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User Input Validation

ASP.NET provides a set of Web server controls called validation controls. These controls provide sophisticated validation on both the client side and the server side depending on the validation settings and the browser capability.


The client-side validation provided by ASP.NET validation controls works only with Internet Explorer 4.0 or higher.


ASP.NET ensures that validations are performed on the server side even if they were already performed at the client side. This ensures that validations are not bypassed if a malicious user tries to circumvent the client-side validation. Of course, if the client-side validation fails, the server-side validation is never performed.

The validation controls are usually associated with input server controls on which the validation needs to be performed. For validation to work properly, the validation control and the input server control must be placed in the same container control.

All the validation controls derive their basic functionality from the BaseValidator abstract class available in the System.Web.UI.WebControls namespace. Table 3.13 lists some of the important members of the BaseValidator class that are inherited by the validation controls.

Table 3.13 Important Members of the BaseValidator Class






Specifies the id of the input server control that needs to be validated.



Specifies how to display the inline error message contained in the Text property. It can be any of the ValidatorDisplay values—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 when the validation fails by the ValidationSummary control. 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 that is displayed by the validation control inline.



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

Each validation control maintains an IsValid property that indicates the status of the validation test. The Page control that hosts all the Web controls also contains a property called IsValid that indicates the status of the validation for the whole page. When each validation control on the Web Form sets its IsValid property to True, the Page.IsValid property also becomes True. If any control has its IsValid property set to False, Page.IsValid will also return False.

ASP.NET provides five validation controls that derive their functionality from the BaseValidator class:

  • RequiredFieldValidator—Ensures that the data is not empty in the input control.

  • RegularExpressionValidator—Ensures that the data in the input control matches a specific pattern.

  • RangeValidator—Ensures that the data is within a specific range in the input control.

  • CompareValidator—Compares the data against a given value.

  • CustomValidator—Uses custom logic to validate data.

You can associate any number of validation controls with a single 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 or equal to the current date (validated through the CompareValidator control) .

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 in order to check 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 the IsValid property to False, indicating that the validation failed. For example, a drop-down list may allow users to select a state. When the page loads, the initial value in this control might 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 as the drop-down list, Select a State. When the validation occurs, the validation control will ensure that the item selected in the drop-down list is not the item set in the InitialValue property of the validation control.

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're not familiar with regular expressions, you can find more information in the .NET Framework documentation.

The RangeValidator Control

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

Table 3.14 shows the important properties of the RangeValidator class.

Table 3.14 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 made to verify that the associated input control's value is of a specified data type. You can make comparisons against different data types such as String, Date, and Integer.

Table 3.15 shows the important properties of the CompareValidator class.

Table 3.15 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.

The CustomValidator Control

The validation controls discussed allow you to handle many different types of validations. However, you might want to perform a validation that cannot be achieved by any of these validation controls. 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 client-side with the help of this validation control.

The validation 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 will be passed two arguments—the first is the custom validator control, and the second argument is an object containing two properties: IsValid and Value. The Value property contains the value that is to be validated and IsValid property is used to return the result of the validation.

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—IsValid and Value. 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

As the name implies, the ValidationSummary control is used to display a summary of all the validation errors on 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 validation fails.

Table 3.16 shows the important properties of the ValidationSummary class.

Table 3.16 Important Properties of the ValidationSummary Class




Specifies the way in which the validation summary will be displayed.


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


Specifies the foreground color in which the message is 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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