User Input Validation

ASP.NET validation controls derive their basic functionality from the BaseValidator abstract class available in the System.Web.UI.WebControls . Table 3.10 lists some of the important members of the BaseValidator class.

Table 3.10. Important Members of the BaseValidator Class

When you set the Enabled property to false for a validation control, the validation on the control is performed neither at the client side nor at the server side. If you want to disable validation only at the client side, you should set the EnableClientScript property of the validation control to false .

When the CausesValidation property is set to false , it prevents client-side and server-side validations from occurring. This property is ideal for the Cancel and Reset buttons in a Web page. The Button, LinkButton, ImageButton, HtmlButton, HtmlInputButton , and HtmlInputImage controls contain this property.

Using the Page.Validate() Method and Page.IsValid Property

As you can see in Table 3.10, each validation control maintains an IsValid property that indicates the status of the validation test. The Page control that hosts the Web controls also contains a property called IsValid that indicates the status of the validation for the whole page. When all the validation controls on the Web form set their IsValid properties to true , Page.IsValid also becomes true . If the validation fails on any of the validation controls, Page.IsValid is false .

The Page class maintains a collection of the validation controls on a Web page that can be accessed through its Validators property. The Page class also contains a Validate() method that invokes the Validate() method of all the validation controls in the page. The Page.Validate() method is used to perform validation programmatically on all the validation controls of a Web page.

ASP.NET validation is performed after page initializationthat is, after the Load event of the Page is raised but before the event-handling code is called. Therefore, you cannot know the status of the validation test in the Page_Load() event handler, and the data posted from the client should not be accessed at this stage of page processing. You should always try to use data posted from the client in event-handling code such as the Click event of the Button control. However, if you need to access client-posted data in the Page_Load() event handler, you can do so by calling the Page.Validate() method.

Validation Web Server Controls

ASP.NET provides the following validation controls that derive their functionality from the BaseValidator class: RequiredFieldValidator , RegularExpressionValidator , RangeValidator , CompareValidator , and CustomValidator .

These validation controls are usually associated with the input server controls on which the validation needs to be performed. For validation to work properly, the validation control and the input server control should be placed in the same container control. The validation controls are usually placed next to the associated input control so that you can display error messages or indicators next to the input control. You can associate any number of validation controls with an input server control. For example, the Date of Hire input field in an Add Employee form cannot be left empty (validated through the RequiredFieldValidator control) and should be less than or equal to the current date (validated through the CompareValidator control). You'll learn more about the validation Web server controls in the following sections.

The RequiredFieldValidator Control

The RequiredFieldValidator control can be used to check whether the input control contains an entry. It makes the associated input control a required field in the Web page and ensures that some input data is passed to it. The control also trims whitespace prior to checking for the required field entry.

The RequiredFieldValidator control contains a special property called InitialValue that can be passed the initial value of the associated input control. During validation, if the input control's validation property contains the same initial value or is empty, it sets IsValid to false , indicating that the validation failed. For example, a drop-down list might allow users to select a state and when the page loads, its initial value could be Select a State . If a RequiredFieldValidator control is associated with the drop-down list control, its InitialValue property can be set to the same initial value of the drop-down list Select a State . When the validation occurs, the validation control ensures that the item selected in the drop-down list is not the item set in the InitialValue property of the validation control.

The RequiredFieldValidator control is the only validation control that ensures that the associated input control is a required field. Other validation controls assume the input control's data to be valid if it is left blank.

The RegularExpressionValidator Control

The RegularExpressionValidator control checks whether the associated input control's validation property matches a specified pattern. This pattern is specified by the ValidationExpression property using a regular expression. If you are not familiar with regular expressions, you can find more information in the Microsoft .NET Framework documentation.

The RangeValidator Control

The RangeValidator control is used to check whether the input control contains a value in the specified range. You can check the range of values against different data types such as String , Date , and Integer .

Table 3.11 shows the important properties of the RangeValidator class.

Table 3.11. Important Properties of the RangeValidator Class

The CompareValidator Control

The CompareValidator control is used to compare the input server control's value against another value. The CompareValidator control can compare against a value specified to the validator control or against the value of another input control. The comparison can be made with different comparison operators such as equal, greater than, and so on. A special comparison operation can be used to verify that the associated input control's value is in the specified data type. You can make comparisons against various data types, such as String , Date , Integer , and so on.

Table 3.12 shows the important properties of the CompareValidator class.

Table 3.12. Important Properties of the CompareValidator Class

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If both the ControlToCompare and ValueToCompare properties are set for a CompareValidator control, the ControlToCompare property takes precedence.

If the Operator property of a CompareValidator is set to DataTypeCheck , the ControlToCompare and ValueToCompare properties are ignored. The validator control tries to convert the input control value to the data type specified by the Type property and sets the IsValid property with the result.