Assembly in Asp.Net

An assembly is a logical unit of code. It physically exists as DLLs or EXEs. One assembly can contain one or more files like image files, text files etc. along with DLLs or EXEs.
Assemblies in the .Net platform are a solution to a problem that has been around as long as libraries. The problem knows as “DLL Hell”, can be caused by one application overwriting a shared library of another application, usually with a different version.
Imagine you write an application with a shared library and then years later create a new application with an updated library. In order not to break old programs, what do you do with the new library? You could give it a different name, store it in a different location, or overwrite the old ones?
.Net assemblies overcome these issues by storing information about themselves, rather than having that information stored in central location (i.e. system registry). This information is called as Assembly Manifest.
Assembly manifest

  • Assembly manifest is a data structure which stores information about an assembly.
  • This information is stored within the assembly file itself.
  • The information includes version information, list of constituent files etc.

What is in the Assembly?
The assembly contains the intermediate code, resources, and the metadata for itself. We can have a look inside the assembly using the ildasm (Intermediate Language Disassembler) tool that comes as part of Visual Studio. To access it you need to open the Visual Studio Command Prompt and type ildasm.exe. To open the Visual Studio Command Prompt goes to:
Start>All Program>Microsoft Visual Studio 2010>Visual Studio Tools>Visual Studio Command Prompt (2010)

Visual Studio Command Prompt
Figure 1: Visual Studio Command Prompt

This will launch a window application that you can use to explore any .Net application. Once launched, you can open any .Net application (File>Open) and view information within the file, including the manifest, types, classes, namespaces, methods and source code.
Visual Studio Command Prompt
Figure 2: Intermediate Language Disassembler

Assembly Types
You can create two types of ASP.Net Assemblies: Private Assemblies and Shared Assemblies.
The assembly which is used only by a single application is called as private assembly. Suppose you created a DLL which encapsulates your business logic. This DLL will be used by your client application only and not by any other application. In order to run the application properly your DLL must reside in the same folder in which the client application is installed. Thus the assembly is private to your application.
Suppose that you are creating a general purpose DLL which provides functionality which will be used by variety of applications. Now instead of each client application having its own copy of DLL you can place the DLL in ‘Global Assembly Cache’. Such assemblies are called shared assemblies.
Creating a Shared Assembly
The procedure for creating a shared assembly is the same as for a private assembly, except that there are a few extra steps involved before you can use it. All shared assemblies need to be strongly named before they can be installed into the global assembly cache. Strongly named assemblies ensure that the assembly has unique manifest consisting of versioning information and public key.
Steps to create shared assemblies:

  • Create your DLL source code.
  • Generate unique assembly name using SN utility.
  • Sign your DLL with the private key by modifying AssemblyInfo file.
  • Compile your DLL.
  • Place the resultant DLL in global assembly cache using GACUtil utility.

What is Global Assembly Cache?
Global Assembly Cache is nothing but a special disk folder where all the shared assemblies will be kept. It is located under :\WinNT\Assembly Folder.
If you are using .Net framework 4.0 then the folder will be : :\WinNT\Microsoft.Net\Assembly.
How do I create unique assembly name?
Microsoft now uses a public-private key pair to uniquely identify an assembly. To generate this cryptographic key pair, use the SN Tool. Go to Visual studio command prompt and use the following syntax to generate a key file.

sn -k “[DriveLetter]:\[DirectoryToPlaceKeyFile]\[KeyFileName].snk”
e.g. sn -k “C:\GACKey\GACkey.snk”

How do I sign my Assembly?
A key is generated, but it is not yet associated with the assembly of the project. To create this association, double-click the AssemblyInfo.vb file in Visual Studio .NET Solution Explorer. This file has the list of assembly attributes that are included by default when a project is created in Visual Studio .NET.
Modify the AssemblyInfo.vb by adding the following AssemblyKeyFile attribute:

[Assembly: AssemblyKeyFile(“C:\\GACKey\\GACKey.snk”)]

