To illustrate the techniques used to implement a web service in Visual Basic .NET using the services classes of the .NET Framework, you will build a simple calculator and then make use of its functions over the Web.
Begin by specifying the web service. To do so, define a class that inherits from System.Web.Services.WebService. The easiest way to create this class is to open Visual Studio and create a new ASP.NET Web Service project. The default name that Visual Studio provides is WebService1, but you might want to choose something more appropriate.
Visual Studio .NET creates a skeleton web service and even provides a .NET Web Service example method for you to replace with your own code, as shown in Example 16-1.
Option Strict On
Imports System
Imports System.Web.Services
<WebService(Namespace := "http://tempuri.org/")> _
Public Class Service1
Inherits System.Web.Services.WebService
#Region " Web Services Designer Generated Code "
Public Sub New( )
MyBase.New( )
'This call is required by the Web Services Designer.
InitializeComponent( )
'Add your own initialization code after the InitializeComponent( ) call
End Sub
'Required by the Web Services Designer
Private components As System.ComponentModel.IContainer
'NOTE: The following procedure is required by the Web Services Designer
'It can be modified using the Web Services Designer.
'Do not modify it using the code editor.
<System.Diagnostics.DebuggerStepThrough( )> Private Sub InitializeComponent( )
components = New System.ComponentModel.Container( )
End Sub
Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
'CODEGEN: This procedure is required by the Web Services Designer
'Do not modify it using the code editor.
If disposing Then
If Not (components Is Nothing) Then
components.Dispose( )
End If
End If
MyBase.Dispose(disposing)
End Sub
#End Region
' WEB SERVICE EXAMPLE
' The HelloWorld( ) example service returns the string Hello World.
' To build, uncomment the following lines then save and build the project.
' To test this web service, ensure that the .asmx file is the start page
' and press F5.
'
'<WebMethod( )> Public Function HelloWorld( ) As String
' HelloWorld = "Hello World"
' End Function
End Class
Create five methods: Add( ), Sub( ), Mult( ), Div( ), and Pow( ). Each takes two parameters of type Double, performs the requested operation, and then returns a value of the same type. For example, here is the code for raising a number to some specified power:
Function Pow(ByVal x As Double, ByVal y As Double) As Double
Dim retVal As Double = x
Dim i As Integer
For i = 0 To (y - 1) - 1
retVal *= x
Next i
Return retVal
End Function 'Pow
To expose each method as a web service, you simply add the <WebMethod> attribute before each method declaration (attributes are discussed in Chapter 8). You are not required to expose all the methods of your class as web methods. You can pick and choose, adding the <WebMethod> attribute only to those methods you want to expose.
That's all you need to do; .NET takes care of the rest.
WSDL and NamespacesYour web service will use a Web Service Description Language (WSDL) XML document to describe the web-callable end points. Within any WSDL document, an XML namespace must be used to ensure that the end points have unique names. The default XML namespace is http://tempuri.org, but you will want to modify this before making your web service publicly available. You can change the XML namespace by using the WebService attribute: <WebService(Namespace := _
"http://www.LibertyAssociates.com/webServices/")>
You can read about attributes in detail in Chapter 8. |
Example 16-2 shows the complete source code for the Calculator web service.
Option Strict On
Imports System
Imports System.Web.Services
<WebService(Namespace := "http://tempuri.org/")> _
Public Class Service1
Inherits System.Web.Services.WebService
#Region " Web Services Designer Generated Code "
#End Region
<WebMethod(Description:="Add two numbers")> Public _
Function Add(ByVal x As Double, ByVal y As Double) As Double
Return x + y
End Function 'Add
<WebMethod(Description:="Subtract two numbers")> Public _
Function Subtract(ByVal x As Double, ByVal y As Double) As Double
Return x - y
End Function 'Sub
<WebMethod(Description:="Multiply two numbers")> Public _
Function Mult(ByVal x As Double, ByVal y As Double) As Double
Return x * y
End Function 'Mult
<WebMethod(Description:="Divide two numbers")> Public _
Function Div(ByVal x As Double, ByVal y As Double) As Double
Return x / y
End Function 'Div
<WebMethod(Description:="Raise a number to a power")> Public _
Function Pow(ByVal x As Double, ByVal y As Double) As Double
Dim retVal As Double = x
Dim i As Integer
For i = 0 To (y - 1) - 1
retVal *= x
Next i
Return retVal
End Function 'Pow
End Class
When you build this project with Visual Studio .NET, a DLL is created in the appropriate subdirectory of your Internet server (e.g., c:\InetPub\wwwroot\VBWSCals\bin\). A quick check of the base directory reveals that a .vsdisco file has also been added.
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If you open a browser to your web service's URL (or invoke the browser by running the program in Visual Studio .NET), you get an automatically generated, server-side web page that describes the web service, as shown in Figure 16-1. Test pages such as this offer a good way to test your web service. (The next section illuminates the seeming hocus-pocus that produces these pages.)
Notice that the description that you added to the WebMethod attribute is used here to provide a description of each method. Clicking a method brings you to a page that allows you to invoke it by typing in parameters and pressing the Invoke button. Figure 16-2 illustrates.
If you type 38 into the first value field and 4 into the second field, you will have asked the web service to raise 38 to the power of 4. The result is an XML page describing the output, as shown in Figure 16-3.
Notice that the URL encodes the parameters of 38 and 4, and the output XML shows the result of 2085136 (38 x 38 x 38 x 38 = 2085136).
A lot of work is being done for you automatically. HTML pages describing your web service and its methods are generated, and these pages include links to pages in which the methods can be tested. How is this done?
As noted earlier, the web service is described in WSDL. You can see the WSDL document by appending ?wsdl to the web service URL, like this:
http://localhost/ProgrammingVBNET/VBWSCalc/Service1.asmx?wsdl
The browser displays the WSDL document, as shown in Figure 16-4.
The details of the WSDL document are beyond the scope of this book, but you can see that each method is fully described in a structured XML format. This is the information used by SOAP to allow the client browser to invoke your web service methods on the server.
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