You need to make sure a user selects an entry in a drop-down list.
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:
CustomValidator control for each drop-down list where you must verify that an item has been selected.
Add Save and Cancel (or equivalently named) buttons.
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:
Add an event handler for the
ServerValidate event whose purpose is to provide the server-side validation to ensure an item has been selected.
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 3.1 for details).
Figure 3-10 shows a typical form with normal output prior to data entry. Figure 3-11 shows the form with validation errors. Examples 3-7, 3-8 through 3-9 show the .aspx and code-behind files for our application that implements ...