How do I place the assembly in shared cache?
You can install the .dll file by using the Gacutil tool or by dragging the .dll file to the appropriate folder. If you use the Gacutil tool, you can use a command that resembles the following:

gacutil -I “[DriveLetter]:\[DirectoryOfDllFile]\[dll File]”

To drag the file, open two instances of Windows Explorer. In one instance, find the location of the .dll file output for your console project. In the other instance, find WinNT\Assembly. Then, drag your .dll file to the Assembly folder.
Hands On…
Now, that we have understood the basics of assemblies let us apply our knowledge by developing a simple shared assembly.
In this example we will create a VB.NET component called SharedAssembly. We will also create a key file named sample.snk. We will sign our component with this key file and place it in Global Assembly Cache.
Step 1: Creating our sample component
Here is the code for the component. It just includes one method which returns a string “Hello World”.

Namespace SharedAssembly
  Public Class Test
    Public Function Helloworld() As String
      Return “Hello World”
    End Function
  End Class
End Namespace

Visual Studio Command Prompt
Figure 3: VB.Net Class File

Step 2: Generate a key file
To generate the key file use the following command at visual studio command prompt:

SN –k c:\sample.snk

Visual Studio Command Prompt
Figure 4 : Generate Key File

Step 3: Sign your component with the key file
To sign the component add the following AssemblyKeyFile attribute in AssemblyInfo.vb file:

[Assembly: AssemblyKeyFile(“C:\\sample.snk”)]

Visual Studio Command Prompt
Figue 5 : AssemblyInfo.vb

Step 4: Host the signed assembly in Global Assembly Cache
To install the assembly in Global Assembly Cache we will use GACUtil Tool. Go to Visual studio command prompt, use the following code :

gacutil –i D:\Test\sikarnarender.com\Assembly\SharedAssembly
\SharedAssembly\bin\Debug\SharedAssembly.dll

Visual Studio Command Prompt
Figure 6: GACUtil.exe

After hosting the assembly just goes to WINNT\Assembly folder and you will find your assembly listed there.

State Management with Context.Items, Asp.net

Passing values between pages is a common task in every asp.net web application.There are many options for state management including Session, Cookies, QueryString etc. Each of these have their strength and weakness and each is designed for different circumstances, it just depends of your requirement. This article will focus on the use of context.items collection to pass variable between different pages.

Assume we have a ecommerce site, the page “ProductList” shows the list of products.When I click on a product, it will redirect the user to “ProductDescription”. How do I tell “ProductDescription” which product to show ?

One possible approach is to use the QueryString and pass the ProductId to the “ProductDescription” page as shown in listing 1 and 2.

Response.Redirect(“ProductDescription.aspx?PrdId=2”)

Listing 1: “ProductList” using the QueryString

Dim PrdId as Integer=Request.QueryString(“PrdId”)

Listing 2: “ProductDescription” using the QueryString

The above approach is totally acceptable. There are several disadvantages of this approach, some of which are as follows:

  • The QueryString is exposed with the URL meaning sensitive data may be revealed.
  • The length of the QueryString is generally limited (e.g. 256 bytes in IE).
  • The QueryString can only contain character data (no objects).
  • Response.Redirect requires a round trip from server to client to server.

A better approach is to store the Product Id in Context.Items collection, then use server.transfer to redirect to the “ProductDescription” as shown in listing 3 and 4.

Context.Items.Add(“PrdId”,2)
Server.Transfer(“ProductDescription.aspx”)

Listing 3: “ProductList” using the Context.Items collection

Dim PrdId as Integer=Context.Items.Item(“PrdId”)

Listing 4: “ProductDescription” using the Context.Items collection

Server.Transfer performs a server-side redirect that is transparent to the user, meaning there is no need of a round trip.The Context.Items collection is also capable of storing objects instead of just plain character data.

It is important to keep in mind that values stored in the Context.Items collection only last for the current request. If I were to do a postback on the “ProductDescription” page, the Context item “Id” would be lost. Most of the time this is convienient since we only need to store that value between pages and no longer.

