Using the CustomValidator Control
Using the CustomValidator ControlIf none of the other validation controls perform the type of validation that you need, you can always use the CustomValidator control. You can associate a custom validation function with the CustomValidator control. The CustomValidator control has three important properties:
The CustomValidator also supports one event:
You associate your custom validation function with the CustomValidator control by handling the ServerValidate event. For example, imagine that you want to validate the length of a string entered into a form field. You want to ensure that a user does not enter more than 10 characters into a multi-line TextBox control. The page in Listing 3.14 contains an event handler for a CustomValidator control's ServerValidate event, which checks the string's length. Listing 3.14. ShowCustomValidator.aspx
The second parameter passed to the ServerValidate event handler is an instance of the ServerValidateEventArgs class. This class has two properties:
In Listing 3.14, if the string represented by the Value property is longer than 10 characters, then the value False is assigned to the IsValid property and validation fails. Otherwise, the value true is assigned to the IsValid property and the input field passes the validation check (see Figure 3.12). Figure 3.12. Validating field length with the CustomValidator control. The ServerValidate event handler in Listing 3.14 is a server-side function. Therefore, validation does not occur until the page is posted back to the web server. If you want to perform validation on both the client (browser) and server, then you need to supply a client-side validation function. Warning If you don't associate a client validation function with a CustomValidator control, then the CustomValidator doesn't render an error message until you post the page back to the server. Because the other validation controls prevent a page from being posted if the page contains any validation errors, you won't see the error message rendered by the CustomValidator control until you pass every other validation check in a page. The page in Listing 3.15 illustrates how you can associate a client-side validation function with the CustomValidator control. This page also checks the length of the string entered into a TextBox control. However, it checks the length on both the browser and server. Listing 3.15. ShowCustomValidatorJS.aspx
Notice that the CustomValidator control in Listing 3.15 includes a ClientValidationFunction property. This property contains the name of a JavaScript function defined in the page's <head> tag. The JavaScript validation function accepts the same two parameters as the server-side validation function. The first parameter represents the CustomValidator control, and the second parameter represents an object that includes both a Value and an IsValid property. The client-side function is nearly identical to the server-side function (with the important difference that it is written in JavaScript). Unlike the RangeValidator, CompareValidator, and RegularExpressionValidator controls, you can validate a form field with the CustomValidator control even when the form field is left blank. The CustomValidator control includes a property named the ValidateEmptyText property. You can use this property to cause the CustomValidator control to validate a form field even when the user hasn't entered a value into the form field. For example, the page in Listing 3.16 contains a TextBox that requires a product code that contains exactly four characters. Listing 3.16. ShowValidateEmptyText.aspx
Notice that the CustomValidator control in Listing 3.16 includes a ValidateEmptyText property which has the value true. If the ValidateEmptyText property was not included, and you submitted the form without entering any data, then no validation error would be displayed. Finally, unlike the other validation controls, you are not required to associate the CustomValidator control with any form field. In other words, you don't need to include a ControlToValidate property. For example, the page in Listing 3.17 contains a timed test. If you don't answer the question within five seconds, then the CustomValidator control displays a validation error message (see Figure 3.13). Figure 3.13. Performing validation against no particular field. Listing 3.17. TimedTest.aspx
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.Web.UI.WebControls.ServerValidateEventArgs) If args.Value.Length > 10 Then args.IsValid = False Else args.IsValid = True End If End Sub </script> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server"> <title>Show CustomValidator</title> </head> <body> <form runat="server"> <div> <asp:Label Text="Comments:" AssociatedControl Runat="server" /> <br /> <asp:TextBox TextMode="MultiLine" Columns="30" Rows="5" Runat="server" /> <asp:CustomValidator ControlToValidate="txtComments" Text="(Comments must be less than 10 characters)" OnServerValidate="valComments_ServerValidate" Runat="server" /> <br /><br /> <asp:Button Text="Submit" Runat="server" /> </div> </form> </body> </html> 