ASP.NET: Creating A Custom Validator Control

Code for Control: /App_Code/LengthValidator.vb

Imports System
Imports System.Web.UI
Imports System.Web.UI.WebControls
Namespace myControls
    ''' <summary>
    ''' Validates the length of an input field
    ''' </summary>
    Public Class LengthValidator
        Inherits BaseValidator
        Private _maximumLength As Integer = 0
        Public Property MaximumLength() As Integer
            Get
                Return _maximumLength
            End Get
            Set(ByVal value As Integer)
                _maximumLength = value
            End Set
        End Property
        Protected Overloads Overrides Function EvaluateIsValid() As Boolean
            Dim value As [String] = Me.GetControlValidationValue(Me.ControlToValidate)
            If value.Length > _maximumLength Then
                Return False
            Else
                Return True
            End If
        End Function
    End Class





Code on the ASPX page

<%@ Register TagPrefix="custom" Namespace="myControls" %>


<form id="form1" runat="server">
<div>
    <asp:Label id="lblComments" Text="Comments:" AssociatedControlID="txtComments" Runat="server" />
    <br />
    <asp:TextBox id="txtComments" TextMode="MultiLine" Columns="30" Rows="2" Runat="server" />
    <custom:LengthValidator
        id="valComments"
        ControlToValidate="txtComments"
        Text="(Must be less than 10 characters)"
        MaximumLength="10"
        Runat="server"
    />
    <br /><br />
    <asp:Button id="btnSubmit" Text="Submit" Runat="server" />
</div>
</form>