Viewstate collection is the solution to retain the values between postbacks.

Callback Combo on Scroll down

You can grab the external JavaScript file representing the jQuery library need for this sample from either of these URLs  – 
http://ajax.googleapis.com/ajax/libs/jquery/1.2.3/jquery.min.js

You have to create two folders as images : to hold the images 1.)dropdown_arrow.gif  2.) spinner.gif and the second folder will be as js : to hold the jquery file.

Onto the code …


CallbackAsyncHandler.ashx : It has all logics to fetch records from database on the bahalf of parameters defined on user control.  It receives all parameter using query string.

<%@ WebHandler Language=”VB” Class=”CallbackAsyncHandler” %>


Imports System.Web
Imports System.Text
Imports System.Data
Imports System.Data.SqlClient


Public Class CallbackAsyncHandler
Implements IHttpHandler


Public Sub ProcessRequest(context As HttpContext) Implements IHttpHandler.ProcessRequest
        Dim Condition As String = Convert.ToString(context.Request.QueryString(“Condition”)).Replace(“#”, “+”)
        Dim CCondition As String = Convert.ToString(context.Request.QueryString(“CCondition”)).Replace(“#”, “+”)
        Dim ValueField As String = Convert.ToString(context.Request.QueryString(“ValueField”)).Replace(“#”, “+”)
        Dim TextField As String = Convert.ToString(context.Request.QueryString(“TextField”)).Replace(“#”, “+”)
        Dim TableName As String = Convert.ToString(context.Request.QueryString(“TableName”)).Replace(“#”, “+”)
        Dim BunchSize As Integer = Convert.ToString(context.Request.QueryString(“BunchSize”)).Replace(“#”, “+”)
        Dim OrderBy As String = Convert.ToString(context.Request.QueryString(“OrderBy”)).Replace(“#”, “+”)
        
        If OrderBy = 1 Then
            OrderBy = ValueField
        Else
            OrderBy = TextField
        End If


        context.Response.Write(PrepareDataSet(Condition, ValueField, TextField, TableName, BunchSize, CCondition, OrderBy))
End Sub


    Private Function PrepareDataSet(ByVal Condition As String, ByVal ValueField As String, ByVal TextField As String, _
                                    ByVal TableName As String, ByVal BunchSize As Integer, ByVal CCondition As String, ByVal OrderBy As String) As String
        
        ‘New SqlConnection(“Data Source=pankajpareek;Initial Catalog=T480;User ID=sa;Password=redtray”)
        Dim conn As System.Data.SqlClient.SqlConnection = New SqlConnection(System.Configuration.ConfigurationManager.AppSettings(“connection”))
        Dim strSql As String = “Select TOP ” & BunchSize & ” ” & ValueField & “,” _
                            & TextField & ” From ” & TableName & ” Where 1=1 ” & CCondition & Condition & ” Order by ” & OrderBy
        Dim da As SqlDataAdapter = New System.Data.SqlClient.SqlDataAdapter(strSql, conn)
        Dim dt As New DataTable
        da.Fill(dt)


        Return BuildRows(dt)
    End Function


Private Function BuildRows(dt As System.Data.DataTable) As String
Dim sb As New StringBuilder()
Dim dr As System.Data.DataRow
If dt.Rows.Count > 0 Then
For i As Integer = 0 To dt.Rows.Count – 1
sb.Append(“<tr class=’mouseOut’ onmouseover=’mouseOver(this)’ onmouseout=’mouseOut(this)’ onclick=’Select(this)’ ctype=’CallbackCombo’>”)
dr = dt.Rows(i)
For j As Integer = 0 To dt.Columns.Count – 1
If j = 0 Then
sb.Append(“<td class=’displayNone’>” & Convert.ToString(dr(j)) & “</td>”)
Else
sb.Append(“<td>” & Convert.ToString(dr(j)) & “</td>”)
End If
Next
sb.Append(“</tr>”)
Next
End If
Return sb.ToString()
End Function


Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
Get
Return False
End Get
End Property
End Class

CallbackCombo.ascx : It keeps all the design & javacript logic. On Vb side it has all property & variables used to define Callback Combo dynamically.

