About ASP.NET
ASP.NET is a server-side Web application framework designed for Web development to produce dynamic Web pages. It was developed by Microsoft to allow programmers to build dynamic web sites, web applications and web services. It was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft's Active Server Pages (ASP) technology. ASP.NET is built on the Common Language Runtime (CLR), allowing programmers to write ASP.NET code using any supported .NET language. The ASP.NET SOAP extension framework allows ASP.NET components to process SOAP messages.
HistoryAfter four years of development, and a series of beta releases in 2000 and 2001, ASP.NET 1.0 was released on January 5, 2002 as part of version 1.0 of the .NET Framework. Even prior to the release, dozens of books had been written about ASP.NET,[1] and Microsoft promoted it heavily as part of its platform for Web services. Scott Guthrie became the product unit manager for ASP.NET, and development continued apace, with version 1.1 being released on April 24, 2003 as a part of Windows Server 2003. This release focused on improving ASP.NET's support for mobile devices.
CharacteristicsASP.NET Web pages, known officially as Web Forms,are the main building block for application development.Web forms are contained in files with a ".aspx" extension; these files typically contain static (X)HTML markup, as well as markup defining server-side Web Controls and User Controls where the developers place all the rc content[further explanation needed] for the Web page. Additionally, dynamic code which runs on the server can be placed in a page within a block <% -- dynamic code -- %>, which is similar to other Web development technologies such as PHP, JSP, and ASP. With ASP.NET Framework 2.0, Microsoft introduced a new code-behind model which allows static text to remain on the .aspx page, while dynamic code remains in an .aspx.vb or .aspx.cs or .aspx.fs file (depending on the programming language used).
DirectivesA directive is special instructions on how ASP.NET should process the page.The most common directive is <%@ Page %> which can specify many attributes used by the ASP.NET page parser and compiler.
Examples[edit] Inline code<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "---//W3C//DTD XHTML 1.0 //EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
// Assign the datetime to label control
lbl1.Text = DateTime.Now.ToLongTimeString();
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Sample page</title>
</head>
<body>
<form id="form1" runat="server">
The current time is: <asp:Label runat="server" id="lbl1" />
</form>
</body>
</html>
The above page renders with the Text "The current time is: " and the current time.
Code-behind solutions<%@ Page Language="C#" CodeFile="SampleCodeBehind.aspx.cs" Inherits="Website.SampleCodeBehind"
AutoEventWireup="true" %>
The above tag is placed at the beginning of the ASPX file. The CodeFile property of the @ Page directive specifies the file (.cs or .vb or .fs) acting as the code-behind while the Inherits property specifies the Class from which the Page is derived. In this example, the @ Page directive is included in SampleCodeBehind.aspx, then SampleCodeBehind.aspx.cs acts as the code-behind for this page:
Source language C#:
using System;
namespace Website
{
public partial class SampleCodeBehind : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
Response.Write("Hello, world");
}
}
}
Source language Visual Basic.NET:
Imports System
Namespace Website
Public Partial Class SampleCodeBehind
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
Response.Write("Hello, world")
End Sub
End Class
End Namespace
In this case, the Page_Load() method is called every time the ASPX page is requested. The programmer can implement event handlers at several stages of the page execution process to perform processing.
User controlsUser controls are encapsulations of sections of pages which are registered and used as controls in ASP.NET.etc
Custom controlsProgrammers can also build custom controls for ASP.NET applications. Unlike user controls, these controls do not have an ASCX markup file, having all their code compiled into a dynamic link library (DLL) file. Such custom controls can be used across multiple Web applications and Visual Studio projects.
Rendering techniqueASP.NET uses a visited composites rendering technique. During compilation, the template (.aspx) file is compiled into initialization code which builds a control tree (the composite) representing the original template. Literal text goes into instances of the Literal control class, and server controls are represented by instances of a specific control class. The initialization code is combined with user-written code (usually by the assembly of multiple partial classes) and results in a class specific for the page. The page doubles as the root of the control tree.
Actual requests for the page are processed through a number of steps. First, during the initialization steps, an instance of the page class is created and the initialization code is executed. This produces the initial control tree which is now typically manipulated by the methods of the page in the following steps. As each node in the tree is a control represented as an instance of a class, the code may change the tree structure as well as manipulate the properties/methods of the individual nodes. Finally, during the rendering step a visitor is used to visit every node in the tree, asking each node to render itself using the methods of the visitor. The resulting HTML output is sent to the client.
After the request has been processed, the instance of the page class is discarded and with it the entire control tree. This is a source of confusion among novice ASP.NET programmers who rely on class instance members that are lost with every page request/response cycle.
State managementASP.NET applications are hosted by a Web server and are accessed using the stateless HTTP protocol. As such, if an application uses stateful interaction, it has to implement state management on its own. ASP.NET provides various functions for state management. Conceptually, Microsoft treats "state" as GUI state. Problems may arise if an application needs to keep track of "data state"; for example, a finite-state machine which may be in a transient state between requests (lazy evaluation) or which takes a long time to initialize. State management in ASP.NET pages with authentication can make Web scraping difficult or impossible.
