CustomValidator with XMLHttpRequest object

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• Download demo project - 22.9 Kb

Introduction

ASP.NET supplies a few validator controls to ensure that user input data will match application needs. Most of them check if control data is not empty or entered data matches specific data type etc. In real world ASP.NET applications, we very often need to check inserted value against database. ASP.NET supplies a CustomValidator which can be useful for this task. CustomValidator uses server-side events where programmer may write code to check whatever or not control value meets business rules. Of course, this kind of validation needs submit. In this article, I will provide an alternate way to solve this problem.

Background

Since the beginning of my career as a web programmer, I wondered if there is any way to get data from web server to HTML page which is open on client machine. This problem is solved in many ways including the use of IFRAME or XML. For me, it was very useful to read this great article: Client Side Validation Using the XMLHTTPRequest Object By Jonathan Zufi, to understand how this things work and how I can use XMLHTTPRequest in ASP.NET server control.

Using the code

In a few sentences, I'll explain how my validator works: I inherit my validator from CustomValidator. CustomValidator has ClientValidateFunction property which may be useful when developers want to validate on client. I insert my function which makes XMLHTTPRequest to SAME PAGE with a few parameters in QueryString. I override onLoad() event. Inside it, I check by QueryString parameters if I should raise custom event (called ClientValidate).

using System;
using System.Data; 
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;
using System.IO;
using System.Web;
using System.Design;
using System.Drawing.Design;
using System.Collections.Specialized;  

namespace Gilat
{
    /// <summary>

    /// GilatValidator makes XMLHTTPRequest

    /// to server to validate without full page refresh

    /// </summary>

    [DefaultProperty("Text"), ToolboxData("<{0}:GilatValidator 
    runat="server"></{0}:GilatValidator>")]
    public class GilatValidator : System.Web.UI.WebControls.CustomValidator
    {
        //function makes XMLHTTPRequest

        const string XMLHTTPRequestScript = @"
        <script language="'javascript'">

            //GilatControlsUtils.RegisterXMLHTTPRequestScript
            function doXMLHTTPRequest(Url)
            {
                var oXMLHTTP = new ActiveXObject(""Microsoft.XMLHTTP"");
    
                oXMLHTTP.open(""POST"", Url, false);

                try
                {
                    oXMLHTTP.send();
                    return oXMLHTTP.responseText;
                }    
                catch(e) 
                {
                    alert(""XMLHTTPRequest failed"");
                    return """";
                }

            }
        </script>";

        string GilatValidatorClientScript = @"
        <script language="'javascript'">
        
        function {0}Validate(source, arguments)
        {{
            var sURL = ""{1}__serverSideRequest=true&__source="" + source.id;

            //add ControlToValidate
            if(source.controltovalidate != undefined)
            {{
                sURL += ""&ControlToValidateValue="" + 
                  document.all(source.controltovalidate).value;                
            }}
        
            var ControlsToValidate = source.ControlsToValidate;
            if(ControlsToValidate != undefined)
            {{
                var arrControlsToValidate = ControlsToValidate.split("";"");
                for(var i=0; i<arrControlsToValidate.length; i++)
                {{
                    var Control = document.all(arrControlsToValidate[i]);
                    if(Control) 
                    {{
                        sURL += ""&"" + Control.id + ""="" + Control.value;
                    }}    
                }} 
            }}

            arguments.IsValid = (doXMLHTTPRequest(sURL) == ""true"");
        }}
        </script>";

        //variable keeps delimited by ';' string of Control ID's

        private string _ControlsToValidate;

        public string ControlsToValidate
        {
            get
            {
                return _ControlsToValidate;
            }
            set
            {
                _ControlsToValidate = value;
            }
        }

        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender (e);
            //add ControlsToValidate as attribute

            //so it will be available at client

            if(ControlsToValidate != string.Empty) 
                this.Attributes.Add("ControlsToValidate", ControlsToValidate);   
            this.ClientValidationFunction = this.ID + "Validate";
            //use existed QueryString collection

            string Url = Page.Request.Url.ToString();
            if(Page.Request.QueryString.ToString() != string.Empty)
                Url += "&";
            else
                Url += "?";
            Page.RegisterClientScriptBlock("GilatValidatorScript", 
               string.Format(GilatValidatorClientScript, ID, Url));  
            if(!Page.IsClientScriptBlockRegistered("XMLHTTPRequestScript"))
            {
                Page.RegisterClientScriptBlock("XMLHTTPRequestScript", 
                                                   XMLHTTPRequestScript);
            }    
        }

        public event 
          System.Web.UI.WebControls.ServerValidateEventHandler ClientValidate;

        /// <summary>

        /// overrides base event

        /// </summary>

        /// <PARAM name="e"></PARAM>

        protected override void OnLoad(EventArgs e)
        {
          base.OnLoad(e);
          //if this page called from client side validation

          //raise new event

          HttpRequest Request = this.Page.Request;

          if(Request.QueryString["__serverSideRequest"] == "true" && 
                Request.QueryString["__source"] == this.ID)
          {
            Page.Response.Clear();   
            ServerValidateEventArgs ServerValidateE = new 
              ServerValidateEventArgs(Request.QueryString["ControlToValidateValue"], 
              false);
            OnCallbackValidation(ServerValidateE); 
            if(ServerValidateE.IsValid)
                Page.Response.Write("true");   
            Page.Response.End(); 
          }
        }

        protected virtual void OnCallbackValidation(ServerValidateEventArgs e) 
        {     
            if (ClientValidate != null) 
            {
                ClientValidate(this, e);
            }  
        }

    }

}

Points of Interest

A few important issues:

1. Events order: ClientValidation event happens after PageLoad event.
2. Inside ClientValidation event, you can't use control values, because I do not pass ViewState.
3. Property ControlsToValidate is the only way to pass control values to server.
4. ControlsToValidate includes any Control ID delimited by ";".

Inside ClientValidate event, you may access these values like:

string MyComboValue = Request.QueryString[MyCombo.ID];

You can still use args.Value (if you use ControlToValidate) and args.IsValid like in normal ServerValidate event. See it in the attached sample project. Of course, validator will work only on browsers with XMLHTTPRequest installed. Another thing: if you're having an exception inside the ClientValidation event, you can't see it on page. But you may enable Application trace and then check it from there.