Design View

<%@ Control Language=”VB” AutoEventWireup=”false” CodeFile=”CallbackCombo.ascx.vb”
    Inherits=”CallbackCombo_CallbackCombo” %>
<asp:Literal ID=”Literal1″ runat=”server”>
</asp:Literal>


<script type=”text/javascript”>
        var prev = 0;
        var ValueField=”<%=ValueField %>”;
        var TextField=”<%=TextField %>”;
        var TableName=”<%=TableName %>”;
        var BunchSize=”<%=BunchSize %>”;
        var OrderBy=”<%=OrderBy %>”;
        var CCondition=”<%=Condition %>”;
        var FolderPath=”<%=FolderPath %>”;
        var DefaultText=”<%=DefaultText %>”;
        var DefaultValue=”<%=DefaultValue %>”;
        
        $(document).ready(
            function() 
            {
                $(“.divProgress”).hide();
                $(“#ImgDropDown”).attr(“src”, FolderPath + “/images/dropdown_arrow.gif”);
                $(“#imgProgress”).attr(“src”, FolderPath + “/images/spinner.gif”);
                
                $(document).click
                (
                    function()
                    {
                        var obj=window.event.srcElement;
                        if($(obj).attr(“ctype”)!=”CallbackCombo”)
                        {
                            ComboListShow=0;
                            $(“#divList”).slideUp(“fast”);
                        }                       
                    }
                )


                $(“#divList”).scroll
                (
                    function() 
                    {
                        if ($(“#tblList”)[0].clientHeight – this.scrollTop == $(this)[0].clientHeight) 
                        {
                            var LastId = $(“#tblList tr:last”).children(“td:first”).html();
                            var trLast = $(“#tblList tr:last”);
                            if (parseInt(LastId, 10) > parseInt(prev, 10)) 
                            {
                                prev = LastId;
                                appendData(” And ” + TextField + ” like ‘” + $(“#” + “<%=txtInput.ClientId %>”).val() + “%’ And ” + ValueField + “>” + LastId);
                            }
                        }
                    }
                );
            }
        );
        function appendData(condition)
        {
            $(“.divProgress”).ajaxStart(function() {
                 $(this).show();
            });
             $(“.divProgress”).ajaxStop(function() {
                 $(this).hide();
            });
            $.post(FolderPath + “/CallbackAsyncHandler.ashx?ValueField=” + escape(ValueField) + “&TextField=” + escape(TextField) + “&TableName=” + escape(TableName)
                                            + “&BunchSize=” + escape(BunchSize) + “&CCondition=” + escape(CCondition) + “&OrderBy=” + escape(OrderBy) 
                                            + “&Condition=” + escape(condition), function(data) {
                 if (data != null && data!=””) 
                 {
                    if($(‘#tblList tr’).length<=0 && DefaultText!=””)
                    {
                        var Temp=”<tr class=’mouseOut’ onmouseover=’mouseOver(this)’ onmouseout=’mouseOut(this)’ onclick=’Select(this)’ ctype=’CallbackCombo’>”;
                        if(DefaultValue!=””)
                        {
                            Temp=Temp + “<td class=’displayNone’>” + DefaultValue + “</td>”
                        }
                        else
                        {
                            Temp=Temp + “<td class=’displayNone’>” + DefaultText + “</td>”
                        }
                        Temp=Temp + “<td>” + DefaultText +  “</td>”;
                        Temp=Temp + “</tr>”
                        $(“#tblList”).append(Temp);
                    }
                    $(“#tblList”).append(data);
                }
            });
        }
</script>


