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Publisher4.4 Consuming a Web Service
In ASP.NET,
consuming a web service is nearly as easy as creating one. ASP.NET
provides a utility called
wsdl.exe
that can create a proxy class, which is a class that knows all of the
necessary details of communicating with the web service via
Figure 4-1, and which can be called from a client
application the same way as any other managed class. In this way, the
proxy class abstracts away the complexities of communication with the
web service.
Consuming a web service in ASP.NET requires four steps:
Locate the WSDL contract for the desired web service.
Create a proxy class by using the
wsdl.exe
command-line utility.
Compile the proxy class.
Create a new instance of the proxy class in the client application
(WinForms, Console, or ASP.NET) and call the desired methods.
In the case of our Qotd_cb web service, you would
execute the following command (again, conveniently saved as a DOS
batch file) to generate a proxy class based on the web service:
wsdl /l:vb /out:Qotd_cb_proxy.vb http://localhost/ASPdotNET_iaN/Chapter_ 4/Qotd_cb.asmx?WSDL
pause
The /l parameter specifies that the proxy class
should be generated in Visual Basic .NET (the default is C#). The
/out parameter specifies the name and, optionally,
the path of the output file. This is important if you are compiling
your proxy class in the same directory as the code-behind class that
implements the web service. In this case, if you do not specify the
output filename, the file
Qotd_cb.vb will be
overwritten. Once the proxy class has been generated, it should be
compiled, and the resulting assembly should be placed in the
bin directory. This can be accomplished using a
command such as the one in the following snippet:
vbc /t:library /r:System.Web.dll,System.dll,System.Web.Services.dll, System.
Xml.dll,System.Data.dll /out:bin\qotd_cb_proxy.dll
qotd_cb_proxy.vb
pause
Remember that all parameters for the
vbc.exe
compiler should be part of the same command; therefore, there should
not be any line breaks if you choose to save the command to a batch
file.
Once you've generated and compiled your proxy class,
using the web service is exactly like using any other .NET class. You
simply create an instance and call the desired methods. Example 4-5 shows the code for a simple ASP.NET page that
consumes the Qotd_cb web service.
Example 4-5. Qotd_cb.aspx
<%@ Page Language="VB" %>
<%@ Import Namespace="aspnetian" %>
<html>
<head>
<title>Quote of the Day Web service example</title>
<script runat="server">
Sub Page_Load(Sender As Object , e As EventArgs )
Dim Quote As New Qotd_cb( )
Dim QuoteNumber As Integer
QuoteNumber = Quote.GetQuoteNumber
Message1.Text = Quote.GetQuote(QuoteNumber)
Message2.Text = Quote.GetAuthor(QuoteNumber)
End Sub
</script>
</head>
<body>
<h1>Demonstration of Quote of the Day Web service</h1>
<form runat="server">
<h4><i>"<asp:Literal id="Message1" runat="server" />"</i></h4>
<h3>--<asp:Literal id="Message2" runat="server" /></h3>
</form>
</body>
</html>
The page imports the aspnetian namespace defined in the
Qotd_cb web service class, creates an instance of
the proxy class, and then calls the GetQuoteNumber method to retrieve
a random quote number. The page then calls the GetQuote and GetAuthor
methods, passing in the quote number each time, and returns the
result to the Text property of two ASP. NET literal controls. The
output of this page is shown in Figure 4-7.
Figure 4-7. Qotd_cb.aspx output
