Microsoft ASP.NET 4 : Caching and State Management - Introduction to Session State

Create a new Empty ASP.NET Web Application. Name it SessionState.

In the default page (Default.aspx), drag a text box to enter a value that will be stored in session state. Add a label to identify the TextBox, too, as shown here:

Drag two buttons and a label onto the form. The first button is for adding data to the session string. The second is to show the session state. Set the Text property of the first button to Add String To Session State. Then, give the button the value AddStringToSessionState as an ID and double-click it in the Designer to insert an event handler in the code-behind file. This button ultimately adds the string from the text box to the page's session state. Doing so can help you distinguish the buttons later on. It doesn't matter what you name the second button (I used the text "Just Submit" and kept the default ID). The first button submits the string to the server to be saved in a local field (for now), and the other button simply performs a postback. This way, you can see the ephemeral nature of page member variables. Name the label LabelShowString. You use it to display the value of the string.

Add a string variable member to the page named sessionString. In the Page_Load handler, set the text box on the page to the value of the string. Then, turn your attention to the event handler for the SubmitString button. Have the handler take the Text property from the TextBox1 and store it in the page member variable. Then, set the LabelShowString label text to the value of the string like so:

public partial class _Default : System.Web.UI.Page
{
  string sessionString;
  protected void Page_Load(object sender, EventArgs e)
  {
    this.LabelShowString.Text = this.sessionString;
  }
  protected void AddStringToSessionState_Click(object sender, EventArgs e)
  {
    this.sessionString = this.TextBox1.Text;
    this.LabelShowString.Text = this.sessionString;
  }
}
    }

Now run the program. Type a string into the text box and click Add String To Session State. When the post goes to the page, the page will show the string in the label like this:

Now click the Just Submit button. What happens? Remember, Page_Load simply looks at the value of the sessionString member variable and stuffs it into the label. Pages (and HTTP handlers in general) are very short-lived objects. They live for the duration of the request and then are destroyed—along with all the data they hold.

The sessionString member variable evaporated as soon as the last request finished. A new sessionString member variable (which was empty) was instantiated as soon as the page was re-created:

You take the data from the text box and store it in a member variable in the Page class. The lifetime of the page is very short. The page lives long enough to generate a response, and then it disappears. Any state you store as data members in the page disappears too. That's why, when you click the Just Submit button, you don't see the string displayed. You do see the string when you click Add String To Session State because the member variable survives long enough to support the button's Click event handler.

Using session state is a way to solve this issue. To show this, add a new label named LabelShowStringAsSessionState to the page. This one will show the data as retrieved from the Session object:

Write code to store the string in session state. Have the AddStringToSessionState take the text from TextBox1 and store it in the Session object. Then, update the Page_Load method to display the value as it came from session state, as shown in bold type in the following code:

public partial class _Default : System.Web.UI.Page
{
  string sessionString;

  protected void Page_Load(object sender, EventArgs e)
  {
    this.LabelShowString.Text = this.sessionString;
    this.LabelShowStringAsSessionState.Text =
     (string)this.Session["sessionString"];
  }

  protected void AddStringToSessionState_Click(object sender, EventArgs e)
  {
    // store in member variable
    this.sessionString = this.TextBox1.Text;

    // store in session state
    this.Session["sessionString"] = this.TextBox1.Text;

    // show member variable
    this.LabelShowString.Text = this.sessionString;

    // show session state
    this.LabelShowStringAsSessionState.Text =
     (string)this.Session["sessionString"];
  }
}

    }

					  

Run the program. Type in a string and click the Add String To Session State button. Both labels should contain data. The LabelShowString label holds data because the SubmitString handler made the member variable assignment. The LabelShowStringAsSessionState label also shows data because the handler stored that text in session state:

Now click the Just Submit button and see what happens:

In this case, the page was simply submitted, causing only the Page_Load to be executed. Page_Load displays both the sessionString member variable (which is empty because it lives and dies with the page) and the data from the Session object (which lives independently of the page).