<script type=”text/javascript”>
        var ComboListShow=0;
        function ShowList()
        {
            if(ComboListShow==1)
            {
                ComboListShow=0;
                $(“#divList”).slideUp(“fast”);
            }
            else
            {
                var UserIdLast = 0;                
                if($(‘#tblList tr’).length<=0)
                {
                    appendData(” And ” + TextField + ” like ‘” + $(“#” + “<%=txtInput.ClientId %>”).val() + “%’ And ” + ValueField + “>0”);
                }
                $(“#divList”).slideDown(“fast”);
                if ($(“#tblList tr.trselect”).length>0)
                {
                    //$(“#divList”).animate({ scrollTop: $(“#tblList tr.trselect”)[0].offsetTop-(14*17) }, “slow”)
                    $(“#divList”).scrollTop($(“#tblList tr.trselect”)[0].offsetTop-(14*17))
                }
                ComboListShow=1;
            }
        }
        function filter()
        {
            prev=0;
            $(“#tblList tr”).remove();
            $(“#” + “<%=hdnValue.ClientId %>”).val(‘0’);
            appendData(” And ” + TextField + ” like ‘” + $(“#” + “<%=txtInput.ClientId %>”).val() + “%’ And ” + ValueField + “>0”);
            $(“#divList”).slideDown(“fast”);
            ComboListShow=1;
        }
        function RemoveCallbackComboRow()
        {
            $(“#tblList tr”).remove();
            $(“#” + “<%=txtInput.ClientId %>”).val(”);
            $(“#” + “<%=hdnValue.ClientId %>”).val(‘0’);
        }
</script>


<style type=”text/css”>
    .dvInput
    {
        border: solid 1px #8AA8FF;
        width: 200px;
        height: 20px;
    }
    .ImgDropDown
    {
        vertical-align: middle;
        width: 13px;
        height: 13px;
        float: right;
        margin-top: 3px;
        cursor: pointer;
    }
    .txtInput
    {
        width: 185px;
        border: none 0px #8AA8FF;
        height: 18px;
    }
    .divList
    {
        height: 200px;
        border: solid 1px #8AA8FF;
        border-top: solid 0px #8AA8FF;
        width: 200px;
        text-align: left;
        overflow-y: auto;
        overflow-x: auto;
    }
    .displayNone
    {
        display: none;
    }
    .divProgress
    {
        width: 100%; /*background-color: red;
            color: white;*/
    }
    .trmouseOver
    {
        background-color: Blue;
        color: White;
    }
    .trmouseOut
    {
        background-color: White;
        color: Black;
    }
    .trselect
    {
        background-color: Blue;
        color: White;
    }
</style>


<script type=”text/javascript” language=”javascript”>
        function mouseOver(Row)
        {
            $(“#tblList tr.trmouseOver”).removeClass(“trmouseOver”).addClass(“trmouseOut”);
            $(Row).removeClass(“trmouseOut”).addClass(“trmouseOver”);
        }
        function mouseOut(Row)
        {
            $(Row).removeClass(“trmouseOver”).addClass(“trmouseOut”);
        }
        function Select(Row)
        {
            $(“#” + “<%=txtInput.ClientId %>”).val($(Row).children(“td:last”).html());
            $(“#” + “<%=hdnValue.ClientId %>”).val($(Row).children(“td:first”).html());
            $(“#tblList tr”).removeClass(“trselect”).addClass(“trmouseOut”);
            $(Row).removeClass(“trmouseOut”).removeClass(“trmouseOver”).addClass(“trselect”);
        }
</script>


<div id=”CallbackCombo” runat=”server” style=”width: auto; height: auto;”>
    <div id=”dvInput” class=”dvInput” ctype=’CallbackCombo’>
        <img id=”ImgDropDown” alt=”” src=”images/dropdown_arrow.gif” class=”ImgDropDown”
            onclick=”ShowList()” ctype=’CallbackCombo’ />
        <asp:TextBox ID=”txtInput” runat=”server” CssClass=”txtInput” ctype=’CallbackCombo’
            onkeyup=”filter()”></asp:TextBox>
        <asp:HiddenField ID=”hdnValue” runat=”server” />
    </div>
    <div class=”divList” id=”divList” style=”display: none; position: absolute; z-index: 1000;
        background-color: White;” ctype=’CallbackCombo’>
        <table id=”tblList” style=”width: 90%; font-family: Arial; font-size: 12px;” ctype=’CallbackCombo’>
        </table>
        <div id=”loading” class=”divProgress” style=”width: 100%”>
            <center>
                <img src=”images/spinner.gif” id=”imgProgress” border=”0″></center>
        </div>
    </div>
</div>

