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Listing 3.20 - LengthValidator.cs, Listing 3.21 - ShowLengthValidator.aspx

Illustrates how you can create a custom validation control that validates the length of a form field.

Listing 3.20 - LengthValidator.cs (C#)
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using System;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace myControls
{

    /// <summary>
    /// Validates the length of an input field
    /// </summary>
    public class LengthValidator : BaseValidator
    {
        private 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;
        }
    }
}

The following page uses the custom LengthValidator custom validator control.

Listing 3.21 - ShowLengthValidator.aspx (C#)
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<%@ 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>