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User Input Validation

ASP.NET provides a set of Web server controls called validation controls that provide sophisticated validation on both the client side and the server side depending on the validation settings and the browser's capabilities.


ASP.NET ensures that validations are performed on the server side even if they were already performed on the client side. This ensures that validations are not bypassed if a malicious user circumvents client-side validation. If the client-side validation fails, the server-side validation is never performed.

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






Specifies the ID of the input server control that needs to be validated. This property should be passed a valid ID. However, it can be empty for a custom validation control.



Specifies how to display the inline error message contained in the Text property. It can be any of the ValidatorDisplay enumeration values, including Dynamic (the space is dynamically added), None (the message is never displayed), and Static (the space is occupied when the validation control is rendered).



Indicates whether the client-side validation is enabled. The default is true.



Indicates whether the validation control is enabled. If false, the validation is never performed.



Represents the error message to be displayed by the ValidationSummary control when the validation fails. If the Text property is not set, this message is displayed inline.



Specifies the foreground color in which the message is displayed when the validation fails. The default value is Color.Red.



Indicates whether the input control passes the validation.



Specifies the text of the error message displayed by the validation control inline.



Performs the validation on the associated input control and then updates the IsValid property with the result of the validation.


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 initialization—that 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




Specifies the upper value of the validation range.


Specifies the lower value of the validation range.


Specifies the data type to be used when comparing the data.

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




-Specifies the input server control against whose value the associated input control is to be validated.


Specifies the comparison operation to be performed.


Specifies the data type to be used when comparing the data.


-Specifies the value against which the associated input control is to be validated.


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.

This example shows how to use the RangeValidator and CompareValidator controls on a Web form:

  1. Add a new Web form (Example3_4.aspx) to the project.

  2. Insert a table by selecting Table, Insert, Table. This opens the Insert Table dialog box. Create a table of two rows and two columns with a border size of 0 in the form. Add two Label controls and two TextBox controls (txtDate and txtTickets) to the table. Drag three Label controls and a Button control (btnSubmit) onto the Web form and arrange the controls.

  3. Place one RangeValidator control (rvDate) on the table next to the txtDate control. Set its ControlToValidate property to txtDate, its Type property to Date, its MinimumValue property to 09/01/2003, its MaximumValue property to 09/30/2003, and its ErrorMessage property to an appropriate error message.

  4. Place one CompareValidator control (cvTickets) on the table next to the txtTickets control. Set its ControlToValidate property to txtTickets, its Type property to Integer, its Operator property to GreaterThanEqual, its ValueToCompare property to 2, and its ErrorMessage property to an appropriate error message.

  5. Set the ASPX page as the start page; then build and run the project. Enter an invalid date and an invalid number of tickets and click the Submit button. You will see the error messages from the RangeValidator and CompareValidator controls (see Figure 3.3). Enter valid values and click the Submit button. The form posts back successfully.

The CustomValidator Control

The CustomValidator control allows you to build a validation control for a custom specification. You can perform any custom validation both at the server side and at the client side with the help of this validation control.

This control exposes a property called ClientValidationFunction that specifies the name of the client script function to be executed for validation on the client side. This custom validation function is passed two arguments: The first one is the custom validator control, and the second argument is an object that contains two properties—IsValid and Value. The Value property contains the value that is to be validated, and the IsValid property is used to set the result of the validation.

Figure 3.3Figure 3.3 Use the RangeValidator control to ensure that a control's value is in the specified range, and use the CompareValidator control to compare the control's value with a specified value.

At the server side, during the validation on the server, the validation control fires a ServerValidate event. An event handler containing the custom validation code is added to this event to perform validation on the server. The event sends a ServerValidateEventArgs object containing event-related data. This object contains two properties: The Value property contains the value of the control that is to be validated, and the IsValid property is used to set the result of the validation.

The ValidationSummary Control

The ValidationSummary control is used to display a summary of all the validation errors of a Web page. It displays the ErrorMessage property of the validation controls in the summary. If the ErrorMessage property is not set, the Text property is displayed as error messages for all the validation controls whose validations fail.

Table 3.13 shows the important properties of the ValidationSummary class.

Table 3.13 Important Properties of the ValidationSummary Class




Specifies the way in which the validation summary is displayed. Values are defined by the ValidationSummaryDisplayMode enumeration and include BulletList (default), List, and SingleParagraph.


Indicates whether the validation summary control should generate client-side script to update itself. The default is true.


Specifies the foreground color in which the error messages are displayed when the validation fails. The default value is Color.Red.


Specifies the header text of the validation summary control.


Indicates whether the validation summary messages should be displayed in a message box. The default is false.


Indicates whether the validation summary messages should be displayed inline in the validation summary control. The default is true.

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