Assembly: System.Web (in system.web.dll)
Use the CustomValidator control to provide a user-defined validation function for an input control. The CustomValidator control is a separate control from the input control it validates, which allows you to control where the validation message is displayed.
Validation controls always perform validation on the server. They also have complete client-side implementation that allows script-enabled browsers (such as Microsoft Internet Explorer 4.0 and later) to perform validation on the client. Client-side validation enhances the validation process by checking user input before it is sent to the server. This allows errors to be detected on the client before the form is submitted, thus avoiding the round trip of information necessary for server-side validation.
To create a server-side validation function, provide a handler for the ServerValidate event that performs the validation. The string from the input control to validate can be accessed by using the Value property of the ServerValidateEventArgs object passed into the event handler as a parameter. The result of the validation is then stored in the IsValid property of the ServerValidateEventArgs object.
To create a client-side validation function, first add the server-side validation function described earlier. Next, add the client-side validation script function to the ASP.NET (.aspx) page.
If you are using Visual Basic Scripting Edition (VBScript), the function must be in this form:
Sub ValidationFunctionName(source, arguments)
If you are using JScript, the function must be in this form:
function ValidationFunctionName(source, arguments)
Use the ClientValidationFunction property to specify the name of the client-side validation script function associated with the CustomValidator control. Because the script function is executed on the client, the function must be in a language that the target browser supports, such as VBScript or JScript.
Like server-side validation, the string from the input control to validate is accessed by using the Value property of the arguments parameter. Return the result of the validation by setting the IsValid property of the arguments parameter.
When using validator controls, you should always check the results of server-side validation before performing any processing. After a postback but before your event methods are called, the page calls the validator controls and aggregates their results into the Page.IsValid property. (You can also call the validator controls explicitly using the Validate method.) In your own code, you should check that the Page.IsValid property returns true before processing input. Even though script-enabled browsers might prevent a postback from occurring on the client if a validation check has failed, you should always also check Page.IsValid in server code before processing validated data.
Multiple validation controls can be used with an individual input control to validate different criteria. For example, you can apply multiple validation controls on a TextBox control that allows the user to enter the quantity of items to add to a shopping cart. You can use a CustomValidator control to ensure that the value specified is less than the amount in inventory and a RequiredFieldValidator control to ensure that the user enters a value into the TextBox control.
If the input control is empty, no validation functions are called and validation succeeds. Use a RequiredFieldValidator control to require the user to enter data in the input control.
It is possible to use a CustomValidator control without setting the ControlToValidate property. This is commonly done when you are validating multiple input controls or validating input controls that cannot be used with validation controls, such as the CheckBox control. In this case, the Value property of the arguments parameter passed to the event handler for the ServerValidate event and to the client-side validation function always contains an empty string (""). However, these validation functions are still called, where appropriate, to determine validity on both the server and client. To access the value to validate, you must programmatically reference the input control you want to validate and then retrieve the value from the appropriate property. For example, to validate a CheckBox control on the server, do not set the ControlToValidate property of the validation control and use the following code for the handler for the ServerValidate event.
For more information on validation controls, see BaseValidator.
The markup rendered by default for this control might not conform to accessibility standards such as the Web Content Accessibility Guidelines 1.0 (WCAG) priority 1 guidelines. For details about accessibility support for this control, see ASP.NET Controls and Accessibility.
The following code example demonstrates how to create a server-side CustomValidator control.
The following code example demonstrates how to create a client-side CustomValidator control.
Windows 98, Windows 2000 SP4, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.