ASP.NET Cookbook

2.5. Requiring that a Drop-Down List Selection Be Made

Problem

You need to make sure a user selects an entry in a drop-down list.

Solution

Add a CustomValidator control to the drop-down list, along with some client-side JavaScript to validate the selection. Next, implement an event handler for the CustomValidator control’s ServerValidate event. Finally, check the Page.IsValid property in the event handler for the control that completes the user’s entry for the page.

In the .aspx file:

1. Add a CustomValidator control for each drop-down list where you must verify that an item has been selected.

2. Add JavaScript to validate the selection on the client side.

3. Add Save and Cancel (or equivalently named) buttons.

4. Set the Save button’s CausesValidation attribute to True to have validation performed when the button is clicked (set it to False for the Cancel button).

In the code-behind class for the page, use the .NET language of your choice to:

1. Add an event handler for the CustomValidator control’s ServerValidate event whose purpose is to provide the server-side validation to ensure an item has been selected.

2. Add code to the event handler for the Save button’s click event to check the Page.IsValid property and verify that all validation was successful (see Recipe 2.1 for details).

Figure 2-9 shows a typical form with normal output prior to data entry. Figure 2-10 shows the form with validation errors. Example 2-7 through Example 2-9 show the .aspx and code-behind files for our application that implements ...