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original error message is embedded in the Exception.Message property, but it’s a rather scary message for the end user.

Figure 25.8

This exception was raised by the Web service’s class.

Maintaining State in Web Services

Like ASP.NET applications, Web services can also maintain state with the Application and Session objects. By default, a Web service doesn’t maintain its state, but you can change the default behavior by setting the EnableSession attribute of the WebMethod tag to True. Listing 25.4 shows the GetMyID method, which returns the ID of the current client. The program sets the ID when requested, but this is for demonstration purposes only. You should probably set this variable when the client goes through the authentication process.

Listing 25.4: Maintaining State with a Web Service

<WebMethod(EnableSession:=True)> Public Function GetMyID() As Guid

If Session(“MyID”) Is Nothing Then

Session(“MyID”) = Guid.NewGuid

End If

Return Session(“MyID”)

End Function

The variable MyID maintains its value through the duration of the session. You can also maintain application-level variables with the Application object.

A Data-Driven Web Service

I’m sure you had enough with trivial examples; let’s move on to something more practical. In this section, we’ll build a Web service that moves DataSets to the client. This is the most practical thing you can do with Web services, and we’ll consume this service from within an ASP application, as well as a VB application. Our Web service will expose two methods, the GetCategories and GetProducts methods. The first DataSet contains the product categories of the Northwind database. This is a very simple DataSet. The GetProducts method returns a DataSet with the Categories and Products tables. As you will see, we’ll be able to bind a DataGrid control to this DataSet and display it hierarchically on the browser.

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Start a new Web service project, name it Products, and drop the Categories and Products tables from the Server Explorer on the Service1.asmx file’s designer surface. Two DataAdapter objects will be created automatically, one for each table. Rename them to DACategories and DAProducts. Then generate two DataSets, the DSCategories and DSProducts DataSets. I need not repeat the entire process here (it’s described in detail in Chapters 21 and 22), so I’ll only show you the SELECT statements for the two DataAdapters. The statement for the DACategories DataAdapter is

SELECT CategoryID, CategoryName, Description

FROM dbo.Categories

Here’s the statement for the DAProducts DataAdapter:

SELECT ProductID, ProductName, SupplierID,

CategoryID, QuantityPerUnit, UnitPrice,

UnitsInStock, UnitsOnOrder, ReorderLevel,

Discontinued

FROM dbo.Products

Once the two DataAdapters are in place, create two DataSets, the DSCategories and DSProducts DataSets. Don’t forget to check the option Add This DataSet To The Designer on the Generate DataSet dialog box. There should be five items on the service’s design surface: the SqlConnection1 object, the two DataAdapter objects, and one instance of each DataSet object (DSCategories1 and DSProducts1), as shown in Figure 25.9.

Figure 25.9

Adding DataSets to a Web service

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This is a good point to change the default name of the Web service from Service1.asmx to Products.asmx. Changing the name of the component on the Solution Explorer isn’t adequate; for some reason, the IDE doesn’t change the name of the class that implements the Web service. Rightclick the design surface of the service and select View Code. The second line in this class is:

Public Class Service1

and you must change it to

Public Class Products

Now add the two methods to the class, using the code from Listing 25.5.

Listing 25.5: The GetCategories and GetPRoducts Methods

<WebMethod()> Public Function GetCategories() As DataSet

DACategories.Fill(DSCategories1, “Categories”)

Return DSCategories1

End Function

<WebMethod()> Public Function GetProducts() As DataSet

DAProducts.Fill(DSProducts1, “Products”)

DACategories.Fill(DSProducts1, “Categories”)

Return DSProducts1

End Function

Each method fills the appropriate DataSet and returns it to the calling application. In our case, the calling application is an application running remotely, but Visual Studio hides all the details from us. We write code as if the Web service’s methods were local functions.

We have built our Web service, so let’s test it. Press F5 and, a few seconds later, you’ll see Internet Explorer displaying the summary of the Products Web service. Click the GetCategories link, and on the following page click the Invoke button (there are no arguments to specify here). You will see the page of Figure 25.10, which shows the rows of the Categories table in XML format. Close this window and then follow the Products hyperlink. This time you’ll see all the categories and all products. The Categories DataSet contains the rows of the Categories table, but the Products table contains the rows of both the Categories and Products tables. The tables of the Products DataSet are not related. You can add a relationship between the two tables in the DataSet designer, but I’ll show you later how to add a relation in the consumer application’s code.

The Web service works. You can stop it and start building a consumer application for this service. Testing a Web service as a stand-alone application is straightforward and helps you make sure the Web service returns the correct result. If you want to call its method from within another application (probably a remote application), you must deploy it on your Web server.

Select Project Copy Project, and on the dialog box that appears (shown in Figure 25.6), specify the name of the Web service’s root folder. Let’s put our Web service in the folder Merchant under the server’s root folder.

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Figure 25.10

Viewing a DataSet on the browser

Now we’re ready to build the test applications. Let’s start with an ASP.NET Web application that will display a single page with a DataGrid control bound to the Categories DataSet. Create a new ASP application project to test the new Web service. The new project is called NWTest, and it’s a single page with a DataGrid control and a Button control on it. The DataGrid’s DataSource property will be set to the GetCategories method at runtime, every time the button is clicked. The application will go out to the Service1 Web service, request the GetCategories method, and then bind the DataSet returned by the method to the DataGrid1.DataSet property. Figure 25.11 shows the NWTest page with the DataGrid populated with the categories. The appearance of the DataGrid control leaves a lot to be desired, but you saw how to customize the control in the previous chapter.

As before, you must add a reference to the Web service through the Project Add Web Reference command. Place a DataGrid control and a Button control on the Web form, and then enter the following code in the button’s Click event handler:

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

ByVal e As System.EventArgs) Handles Button1.Click

Dim WS As New localhost.Service1()

DataGrid1.DataSource = WS.GetCategories

DataGrid1.DataBind()

End Sub

Consuming the Products Web Service in VB

Now we’ll build a Windows application to consume the Products Web service. The .NET version of the DataGrid control can display related tables, and we’ll take advantage of this feature of the control. As you recall, the GetProducts method returns a DataSet that contains two tables, but no relationship between them. The test project is called WSDataSet, and you will find it on the CD. The main form of the application is shown in Figure 25.12.

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Figure 25.11

Binding a DataGrid Web control to a method of a Web service

Figure 25.12

The WSDataSet project consumes the Products Web service.

As usual, add a reference to the Web service, then enter the code shown in Listing 25.6 behind the Populate Grid button.

Listing 25.6: Binding a Windows DataGrid Control to a Member of a Web Service

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

ByVal e As System.EventArgs) Handles Button1.Click Dim WS As New localhost.Service1()

Dim localDS As DataSet localDS = WS.GetProducts