1042 Chapter 23 INTRODUCTION TO WEB PROGRAMMING

Using Cookies

To access and manipulate cookies, use the Cookies collection. This collection is a property of the Response and Request objects and contains all the cookies currently on the client. To send a new cookie to the client, add it to the Cookies collection of the Response object.

Figure 23.11 shows the Cookies Web project, which adds three cookies to the client and then reads them back. Click the Add button to add the corresponding cookie and the Delete button to delete it. Click the Read All Cookies button to read the cookies currently on the client. The values of all cookies (not necessarily three) will appear on a Label control at the bottom of the form.

Figure 23.11

The Cookies Web application demonstrates how to use the Cookies collection.

To add a cookie, you must first create it; the simplest method of doing so is to call the HttpCookie object’s constructor passing the name of the cookie as argument:

Dim cookie As New HttpCookie(“User”)

Then you can access the properties of the cookie object. The Value property is the cookie’s value, and the Expires property is a date/time value that determines when the cookie will expire. If you don’t set this property explicitly from within your code, the cookie will expire at the end of the session. You can also create a cookie with multiple names and values (an array type of cookie). To set up this cookie, use the Values property, which is a collection of names and keys, all stored in a single cookie. The code in Listing 23.6 creates a new cookie and adds it to the Response object’s Cookies collection.

Listing 23.6: Adding a Cookie

Private Sub AddCookie1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles AddCookie1.Click Dim cookie As New HttpCookie(“User”)

cookie.Value = txtCookie1.Text Response.Cookies.Add(cookie)

End Sub

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HANDLING MULTIPLE FORMS IN WEB APPLICATIONS 1043

Note It’s possible to add new cookies to the Request object’s Cookies collection. The Request object, however, doesn’t affect the information sent out to the client. It represents the incoming information, and any cookies you add to the Request.Cookies collection will never be sent to the client. Use the Response.Cookies collection to send cookies out to the client and the Request.Cookies collection to read the cookies from the client.

To delete a cookie, call the Remove method of the Cookies collection passing the name of the cookie to be deleted. Listing 23.7 demonstrates the method that removes the “User” cookie from the client computer.

Listing 23.7: Deleting a Cookie

Private Sub DelCookie1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles DelCookie1.Click

Request.Cookies.Remove(“User”)

End Sub

The Read All Cookies button (Listing 23.8) goes through each item of the Request.Cookies collection and prints its Name and Value properties on a table. As you can see, in addition to the cookies you’re adding to the client, there is one more cookie, the ASPSession_ID cookie. This is a string that uniquely identifies the current session.

Listing 23.8: Reading All Cookies

Private Sub GetCookies_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles GetCookies.Click Dim i As Integer

Dim cookie As HttpCookie Label1.Text = “<TABLE>”

For i = 0 To Request.Cookies.Count - 1 cookie = Request.Cookies.Item(i)

Label1.Text = Label1.Text & “<TR><TD><B>” & cookie.Name.ToString & _ “</B></TD><TD>” & cookie.Value.ToString & “</TD></TR>”

Next

Label1.Text = Label1.Text & “</TABLE>” End Sub

Handling Multiple Forms in Web Applications

The last topic in this chapter is the handling of multiple pages. Any nontrivial Web application consists of multiple pages; you should be able to invoke a Web page from within another, and even share parameters between two pages (in VB terms, pass arguments to the page you’re calling). The simplest method of invoking one page from within another is through a hyperlink. Just place a Hyperlink control on the form and set its NavigateUrl property to the ID of the form you want to invoke. If you locate this property in the Properties window, you will see a button with an ellipsis

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1044 Chapter 23 INTRODUCTION TO WEB PROGRAMMING

next to its setting. Click this button, and the Select URL dialog box appears (Figure 23.12). All the WebForms of the current project will be displayed, and you can select one by clicking its name. Or you can click the Browse button to locate any other WebForm on your server. This technique will work fine, as long as you don’t need to pass any arguments to the page you’re invoking. The new page will be sent down to the client, and it can retrieve status information through cookies or through session variables.

Figure 23.12

Use the Select URL dialog box to specify the destination of a Hyperlink control.

If you want to pass a few values to the new page, so that it can populate some of its controls or perform some other tasks before it’s sent to the client, you can use either the Response.Redirect or the Response.Execute method. Let’s look at the Redirect method first. This method accepts as argument the ID of the page to which you’re redirecting the user. As you recall from our earlier discussion, the URL may contain parameter values. You can easily pass parameter values to the new page by appending them to its URL. You must separate the parameters from the URL with a question mark and delimit each pair or parameter name/value with the ampersand symbol (&). In addition, you must URL encode the string with the parameters by calling the Server.UrlEncode method.

Let’s say the current page contains three TextBox controls (among other controls), and you want to pass the values of these parameters to the second page. The following statements prepare the URL of the page WebForm3.aspx, attach the three parameters, and then pass this string to the Redirect method:

Dim params As String

params = “Var1=” & Server.UrlEncode(TextBox1.Text) & “ & “

params = params & “Var2=” & Server.UrlEncode(TextBox2.Text) & “ & “ params = params & “Var3=” & Server.UrlEncode(TextBox3.Text) Response.Redirect(“WebForm3.aspx?” & params)

You can then read the values of the parameters from within the WebForm3 page’s code and use them to initialize the page. The following statements in the page’s Load event handler display the names and values of the parameters with the Response.Write method:

Private Sub Page_Load(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles MyBase.Load

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HANDLING MULTIPLE FORMS IN WEB APPLICATIONS 1045

‘Put user code to initialize the page here Response.Write(Request.QueryString(“Var1”))

Response.Write(Request.QueryString(“Var2”))

Response.Write(Request.QueryString(“Var3”))

End Sub

In an actual application, you’ll use these variables in slightly more complicated calculations. This example demonstrates how to access the values of the parameters, and then you can do anything you want with them.

The Execute method is different in the sense that it doesn’t actually display the new page. It accepts the URL of a page, executes it, and appends the HTML code generated by the page to the current page. The safest use of the Execute method is to read the output it generates into a StringWriter object and place it on a Label or TextBox control on the current page. The following statements call the page ShowBasket.aspx and append the output generated by this page to the current page:

Dim pageOut As New System.IO.StringWriter()

Server.Execute(“ShowBasket.aspx”, pageOut)

Response.Write(“<H2>Your basket contains the following items:</H2>”

Response.Write(“<BR>”)

Response.Write(pageOut.ToString)

The pageOut variable receives the output of the ShowBasket.aspx page and sends it to the client. The Response.Write method doesn’t clear the current page; it simply appends its output so the contents of the pageOut variable will appear at the bottom of the current page.

To access the controls of the current WebForm from within the page that’s called with the Execute method, use a loop like the following:

Private Sub Page_Load(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles MyBase.Load

‘Put user code to initialize the page here Dim key, value As String Response.Write(Request.QueryString)

For Each key In Request.Form.Keys

Response.Write(key & “ = “ & Request.Form.Item(key))

Response.Write(“<BR>”)

Next

End Sub

Compared to VB, this method of passing variables between pages is awkward. It’s true that ASP hides many of the mundane operations you would normally have to code yourself, but this aspect of ASP.NET isn’t as polished as it could be. Keep in mind, however, that Web pages don’t communicate directly with one another. The WebMultiplePage project on the CD demonstrates how to call a WebForm from within another form and pass parameters to it.

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