ApplicationApplication state is held by a collection of shared user-defined variables. These are set and initialized when the Application_OnStart event fires on the loading of the first instance of the application and are available until the last instance exits. Application state variables are accessed using the Applications collection, which provides a wrapper for the application state. Application state variables are identified by name.
Session stateServer-side Session state is held by a collection of user-defined session variables that are persistent during a user session. These variables, accessed using the Session collection, are unique to each session instance. The variables can be set to be automatically destroyed after a defined time of inactivity even if the session does not end. Client-side user session is maintained by either a cookie or by encoding the session ID in the URL itself.
ASP.NET supports three modes of persistence for server-side session variables:
In-Process Mode
The session variables are maintained within the ASP.NET process. This is the fastest way; however, in this mode the variables are destroyed when the ASP.NET process is recycled or shut down.
ASP State Mode
ASP.NET runs a separate Windows service that maintains the state variables. Because state management happens outside the ASP.NET process, and because the ASP.NET engine accesses data using .NET Remoting, ASPState is slower than In-Process. This mode allows an ASP.NET application to be load-balanced and scaled across multiple servers. Because the state management service runs independently of ASP.NET, the session variables can persist across ASP.NET process shutdowns. However, since session state server runs as one instance, it is still one point of failure for session state. The session-state service cannot be load-balanced, and there are restrictions on types that can be stored in a session variable.
SqlServer Mode
State variables are stored in a database, allowing session variables to be persisted across ASP.NET process shutdowns. The main advantage of this mode is that it allows the application to balance load on a server cluster, sharing sessions between servers. This is the slowest method of session state management in ASP.NET.
ASP.NET session state enables you to store and retrieve values for a user as the user navigates ASP.NET pages in a Web application. HTTP is a stateless protocol. This means that a Web server treats each HTTP request for a page as an independent request. The server retains no knowledge of variable values that were used during previous requests. ASP.NET session state identifies requests from the same browser during a limited time window as a session, and provides a way to persist variable values for the duration of that session. By default, ASP.NET session state is enabled for all ASP.NET applications.
Alternatives to session state include the following:
Application state, which stores variables that can be accessed by all users of an ASP.NET application.
Profile properties, which persists user values in a data store without expiring them.
ASP.NET caching, which stores values in memory that is available to all ASP.NET applications.
View state, which persists values in a page.
Cookies.
The query string and fields on an HTML form that are available from an HTTP request.
For a comparison of different state-management options, see ASP.NET State Management Recommendations. Session
View stateView state refers to the page-level state management mechanism, utilized by the HTML pages emitted by ASP.NET applications to maintain the state of the Web form controls and widgets. The state of the controls is encoded and sent to the server at every form submission in a hidden field known as __VIEWSTATE. The server sends back the variable so that when the page is re-rendered, the controls render at their last state. At the server side, the application may change the viewstate, if the processing requires a change of state of any control. The states of individual controls are decoded at the server, and are available for use in ASP.NET pages using the ViewState collection.
The main use for this is to preserve form information across postbacks. View state is turned on by default and normally serializes the data in every control on the page regardless of whether it is actually used during a postback. This behavior can (and should) be modified, however, as View state can be disabled on a per-control, per-page, or server-wide basis.
Developers need to be wary of storing sensitive or private information in the View state of a page or control, as the base64 string containing the view state data can easily be de-serialized. By default, View state does not encrypt the __VIEWSTATE value. Encryption can be enabled on a server-wide (and server-specific) basis, allowing for a certain level of security to be maintained.
Server-side cachingASP.NET offers a "Cache" object that is shared across the application and can also be used to store various objects. The "Cache" object holds the data only for a specified amount of time and is automatically cleaned after the session time-limit elapses.
OtherOther means of state management that are supported by ASP.NET are cookies, caching, and using the query string.
Template engineWhen first released, ASP.NET lacked a template engine. Because the .NET Framework is object-oriented and allows for inheritance, many developers would define a new base class that inherits from "System.Web.UI.Page", write methods there that render HTML, and then make the pages in their application inherit from this new class. While this allows for common elements to be reused across a site, it adds complexity and mixes source code with markup. Furthermore, this method can only be visually tested by running the application – not while designing it. Other developers have used include files and other tricks to avoid having to implement the same navigation and other elements in every page.
ASP.NET 2.0 introduced the concept of "master pages", which allow for template-based page development. A Web application can have one or more master pages, which, beginning with ASP.NET 2.0, can be nested.[10] Master templates have place-holder controls, called ContentPlaceHolders to denote where the dynamic content goes, as well as HTML and JavaScript shared across child pages.
Child pages use those ContentPlaceHolder controls, which must be mapped to the place-holder of the master page that the content page is populating. The rest of the page is defined by the shared parts of the master page, much like a mail merge in a word processor. All markup and server controls in the content page must be placed within the ContentPlaceHolder control.
When a request is made for a content page, ASP.NET merges the output of the content page with the output of the master page, and sends the output to the user.
The master page remains fully accessible to the content page. This means that the content page may still manipulate headers, change title, configure caching etc. If the master page exposes public properties or methods (e.g. for setting copyright notices) the content page can use these as well.
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