Code View : 
Partial Class CallbackCombo_CallbackCombo
    Inherits System.Web.UI.UserControl


    Private _ValueField As String
    Private _TextField As String
    Private _TableName As String
    Private _BunchSize As Integer
    Private _FolderPath As String
    Private _Condition As String
    Private _OrderBy As OrderByType
    Private _DefaultText As String
    Private _DefaultValue As String


    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Literal1.Text = “<script src='” & FolderPath & “/js/jquery-1.2.6.min.js’ type=’text/javascript’></script>”
    End Sub


    Public Property DefaultValue() As String
        Get
            Return _DefaultValue
        End Get
        Set(ByVal value As String)
            _DefaultValue = value
        End Set
    End Property


    Public Property DefaultText() As String
        Get
            Return _DefaultText
        End Get
        Set(ByVal value As String)
            _DefaultText = value
        End Set
    End Property


    Public Property ValueField() As String
        Get
            Return _ValueField
        End Get
        Set(ByVal value As String)
            _ValueField = value
        End Set
    End Property


    Public Property TextField() As String
        Get
            Return _TextField
        End Get
        Set(ByVal value As String)
            _TextField = value
        End Set
    End Property


    Public Property TableName() As String
        Get
            Return _TableName
        End Get
        Set(ByVal value As String)
            _TableName = value
        End Set
    End Property


    Public Property BunchSize() As Integer
        Get
            Return _BunchSize
        End Get
        Set(ByVal value As Integer)
            _BunchSize = value
        End Set
    End Property


    Public Property FolderPath() As String
        Get
            Return _FolderPath
        End Get
        Set(ByVal value As String)
            _FolderPath = value
        End Set
    End Property


    Public Property OrderBy() As OrderByType
        Get
            Return _OrderBy
        End Get
        Set(ByVal value As OrderByType)
            _OrderBy = value
        End Set
    End Property


    Public Property Condition() As String
        Get
            Return _Condition
        End Get
        Set(ByVal value As String)
            _Condition = value
        End Set
    End Property


    Public ReadOnly Property SelectedText() As String
        Get
            Return txtInput.Text
        End Get
    End Property


    Public ReadOnly Property SelectedValue() As String
        Get
            Return hdnValue.Value
        End Get
    End Property


    Public Enum OrderByType
        TextField = 0
        ValueField = 1
    End Enum


End Class

References : 
Load Content While Scrolling With jQuery
Implementing infinite scrolling with jQuery


Progressive loading in Gridview on Scroll down

In contrast to the Pagination patterns, the Continuous Scrolling pattern has no natural break. When using pagination patterns, a decision to only show a subset of data at a time and then let the user request more data if wanted is chosen. With the Continuous Scrolling, new data is automatically retrieved as the user has scrolled to the bottom of the page. It thus appears as if the page has no end, as more data will be loaded and inserted into the page each time the user scrolls to the bottom of page.


The minimalistic code sample explained here will show how to fetch records asynchronously on scrolling from a hypothetical Users table after the first batch is initially fetched. A Generic Handler (which is similar to an ASPX page but lacks HTML tags) is used to retrieve records asynchronously and hand it over to the parent page. So the sample basically consists of  the following 2 files which you will have to copy to a ASP.NET 2.0 Website in VS.NET 2005 or VS.NET 2008 –
FetchOnDemand.aspx
AsyncHandler.ashx

All the JavaScript magic to track when the user reaches the end of the scroll bar (in this example we simulate DIV scrolling  by restricting it’s height & setting overflow:auto style for the DIV tag) is done effortlessly by jQuery.

