If Not IsPostBack Then message += "Page first accessed at " + DateTime.Now + ".<br/>" End If in Visual Studio .NET

Drawing QR-Code in Visual Studio .NET If Not IsPostBack Then message += "Page first accessed at " + DateTime.Now + ".<br/>" End If

Then you assign the string to the Text property of the label:

That s all easy enough, but you need that string for the next time the page is posted, so you need to store it in the state bag. You simply create a new item in the dictionary, give it a name, and then assign the string to it:

Now you need to account for when the page is posted back, so you have to go back up and add an Else clause to your If statement. This time, you want to retrieve the previous message first, so you get it back from the state bag, and use CType to convert it to a string. Then you can add the rest of the message just as you did in the first half of the If:

Else message = CType(ViewState("message"), String) + "Page posted back at " _ + DateTime.Now + ".<br/>"

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The statements to assign the message to the label and then store the message back in the state bag happen outside the If, so there s no need to repeat them here. The entire event handler is shown in Example C-22.

Example C-22. The event handler for the Page_Load event in Exercise 7-3

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) _ Handles Me.Load Dim message As String = "" If Not IsPostBack Then message += "Page first accessed at " + DateTime.Now + ".<br/>" Else message = CType(ViewState("message"), String) + "Page posted back at " _ + DateTime.Now + ".<br/>" End If lblMessage.Text = message ViewState("message") = message End Sub

As a bonus, try adding EnableViewState = "False" to the Page directive, and then run the application again. Instead of appending each new line to the string, every postback will overwrite the existing string. Exercise 7-3. For this exercise, you need to use the Session dictionary instead of the state bag. Start by adding the two new buttons to Default.aspx, and give each of them a handler that uses Response.Redirect to point to SecondPage.aspx and ThirdPage.aspx. You ll need to make some changes to the code-behind file of Default.aspx. First, replace the ViewState methods with Session. In addition, you have to make a change to the first half of the If statement:

If Not IsPostBack Then message += "Page first accessed at " + DateTime.Now + ".<br/>"

If you navigate to another page, and then return to the Home page, the message will be defined again as a blank string. If that happens, you need to retrieve Session("message") from the session state, even the first time the page loads, when Session("message") will be empty:

If Not IsPostBack Then message = CType(Session("message"), String) + _ "Home page first accessed at " + DateTime.Now + ".<br/>"

The entirety of the code-behind for Default.aspx should now look like Example C-23.

Example C-23. The code-behind file for Default.aspx in Exercise 7-3

Partial Class _Default Inherits System.Web.UI.Page Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)_ Handles Me.Load Dim message As String = "" If Not IsPostBack Then message = CType(Session("message"), "Home page first accessed Else message = CType(Session("message"), "Home page posted back at End If lblMessage.Text = message Session("message") = message End Sub Protected Sub btnPage2_Click(ByVal sender As Object, ByVal e As _ System.EventArgs) Handles btnPage2.Click Response.Redirect("SecondPage.aspx") End Sub Protected Sub btnPage3_Click(ByVal sender As Object, ByVal e As _ System.EventArgs) Handles btnPage3.Click Response.Redirect("ThirdPage.aspx") End Sub End Class

String) + _ at " + DateTime.Now + ".<br/>" String) + _ " + DateTime.Now + ".<br/>"

Now create SecondPage.aspx and ThirdPage.aspx. These two pages need the same controls as Default.aspx, and the same code-behinds, but be sure to change the messages and the Response.Redirect targets accordingly. This time, if you add EnableViewState = "False" to the Page directive, the application doesn t change, because the string is stored in state instead. If you disable session state and try to run the application, you ll get an error.

8: Errors, Exceptions, and Bugs, Oh My!