Create a new validation control by deriving a new control from BaseValidator : BaseValidator « Validation « ASP.NET Tutorial

using System;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace myControls
{
    public class LengthValidator : BaseValidator
    {
        int _maximumLength = 0;

        public int MaximumLength
        {
            get { return _maximumLength; }
            set { _maximumLength = value; }
        }

        protected override bool EvaluateIsValid()
        {
            String value = this.GetControlValidationValue(this.ControlToValidate);
            if (value.Length > _maximumLength)
                return false;
            else
                return true;
        }
    }
}

<%@ Page Language="C#" %>

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

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Show Length Validator</title>
</head>
<body>
    <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>
</body>
</html>

8.1.BaseValidator
	8.1.1.	Create a new validation control by deriving a new control from BaseValidator