Required Field Validator Control Sample

The required field validator control described here extends the base validator control described in the Base Validator Control Sample. This validator adds the following functionality to the base class:

  • It exposes a property named InitialValue that allows the page developer to set an initial value for the required field.
  • It overrides the EvaluateIsValid method (defined as an abstract method in BaseDomValidator) to provide logic to determine whether the value of the required field has changed when the user moves to another field.
  • It overrides AddAttributesToRender (inherited from WebControl) to provide a client-side handler for the evaluation logic. The client-side handler is a function defined in the script library.

To compile and build this sample, see the instructions in Validator Control Samples. For an overview of client-side scripting in server controls, see Client-Side Functionality in a Server Control.

// RequiredDomValidator.cs.
namespace DomValidators {

    using System.ComponentModel;
    using System.Diagnostics;
    using System;
    using System.Web;
    using System.Web.UI;
    using System.Web.UI.WebControls;

    ToolboxData("<{0}:RequiredDomValidator runat=server ErrorMessage=\"RequiredDomValidator\"></{0}:RequiredDomValidator>")
    public class RequiredDomValidator : BaseDomValidator {

        Description("Initial Value")
        public string InitialValue {
            get { 
                object o = ViewState["InitialValue"];
                return((o == null) ? String.Empty : (string)o);
            set {
                ViewState["InitialValue"] = value;

        protected override void AddAttributesToRender(HtmlTextWriter writer) {
            if (RenderUplevel) {
                writer.AddAttribute("evaluationfunction", "RequiredFieldValidatorEvaluateIsValid");
                writer.AddAttribute("initialvalue", InitialValue);

        protected override bool EvaluateIsValid() {

            // Get the control value; return true if it is not found.
            string controlValue = GetControlValidationValue(ControlToValidate);
            if (controlValue == null) {
                Debug.Fail("Should have been caught by PropertiesValid check.");
                return true;

            // See if the control has changed.
[VisualĀ Basic]
' RequiredDomValidator.vb
Option Explicit
Option Strict

Imports System.ComponentModel
Imports System.Diagnostics
Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls

Namespace DomValidators

   <ToolboxData("<{0}:RequiredDomValidator runat=server ErrorMessage=""RequiredDomValidator""></{0}:RequiredDomValidator>")> _
   Public Class RequiredDomValidator
      Inherits BaseDomValidator

      <Bindable(True), _
         Category("Behavior"), _
         DefaultValue(""), _
         Description("Initial Value")> _
      Public Property InitialValue() As String
            Dim o As Object = ViewState("InitialValue")
            If o Is Nothing Then
               Return String.Empty
               Return CStr(o)
            End If
         End Get
            ViewState("InitialValue") = value
         End Set
      End Property
      Protected Overrides Sub AddAttributesToRender(writer As HtmlTextWriter)
         If MyBase.RenderUplevel Then
            writer.AddAttribute("evaluationfunction", "RequiredFieldValidatorEvaluateIsValid")
            writer.AddAttribute("initialvalue", InitialValue)
         End If
      End Sub
      Protected Overrides Function EvaluateIsValid() As Boolean
         ' Get the control value; return true if it is not found.
         Dim controlValue As String = GetControlValidationValue(MyBase.ControlToValidate)
         If controlValue Is Nothing Then
            Debug.Fail("Should have been caught by PropertiesValid check.")
            Return True
         End If
         ' See if the control has changed.
         Return Not controlValue.Trim().Equals(InitialValue.Trim())
      End Function
   End Class
End Namespace

See Also

Validator Control Samples | Base Validator Control Sample | Regular Expression Validator Control Sample | Script Library for Validator Sample | Configuration File for Validator Sample | Test Page for Validator Sample | Developing a Validator Control