Question:
Is there any way one can call web services remotely? Suppose I have a web
service which returns a DataSet. I want to call this web service from another
URL and use the returned DataSet to display or enter it into my database.
- Shoaib Pervaiz
Answer:
The .NET Framework has made creating and consuming web services quite easy
for developers. Using built in technology you can create a function, declare
it as a WebMethod and you have a web service. On the other side of that,
consuming web services is just as easy. Using another stock .NET piece of
technology, the WebServiceUtil.exe, you can create a DLL for consuming a
web service. With that you can treat the data returned to you from the web
service almost as if it were your own data.
In this tutorial you will work with a web service that exposes a DataSet, and
write an ASP.NET page to consume the web service from another domain.
The Web Service
The first thing you need is a web service to consume. I will be working with
the ASPNextGen.com Tutorials Service, which can be found in .NETWebServ.
If you would like, feel free to work with any web service, but your code
may differ from mine.
The ASPNextGen.com Tutorial Service returns a DataSet of tutorials, a summary
of each tutorial, and a URL for the full tutorial.
When a web service is exposed, a Service Description Language (SDL)
file is generated (either pre-generated, or generated on demand by adding
?SDL to the URL for the .asmx
file). The SDL is the contract between a provider and a consumer on how a
web service is exposed and how it is to be consumed. The SDL for the ASPNextGen.com
Tutorials Service is at:
http://www.aspnextgen.com/services/tutorials.asmx?SDL
The SDL file is an XML file that indicates the schema, the method(s) to call,
and the data type(s) returned.
The SDL is critical for consuming a web service, as it details
all the information you need to consume the service.
The Consumer
To build an ASP.NET page that consumes this service we must create
a proxy client class. The proxy client class will provide the method(s) we
need to call in order to consume the web service and use the DataSet returned.
The .NET Framework provides a tool for building proxy client classes easily
- WebServiceUtil.exe. You use this tool on the command line. You pass
into it the path to the SDL file, and the namespace and location for the
proxy client.
To create a proxy client, go to the command line on a machine
with the .NET Framework installed and enter the following with no line breaks
(I'll explain it in a moment):
C:>
webserviceutil /c:proxy
/pa:http://www.aspnextgen.com/services/tutorials.asmx?SDL
/l:vb /n:codejunkies.aspnextgen.services
/o:[enter the path to your ASP.NET application]\aspnextgen_proxy.vb |
After entering the command above you should have a Visual Basic
file in your ASP.NET application's directory named aspnextgen_proxy.vb.
The file created is a class file using the namespace codejunkies.aspnextgen.services.
Here's how the WebServiceUtil.exe command worked:
/c:proxy
/c: is the shorthand for command. This parameter indicates that you
are using WebServiceUtil.exe to create a proxy client. Other possibilities
are discovery, getSDL,
makeSDL, or template.
/pa:http://www.aspnextgen.com/services/tutorials.asmx?SDL
/pa: is shorthand for path. This parameter specifies the path to an
SDL file for the web service you are building a proxy client for. The path
could be a local path to a SDL file, or a URI to a SDL for a remote web service.
/l:vb
/l: is shorthand for language. This could be Visual Basic, C#, JScript
or any other language with a supplied CodeGenerator. I chose vb because I
think more people can understand the Visual Basic code. Feel free to use
any language.
/n:codejunkies.aspnextgen.services
/n: is shorthand for namespace. This parameter defines the namespace
the proxy class will use. It is typically accepted to identify namespaces
with the name of the company, followed by the purpose of the namespace, such
as Microsoft.VisualBasic.
/o:[enter the path to your ASP.NET application]\aspnextgen_proxy.vb
/o: is shorthand for output. This is the directory where the proxy
class file should be created. For my system I used /o:D:\myProject\aspnextgen_tutorials.vb
If you open the aspnextgen_tutorials.vb
proxy class file you will find the code that WebServiceUtil.exe generated
to consume the ASPNextGen.com Tutorials Service.
