Web Dev with ASP.NET
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.Characteristics
ASP.NET Web pages, known officially as Web Forms, are the main building blocks for application development.[3] 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)User controls
User controls are encapsulations of sections of pages which are registered and used as controls in ASP.NET, etc.Custom controls
Programmers 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 technique
ASP.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 the class instance members that are lost with every page request/response cycle.
State management
ASP.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.Application
Application state is held by a collection of shared user-defined variables. These are set and initialized when theApplication_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.[6]Session state
Server-side session state is held by a collection of user-defined session variables that are persistent during a user session. These variables, accessed using theSession
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.[6]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.
- State server 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.
- SQL Server 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.
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.
View state
View 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 caching
ASP.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.Other
Other means of state management that are supported by ASP.NET are cookies, caching, and using the query string.Template engine
When 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.