ViewState

Another approach to saving data for the user, is the ViewState. As described elsewhere in this tutorial, the ViewState allows ASP.NET to repopulate form fields on each postback to the server, making sure that a form is not automatically cleared when the user hits the submit button. All this happens automatically, unless you turn it off, but you can actually use the ViewState for your own purposes as well. Please keep in mind though, that while cookies and sessions can be accessed from all your pages on your website, ViewState values are not carried between pages. Here is a simple example of using the ViewState to carry values between postbacks:
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!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>ViewState</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:TextBox runat="server" id="NameField" />
        <asp:Button runat="server" id="SubmitForm" onclick="SubmitForm_Click" text="Submit & set name" />
        <asp:Button runat="server" id="RefreshPage" text="Just submit" />
        <br /><br />
        Name retrieved from ViewState: <asp:Label runat="server" id="NameLabel" />
    </form> 
</body>
</html>
And the CodeBehind:
using System;
using System.Data;
using System.Web;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if(ViewState["NameOfUser"] != null)
            NameLabel.Text = ViewState["NameOfUser"].ToString();
        else
            NameLabel.Text = "Not set yet...";
    }

    protected void SubmitForm_Click(object sender, EventArgs e)
    {
        ViewState["NameOfUser"] = NameField.Text;
        NameLabel.Text = NameField.Text;
    }
}
Try running the project, enter your name in the textbox and press the first button. The name will be saved in the ViewState and set to the Label as well. No magic here at all. Now press the second button. This one does nothing at all actually, it just posts back to the server.

As you will notice, the NameLabel still contains the name, but so does the textbox. The first thing is because of us, while the textbox is maintained by ASP.NET it self. Try deleting the value and pressing the second button again. You will see that the textbox is now cleared, but our name label keeps the name, because the value we saved to the ViewState is still there!

ViewState is pretty good for storing simple values for use in the form, but if you wish to save more complex data, and keep them from page to page, you should look into using cookies or sessions, as described in the previous chapters.
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