox1" />

Because the page is based on a Master Page, the HTML name and id attributes have been prefixed with the names of their naming container, the ContentPlaceHolder1 in this case.

Now how do you get the correct client id inside the call to WebForm_AutoFocus? The trick here is to use the controls ClientID property that returns its long client name. If you change the code in the code behind to the following, the DefaultFocus will work:

Page.Form.DefaultFocus = TextBox1.ClientID

You can apply the same technique to set the DefaultButton but then you should use its UniqueID property:

Page.Form.DefaultButton = Button1.UniqueID

This way, you end up with a form that has the focus on the TextBox1 by default and when you press Enter, the form will post back to the server and the code in the event handler for Button1.Click will fire.

Although in normal pages (not based on a Master Page) you can set the DefaultButton and DefaultFocus properties as a string as you saw in the first code example, it's recommend not to do this. When you always use ControlName.UniqueID or ControlName.ClientID you get compile time checking of the control's name. If you rename the control in the markup of the page (which is a good idea when you call buttons Button1 ;-) ) the compiler will see the change and highlight the offending code; a benefit you don't have when you use text strings.