Validation Controls

Validation is the process of making sure that the user enters correct information into a form. Validation controls provided by .NET Framework work in the client browser if the browser supports Javascript and DHTML and checks the data the user entered before sending it to the server. All the validation takes place in the browser and nothing is sent back to the server. If the browser doesn't support DHTML and scripting then validation is done on the server. All validation controls in the .NET Framework are derived from the BaseValidator class. This class serves as the base abstract class for the validation controls and provides the implementation for all validation controls that derive from this class.

The validation controls that are provided by the .NET Framework are as follows:

RequiredFieldValidator
CompareValidator
RangeValidator
RegularExpressionValidator
CustomValidator

Common to all the above said controls are the ErrorMessage and ControlToValidate properties. The ErrorMessage property is used to set an error to be displayed and the ControlToValidate property is used to set the control you want to check.

RequiredFieldValidator

Simplest of all, the RequiredField validator makes sure that the user enters data into a form. For example, on a registration form you might want your users to enter their date of birth in a textbox. If they leave this field empty, this validation control will display an error. The class hierarchy for this control is as follows:

Object
 Control
  WebControl
   Label
    BaseValidator
     RequiredFieldValidator

Notable property of the RequiredFieldValidator is the InitialValue property which sets an initial value in the control.

CompareValidator

Comparison validators are used to compare the values entered by the user into a control (textbox) with the value entered into another control or with a constant value. We indicate the control to validate by setting the ControlToValidate property and if we want to compare a specific control with another control we need to set the ControlToCompare property to specify the control to compare with. The class hierarchy for this control is as follows:

Object
 Control
  WebControl
   Label
    BaseValidator
     BaseCompareValidator
      CompareValidator

RangeValidator

Range Validators are used to test if the value of a control is inside a specified range of values. The three main properties of this control are the ControlToValidate property which contains the control to validate and Maximum and Minimum values which hold the maximum and minimum values of the valid range. The class hierarchy for this control is as follows:

Object
 Control
  WebControl
   Label
    BaseValidator
     BaseCompareValidator
      RangeValidator

RegularExpressionValidator

RegularExpression validators are used to check if the value in a control matches a pattern defined by the regular expression. Notable property for this control is the ValidationExpression property which allows us to select a predefined expression which we want to match with the data entered in a control. The class hierarchy for this control is as follows:

Object
 Control
  WebControl
   Label
    BaseValidator
     RegularExpressionValidator

CustomValidator

Custom validators are used to perform our own validation for the data in a control. For example, you can check the value entered by a user is even or odd with this control which is not possible with any of the above mentioned validation controls. You write the script for the validation using Javascript or VBScript and associate that script function to the ClientValidationFunction property of this control. The class hierarchy for this control is as follows:

Object
 Control
  WebControl
   Label
    BaseValidator
     CustomValidator

ValidationSummary

Validation summary control is used to display a summary of all validation errors (from all validation controls) on a Web page. This class is supported by the ValidationSummary class and the hierarchy is as follows:

Object
 Control
  WebControl
   ValidationSummary

Notable property of this control is the DisplayMode property which allows us to display the errors in a specific format, example, as a list, paragraph or as a bulletlist.

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