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To delete a row, we call the DataRow’s Delete method. The code behind the Delete Product button executes the following statement:

DsProducts1.Products(Me.BindingContext(DsProducts1, _

“Products”).Position).Delete()

The expression

Me.BindingContext(DsProducts1, “Products”).Position

returns the index of the selected row in the DataTable—an integer value. DSProducts1.Products is a DataTable object that represents the Products table in the DataSet. By calling this object with a numeric value as argument, we isolate a DataRow object, which represents the selected row in the Products table. Finally, we call the Delete method on this row to delete it. The row isn’t actually removed from the DataSet. It will be removed only when we call the DataSet’s AcceptChanges method—and this method must be called only after the underlying row has been successfully removed from the underlying table.

Open the DataEntry project on the CD and experiment with its code. See how the ComboBox controls were bound to the DataSet so that they always display the selected product’s category and supplier. You can add more features to the application, starting with a better navigational model.

The rows on the ListBox control are displayed in the same order as they were retrieved from the original table. In a “real” table with thousands of products, you wouldn’t want to read the entire table into the DataSet. You should allow the user to specify the subset of the products they’re interested in and upload only the matching rows to the client.

Building and Using Custom DataSets

In this section, we’ll build a DataSet that’s not connected to a data store. We’ll create a new DataSet by specifying its schema with the XML Schema tools of the Toolbox. Then we’ll make this DataSet the data source of a DataGrid control, and we’ll be able to add data directly on the grid. With a few statements, we’ll be able save the contents of the DataSet to a disk file and load it back from the disk file into the application.

Start a new project (it’s the XMLGrid project on the CD) and add an XMLSchema component to the project. Right-click the project’s name and, from the context menu, select Add New Item. Select the XML Schema template, specify a name for the new component (or accept the default name), and click OK to close the dialog box. An XMLSchema component will be added to the project; its default name is XMLSchema1.xsd. Double-click its icon to open it on the design surface.

Open the Toolbox, which now contains the XML Schema tools, and double-click the Element tool to add a new element to the XMLSchema. An XML element is equivalent to the table of a regular DataSet. The new XML element will be depicted on the design surface by an empty box. The default element’s name is element1; change it to GridData. Each XML element has one or more attributes, so let’s add a few. Open the Toolbox and drag the icon of the Attribute tool on the existing element. By default, new attributes are named attribute1, attribute2, and so on, and their default type is “string.” Figure 22.5 shows an XML element with four attributes and the XML Toolbox.

BUILDING AND USING CUSTOM DATASETS 993

Figure 22.5

Designing an XML schema

The box with the attribute’s type is a drop-down list of all the data types you can assign to an attribute. The schema shown in Figure 22.5 contains an element with three string attributes and one positive Integer attribute. It’s like a table with four columns—three string columns and one numeric column. As you will see shortly, if you attempt to assign a value of the incorrect type to an attribute, the attribute’s current value won’t change.

So far you’ve created the schema of the DataSet—the information that came from the database in the previous examples. Now we must use the XMLSchema component to create a DataSet object. Right-click somewhere on the XMLSchema design surface and, from the context menu, select Generate Dataset. Switch to the form and add a DataSet object. The Add Dataset dialog box pops up, prompting you for the dataset’s schema information, as shown in Figure 22.6. Click OK and a new typed DataSet will be added to the project.

Figure 22.6

Adding a typed

DataSet to a project

At this point, you’re ready to use the custom DataSet. Place a DataGrid control on the form, set the control’s DataSource property to xmlschema11.GridData (you’ll select this item from a list), and run the project. You can optionally set some of the DataGrid control’s attributes. You have a DataGrid control that you can put any type of information on or use as a data-entry tool.

994 Chapter 22 PROGRAMMING THE ADO.NET OBJECTS

You can also add multiple elements to the XMLSchema and relate them to one another. Create each element separately and then add the appropriate relationships with the Relation tool.

Figure 22.7 shows the form of a simple application that allows you to populate a DataGrid control and persists the data to a file. It can also read the data from the disk file.

