State Management in ASP.NET
A new instance of the Web page class is created each time the page is posted to the server.
In traditional Web programming, all information that is associated with the page, along with the controls on the page, would be lost with each roundtrip.
The Microsoft ASP.NET framework includes several options to help you preserve data on both a per-page basis and an application-wide basis. These options can be broadly divided into the following two categories:
Client-Side State Management Options
Server-Side State Management Options
Client-Side State Management
Client-based options involve storing information either in the page or on the client computer.
Some client-based state management options are:
Hidden fields
View state
Cookies
Query strings
Hidden Fields
A hidden field is a control similar to a TextBox control.
A hidden field does not render in a Web browser. A user cannot type anything into it.
Hidden fields can be used to store information that needs to be submitted along with the web page, but should not be displayed on the page.
Hidden fields can also be used to pass session information to and from forms, transparently.
To pass the information from one page to another, you can invoke the Server.Transfer method on the Click event of a Button control, as shown in the following example:
protected void Button1_Click(object sender, EventArgs e)
{
Server.Transfer("Page2.aspx");
}
The target page can access the passed information by referring to individual controls on the source page, as shown in the following example:
String name = Request.Form["TextBox1"];
String color = Request.Form["HiddenField1"];
View State
Each Web page and controls on the page have a property called ViewState.
This property is used to automatically save the values of the Web page and each control on the Web page prior to rendering the page.
The view state is implemented using a hidden form field called _VIEWSTATE.
This hidden form field is automatically created in every Web page.
When ASP.NET executes a Web page on the Web server, the values stored in the ViewState property of the page and controls on it are collected and formatted into a single encoded string.
The encoded string is:
Assigned to the Value attribute of the hidden form field, _VIEWSTATE.
Sent to the client as part of the Web page.
During postback of a Web page to itself, one of the tasks performed by ASP.NET is to restore the values in _VIEWSTATE.
Enabling and disabling view state
By default, the view state is enabled for a Web page and the controls on the Web page.
You can enable or disable view state for a page by setting the EnableViewState property of a Web page, as shown in the following example:
<%@ Page Language="C#"AutoEventWireup="true"EnableViewState="false"CodeFile="Page1.aspx.cs"Inherits="Page1" %>
You can enable or disable view state for a control by setting its EnableViewState property to false.
When view state is disabled for a page, the view state for the controls on the page is automatically disabled.
Cookies
Cookies
Cookies are:
Used to store small pieces of information related to a user's computer such as its IP address, browser type, operating system, and Web pages last visited.
Sent to a client computer along with the page output.
Types of cookies
Temporary Cookies
Exist in the memory space of a browser.
Also known as session cookies.
Are useful for storing information required for only a short time.
Persistent Cookies
Are saved as a text file in the file system of the client computer.
Are used when you want to store information for a longer period.
Are useful for storing information required for only a short time.
Creating Cookies
Response.Cookies["userName"].Value = "Peter";
Response.Cookies["userName"].Expires = DateTime.Now.AddDays(2);
Reading Cookies
You can access the value of a cookie using the Request built-in object.
if (Request.Cookies["userName"].Value != null)
{
Label1.Text = Request.Cookies["userName"].Value;
}
Query String
A query string:
Provides a simple way to pass information from one page to another.
Is the part of a URL that appears after the question mark (?) character.
You can pass data from one page to another page in the form of a query string using the Response.Redirect method, as shown in the following example:
Response.Redirect("BooksInfo.aspx?Category=fiction&Publisher=Sams");
Server-Side State Management
There are situations where you need to store the state information on the server side.
Server-side state management enables you to manage application-related and session-related information on the server.
ASP.NET provides the following options to manage state at the server side:
Application state
Session state
Application State
ASP.NET provides application state as a means of storing application-specific information such as objects and variables.
The following describes the information in the application state:
Is stored in a key-value pair.
Is used to maintain data consistency between server round trips and among pages.
Application state is created the first time a user accesses any URL resource in an application.
After an application state is created, the application-specific information is stored in it.
Storing and Reading information in application state
You can add application-specific information to an application state by creating variables and objects and adding them to the application state.
For example:
Application ["MyVariable"] = "Hello";
You can read the value of MyVariable using the following code snippet:
stringval = (string) Application["MyVariable"];
Removing information from application state
You can remove an existing object or variable, such as MyVariable from an application state using the following code snippet:
Application.Remove(["MyVariable"]);
You can also remove all the application state variables and objects by using the following code snippet:
Application.RemoveAll();
Synchronizing application state
Multiple pages within an ASP.NET web application can simultaneously access the values stored in an application state, that can result in conflicts and deadlocks.
To avoid such situations, the HttpApplicationState class provides two methods, Lock() and Unlock().
These methods allow only one thread at a time to access application state variables and objects.
Session State
In ASP.NET, session state is used to store session-specific information for a web application.
The scope of session state is limited to the current browser session.
Session state is structured as a key-value pair for storing session-specific information that needs to be maintained between server round trips and between requests for pages.
Session state is not lost if the user revisits a Web page by using the same browser window.
However, session state can be lost in the following ways:
When the user closes and restarts the browser.
When the user accesses the same Web page in a different browser window.
When the session times out because of inactivity.
When the Session.Abandon() method is called within the Web page code.
Each active ASP.NET session is identified and tracked by a unique 120-bit SessionID string containing American Standard Code for Information Interchange (ASCII) characters.
You can store objects and variables in a session state.
You can add a variable, MyVariable with the value HELLO in the session state using the following code snippet:
Session["MyVariable"]="HELLO";
You can retrieve the value of the variable, MyVariable, using the following code snippet:
String val = (string)Session["MyVariable"];
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