Dealing with Stateless Programming

You can save values to this object Application"ItemName" = value by using an instance of the HttpApplicationState class, but all those people who are in the application at that time wil

Dealing with Stateless Programming

One of the big differences lies in the fact that Web pages are still inherently stateless This means that when you move from page to page, you tend to lose the data that the pages use Although this is not a big problem when you are creating simple Web pages that perform easy tasks, when you begin creating real applications, especially dealing with databases or data that users input, this factor can be a big hassle

State Management on the Client

In the past, the statelessness of Web pages was dealt with to an extent in a number of ways on the client side:

• Cookies Small files that a Web application creates Cookies store data on the local

machines of those who are accessing the Web application A couple of problems exist with just using cookies First, cookies are stored on the local machine, so the machine (or browser) must allow them Second, the type of data that can be stored

in cookies is limited

• Query strings Information that is appended to the end of a page's URL You will

see examples of this when you're creating hyperlinks and calling new Web pages

• Hidden fields An HTML field that is hidden to the user These are not visible to the class module either

• Control.ViewState property Provides a dictionary object for retaining values between multiple requests for the same page Note that this is the method that the page uses to preserve page and control property values between round trips

Although some of these management techniques have been used in ASP, they are still valid in NET, and they are useful given the correct circumstances When you're

developing in ASP.NET, some additional solutions are available for state management Those solutions are handled on the server side

Server-Side Solution to State Management

Depending on the information that you want to retain, NET has a number of ways to handle state management Although you could use the methods just mentioned, a few other options are available as well:

• Application state You can save values to this object Application("ItemName") =

value by using an instance of the HttpApplicationState class, but all those people who are in the application at that time will be able to see it

• Session state This option allows you to maintain state for the individual session

This is discussed further in the next section

• Database support SQL Server helps you to maintain state by adding values to the

Temp database on the server

Using the Session Object

The Session object is pretty straightforward to work with When you assign the variable, you will use the following syntax:

Session("SessionVariableName") = Value

SessionVariableName is in fact used inside the quotes It can be whatever you want to call it

Value can be any type of variable, including a DataTable object, as you will see in the steps that follow

Sometimes, you can stash the data table and a Boolean variable to the Session object Following is one of the lines of code that stores the data table:

Session("MyLookupData") = mdtLookupData

MyLookupData is the name of the entry that is created in the Session object

mdtLookupData is the variable name of the DataTable object, declared at the module level

When you're reading a variable, you will turn the statement around and add a CType() function to convert the value to the desired type

Variable = CType(Session("SessionVariableName",Type)

In the instance of a data table, such as the one used in this How-To, you would see something like this:

mdtLookupData = CType(Session("MyLookupData"), DataTable)

Note that you can use other conversion functions besides CType() If you don't convert the value, then the Object type will be returned

Another task that is necessary to perform is testing whether the entry in the Session object has been made You do this by testing the Session entry against the Nothing keyword, as seen here:

'Put user code to initialize the page here

If Not (Session("MyLookupData") Is Nothing) Then

mdtLookupData = CType(Session("MyLookupData"), DataTable)

End If

This code is used in the Load event of the page If the entry has been created in the Session object, then the data is loaded into the variable You can also use the opposite as well If the entry in the Session object is Nothing, then create the entry

This is just a quick and simple way of taking advantage of the Session object There are probably hundreds of other ways to take advantage of the Session object For more information on this, check out Session State in the NET Framework Developer's Guide, which is part of the help provided by Visual Studio Type in "state management in

ASP.NET" for the index to locate the Session object topic

You can find all of the examples in this chapter in the Solution called Visual Basic .NET-Chapter 5 on the Web site