Figure 22.7

The main form of the

XMLGrid project

To persist the data on the grid to a disk file, insert the code of Listing 22.15 in the Persist Data button’s Click event handler. The Read Data button reads the data from the same file and populates the DataGrid control, and the code behind it is shown in Listing 22.16.

Listing 22.15: Persisting DataSets as XML Files

Private Sub Button1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles Button1.Click

Dim FS As Stream

FS = New FileStream(“C:\GRID.XML”, FileMode.OpenOrCreate)

XmlSchema11.WriteXml(FS)

FS.Close()

End Sub

Listing 22.16: Populating a DataGrid Control with XML Data

Private Sub Button2_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles Button2.Click

Dim FS As Stream

FS = New FileStream(“C:\GRID.XML”, FileMode.Open)

XmlSchema11.Clear()

XmlSchema11.ReadXml(FS)

FS.Close()

End Sub

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You should also import the System.IO namespace, so that you won’t have to fully qualify the FileStream objects. The XMLGrid project demonstrates how to design schemas for your DataSets. You can also create generic DataSets and use them as data-entry tools for many operations.

If you open the GRID.XML file created by this application, you’ll see that it contains XML code. Figure 22.8 shows the XML generated by the data shown in Figure 22.7, opened with Internet Explorer.

Figure 22.8

Viewing an XML file with Internet Explorer

You may wish to explore XML in depth, as it’s one of the hottest topics in the industry today. ADO.NET uses XML to format the data as well as describe the structure of its DataSets, and you can take advantage of this to move DataSets between computers or between applications. You’re actually taking advantage of XML without having to write a single line of XML code.

Summary

You’ve learned a lot about ADO.NET in Chapters 21 and 22, but there’s even more to learn. ADO.NET is one of the most elaborate and complex components of .NET, and it couldn’t be exhausted in two chapters. These chapters were meant to help you familiarize yourself with the visual database tools, understand the structure of a DataSet, learn how to navigate the related tables of the DataSet and the rows of the individual DataTables, and know how to execute commands against the database. You should also be able to improve the code generated by the DataForm wizard and generate more robust and user-friendly applications.

Part VI

VB.NET on the Web

In this section:

Chapter 23: Introduction to Web Programming

Chapter 24: Accessing Data on the Web

Chapter 25: XML Web Services

Chapter 23

Introduction to

Web Programming

If there is one technology that caught on overnight and has affected more computer users than any other, it is the World Wide Web—the set of all public Web sites and the documents they can provide to clients. The computers that host Web sites are called servers; their service is to provide the documents to the clients that request them. Clients are the millions of personal computers connected to the Internet. To exploit the Web, all you need is a browser, such as Internet Explorer, that can request documents and render them on your computer.

This chapter is a compendium of information on how to apply the knowledge you acquired in previous chapters to the Web—or, to use a popular term, how to leverage your knowledge of Visual Basic by applying it to the Web. To do so, you need a basic understanding of Hypertext Markup Language (HTML), the language used to build Web documents, and a good understanding of how the clients interact with the servers on the Internet. Building a Web application with Visual Studio is very similar to building a Windows application, but there are many differences you should be aware of.

The Internet is a global, distributed network of computers that use a common protocol to communicate: Transmission Control Protocol/Internet Protocol (TCP/IP). TCP/IP is a simple protocol because it had to be implemented consistently on all computers and operating systems. Indeed, TCP/IP is a truly universal protocol, but you needn’t know much about it. It’s there when you need it and allows your computer to connect to any other computer on the Internet.

If TCP/IP enables any two computers on the Internet to talk to each other, why do we need any other protocol? Hypertext Transfer Protocol (HTTP) is the protocol of the Web. Whereas TCP/IP allows two computers to connect on the hardware level, HTTP is the language servers and clients use to exchange information. HTTP is optimized for requesting and supplying HTML documents. The Internet is more than the Web (although it seems the Web is taking over). To exchange files through the Internet, for example, computers use File Transfer Protocol (FTP). The protocol that is used depends on the type of information to be exchanged. All other protocols, however, run on top of TCP/IP.