You can grab the external JavaScript file representing the jQuery library need for this sample from either of these URLs  –
http://code.jquery.com/jquery-latest.js
http://ajax.googleapis.com/ajax/libs/jquery/1.2.3/jquery.min.js

You can copy and place the external JavaScript file in a JS folder under the ASP.NET 2.0 website you create in VS.NET for running this code sample or you may even choose to reference it directly in the code. The jQuery library will be used in the parent page (FetchOnDemand.aspx) to trigger fetching of new records dynamically on scrolling to the bottom. The parent page communicates the unique Id related to the last row that was previously fetched to a Generic Handler (AsyncHandler.ashx). The Handler in turn returns a batch of new records asynchronously as a HTML table so that it can be injected at the end of table rendered by the GridView in the parent page.

Onto the code …


FetchOnDemand.aspx displays the first 20 records through a GridView. jQuery helps us get the last User Id from the last row of the table rendered from the GridView. We pass this as parameter to the Generic Handler’s querystring -“AsyncHandler.ashx?lastUserId=

Design View :
<%@ Page Language=”C#” AutoEventWireup=”true” CodeFile=”FetchOnDemand.aspx.cs” Inherits=”FetchOnDemand” %>


<!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 runat=”server”>
    <title>Untitled Page</title>
    <style type=”text/css”>
        body
        {
            font-family: Verdana, Arial, Helvetica, sans-serif;
            font-size: 11px;
            color: #666666;
        }
        .divLeft
        {
            height: 500px;
            border: solid 1px black;
            width: 1300px;
            text-align: left;
            overflow: auto;
        }
        .divProgress
        {
            width: 100%;
            /*background-color: red;
            color: white;*/
        }
        .tblHeader
        {
            font-weight: bold;
            text-align: left;
            background-color: gray;
            color: black;
        }
        td
        {
            text-align: center;
        }
    </style>


    <script src=”js/jquery-1.2.6.min.js” type=”text/javascript”></script>


    <script type=”text/javascript”>
            //following code utilizes jQuery 1.2.6
            var prev = 0;
             $(document).ready(
             
             //DIV showing the message “Loading…” is hidden initially
            //The message will be shown when records are fetched with AJAX 
            //when user has scrolled to the bottom of the DIV scrollbar 
            function() {
                 $(“.divProgress”).hide();


                 $(window).scroll(
                
            function() {
            //triggering point is when the difference of the heights of the TABLE 
            //and DIV match the DIV’s scrollTop value
            //if ($(“#tblOrders”).height() – ($(window).scrollTop()-50) == $(window).height()) {
            if ($(window).scrollTop() == $(document).height() – $(window).height()){
            //progress bar         
            $(“.divProgress”).ajaxStart(function() {
                 $(this).show();
            });
             $(“.divProgress”).ajaxStop(function() {
                 $(this).hide();
            });


            //get last Order Id to track next fetch
            var UserIdLast = $(“#tblOrders tr:last”).children(“td:first”).html();


            //get last table row in order to append the new result set increment
            var trLast = $(“#tblOrders tr:last”);
            if (parseInt(UserIdLast, 10) > parseInt(prev, 10)) {
                prev = UserIdLast;
                 //make a async call to fetch the incremental results     
                $.post(“AsyncHandler.ashx?lastUserId=” + UserIdLast, function(data) {
                     if (data != null) {
                         //append new result set to last row
                        trLast.after(data);
                    }
                });
            }
        }
    });
    });
    </script>