'------------------------------------------------------------------------------
' <autogenerated>
' This class was generated by a tool.
' Runtime Version: 1.0.2204.21
'
' Changes to this file may cause incorrect behavior and will be
lost if
' the code is regenerated.
' </autogenerated>
'------------------------------------------------------------------------------
Imports System.Xml.Serialization
Imports System.Web.Services.Protocols
Imports System.Web.Services
Namespace codejunkies.aspnextgen.services
Public Class Tutorials
Inherits System.Web.Services.Protocols.SoapClientProtocol
Public Sub New()
MyBase.New
Me.Url = "http://www.aspnextgen.com/services/tutorials.asmx"
End Sub
Public Function <System.Web.Services.Protocols.SoapMethodAttribute("http://tempuri.org/GetTutorialsSummary")>
GetTutorialsSummary() As System.Data.DataSet
Dim results() As Object = Me.Invoke("GetTutorialsSummary",
New Object(0) {})
Return CType(results(0),System.Data.DataSet)
End Function
Public Function BeginGetTutorialsSummary(ByVal callback
As System.AsyncCallback, ByVal asyncState As Object) As System.IAsyncResult
Return Me.BeginInvoke("GetTutorialsSummary",
New Object(0) {}, callback, asyncState)
End Function
Public Function EndGetTutorialsSummary(ByVal asyncResult
As System.IAsyncResult) As System.Data.DataSet
Dim results() As Object = Me.EndInvoke(asyncResult)
Return CType(results(0),System.Data.DataSet)
End Function
End Class
End Namespace
|
As you can see, the codejunkies.aspnextgen.services
namespace is declared, and a class named Tutorials
has been created. In the Tutorials class
there is a New() sub procedure and three
functions, GetTutorialsSummary(), BeginGetTutorialsSummary()
and EndGetTutorialsSummay(). The GetTutorialsSummary()
function is the one we will call to consume the web service. From here you
can consume the web service just as you would call any function in a DLL...of
course, the DLL must be compiled and placed in your bin directory. You can
compile this class in the same DLL as the rest of your project. The namespace
codejunkies.aspnextgen.services will be in
the DLL and the functions will be exposed.
The ASP.NET Page
Once you have compiled the DLL you can consume the service. Just
as with other calls to methods in a DLL, you need to import the namespace,
in this case codejunkies.aspnextgen.services,
and instantiate the class, in this case the Tutorials
class. Once you have a variable representing the class you can call the method
to consume the web service.
Here is a simple page to render a DataGrid with all the output
from the ASPNextGen.com Tutorials Service.
<%@ Page Language="vb" %>
<%@ Import Namespace="codejunkies.aspnextgen.services"
%>
<%@ Import Namespace="System.Data" %>
<html>
<head>
<script runat="server">
Sub Page_Load(Source As Object, E As EventArgs)
Dim ds As New DataSet
Dim Service As Tutorials = New Tutorials
Ds = Service.GetTutorialsSummary()
dg.DataSource = ds.Tables("Tutorials").DefaultView
dg.DataBind()
End Sub
</script>
</head>
<body bgcolor="#FFFFFF">
<form method="post" runat="server">
<asp:DataGrid id="dg" Runat="server"
AlternatingItemStyle-BackColor="#eeeeee"
ShowHeaders="True" />
</form>
</body>
</html> |
This is what the rendered page looks like:
Summary
The .NET Framework has made creating and consuming web services
quite easy. Tools, such as WebServiceUtil.exe, are included in the framework
to enable developers to quickly develop rich, data-driven applications. In
the case of the example in this tutorial, you learned how to consume a web
service that returns an ADO.NET DataSet. Once you have the DataSet in your
application you can do virtually anything with it. You can change the properties
of the DataGrid to alter the output, you can insert the data into a database,
you can iterate through the data looking for specific parameters, etc. Virtually
anything is possible.