</head>
<body>
    <h3>
        This is a demo to show Continous Scrolling UI Pattern</h3>
    <form id=”form1″ runat=”server”>
    <%–<div class=”divLeft”>
    </div>–%>
    <asp:GridView ID=”tblOrders” runat=”server” AutoGenerateColumns=”false” CellPadding=”2″
        Width=”100%”>
        <HeaderStyle CssClass=”tblHeader” />
        <Columns>
            <asp:BoundField DataField=”UserID” HeaderText=”UserID” InsertVisible=”False” ReadOnly=”True”
                SortExpression=”UserID”></asp:BoundField>
            <asp:BoundField DataField=”LoginName” HeaderText=”LoginName” SortExpression=”LoginName”>
            </asp:BoundField>
            <asp:BoundField DataField=”FirstName” HeaderText=”FirstName” SortExpression=”FirstName”>
            </asp:BoundField>
            <asp:BoundField DataField=”LastName” HeaderText=”LastName” SortExpression=”LastName”>
            </asp:BoundField>
            <asp:BoundField DataField=”EmailAddress” HeaderText=”EmailAddress” SortExpression=”EmailAddress”>
            </asp:BoundField>
            <asp:BoundField DataField=”TelephoneNo” HeaderText=”TelephoneNo” SortExpression=”TelephoneNo”>
                <ItemStyle Width=”50px” />
            </asp:BoundField>
        </Columns>
    </asp:GridView>
    <div class=”divProgress”>
        Loading….
    </div>
    </form>
</body>
</html>

Code View : 
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

public partial class FetchOnDemand : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection con=new SqlConnection(“ConnectionString”);
        string strSql = “Select TOP 20          UserId,LoginName,FirstName,LastName,EmailAddress,TelephoneNo From Users”;
        SqlDataAdapter dap= new SqlDataAdapter(strSql, con);
        DataTable dt=new DataTable();
        dap.Fill(dt);

        tblOrders.DataSource = dt;
        tblOrders.DataBind();
    }
}

AsyncHandler.ashx keeps the weight of the response lighter than an equivalent ASPX page. It takes a UserId value & returns the next twenty  records formatted in a HTML table so that it can be readily appended to the table in the parent view containing the bigger set of records.

 

<%@ WebHandler Language=”C#” Class=”AsyncHandler” %>


using System;
using System.Web;
using System.Text;
using System.Data;
using System.Data.SqlClient;


public class AsyncHandler : IHttpHandler
{
    
    public void ProcessRequest (HttpContext context) 
    { 
        //REMOVE BELOW LINE if you are using it in a real application 
        //It is here to simulate the delay while fetching results
        System.Threading.Thread.Sleep(2000);


        //The last OrderId is used to get the next increment
        string lastUserId = Convert.ToString(context.Request.QueryString[“lastUserId”]);


        //The PrepareDataSet method stuffs the DataSet into a HTML table 
        context.Response.Write(PrepareDataSet(lastUserId));
    }

    private string PrepareDataSet(string _UserId) 
    { 
        System.Data.SqlClient.SqlConnection conn = new SqlConnection(“ConnectionString”); 
        string strSql = “Select TOP 20 UserId,LoginName,FirstName,LastName,EmailAddress,TelephoneNo From Users Where UserId >” + _UserId;
        SqlDataAdapter da = new System.Data.SqlClient.SqlDataAdapter(strSql,conn); 
        DataSet ds = new System.Data.DataSet(); 
        da.Fill(ds, “Users”); 


        //The BuildRows method prepares a HTML table & stuffs the resultset into it 
        return BuildRows(ds.Tables[0]);
    }
     
    private string BuildRows(System.Data.DataTable dt) 
    { 
        StringBuilder sb = new StringBuilder();
        System.Data.DataRow dr; 
        if (dt.Rows.Count > 0) 
        { 
            for (int i = 0; i < dt.Rows.Count;i++) 
            { 
                sb.Append(“<tr class=’tblRow’>”); 
                dr = dt.Rows[i]; 
                for (int j = 0; j < dt.Columns.Count; j++) 
                { 
                    sb.Append(“<td>” + dr[j] + “</td>”); 
                } 
                sb.Append(“</tr>”); 
            } 
        } 
        return sb.ToString(); 
    } 


    public bool IsReusable 
    { 
        get 
        { 
            return false; 
        } 
    }
}

References : 
Load Content While Scrolling With jQuery
Implementing infinite scrolling with jQuery