Creating Web Services in ASP.NET
To understand the meaning of term "Web Services" we will take an
analogous example from the world of COM. If you are programmer
working with Microsoft technologies, you have probably used third
party components or at least components developed for your own
applications. The COM components you developed basically provide
"services" to your application. For example a component developed
for a banking application might be providing services such as loan
calculation, interest calculation etc.
If you needed the same business logic in a distributed way (in
many places around the web) this component will be of great use to
you. Components also isolate your business logic from the rest of
the application.
Web services offer similar functionality. Web services offer
"services" over the "web". The communication between your ASP.NET
pages and web service is via HTTP protocol using SOAP standards
(Simple Object Access Protocol).
In simple terms it works as follows:
- Your ASP.NET
page requests a service from a Web Service component
- Your request
is converted into SOAP format (which is in XML)
- The web
service processes your request and sends back the result again in
an XML format over SOAP
Fortunately, as an ASP.NET developer you are isolated from the
pain of SOAP internals. All of that is handled for you!
The following points need to be noted about web services:
- Web services
are written as plain text files with an extension of .asmx
- You then
convert this .asmx file first into VB or C# source code using a
utility provided with ASP.NET called WebServiceUtil.exe
- You then
compile the source code into a DLL file which makes the "Web
Service" component
Service description language or SDL is nothing but an XML
document describing the web service. It is analogous to Interface
Definition Language (IDL) or Type Libraries(TLB). It gives details
like service method names, their parameter data types, return
values etc.
In this section we will create a simple web service called
Greeting Service. This web service will provide various greeting
messages. You will need to pass an integer indication the type of
message you want e.g. Birthday Greetings, New Year Greetings etc.
Following are the steps involved in creating a web service:
- Create .asmx
file containing source code for the service ( GreetingService.asmx
in our case)
- Convert .asmx
file into VB or C# source code file
- Compile VB or
C# source code thus formed
- Deploy the
resulting DLL to the applications bin folder
Create GreetingService.asmx file containing source code for the
service
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<%@ webservice language="VB" class="greetings"%>
imports system.web.services
public Class greetings
public function
<WebMethod()>getMsg(id as integer)as string
select case id
case 101
getMsg="Happy Birthday"
case 102
getMsg="Happy New Year"
case else
getMsg="Seasons Greetings"
end select
end function
end class
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Let us dissect the code:
- Webservice
source code is stored in simple text files having extension
.asmx. In our case we created a file called
GreetingService.asmx
- <%@
webservice language="VB" class="greetings"%>
The webservice directive indicates that this is a web
service. Language attribute specifies the language used to write
the web service. We have used VB in our example. The class
attribute specifies the class name that constitutes the web
service. One .asmx file can have multiple class files (some of
which may be supporting classes). Out of available classes
the class specified in the class attribute is used for
creating the web service.
- public
function <WebMethod()>getMsg(id as integer)as string
This line marks getMsg() method as the "web callable" method. This
method returns the greeting message based on id we pass
Now that we have our web service source code file ready, we need
to convert it into VB source code. To do this ASP.NET comes with a
utility called WebServiceUtil.exe. This utility is used for other
purposes as well but we will consider its use only to generate VB
source code. Type in the following command at the DOS prompt:
WebServiceUtil /c:proxy
/pa:http://localhost/mywebapp/GreetingService.asmx?sdl /l:VB
/n:GreetingService
Here,
- /c:proxy represents that we want to create proxy for the
client
- /pa:http://localhost/mywebapp/GreetingService.asmx?sdl We
need to supply the SDL of the web service. This can be achieved by
appending SDL to the path of our .asmx file. You can even invoke
this URL in browser and view the resulting XML code.
- /l:VB Specifies that the resulting code should be
generated in VB
- /n:GreetingService Indicates that name for the resulting
namespace should be GreetingService
After executing above command you will get a file called
greetings.vb and it contains something like this :
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'--------------------------------------------------------
' <autogenerated>
' This class was generated by a
tool.
' Runtime Version: 2000.14.1812.10
'
' 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 GreetingService
Public Class Greetings
Inherits
System.Web.Services.Protocols.SoapClientProtocol
Public Sub New()
MyBase.New
Me.Path =
"http://localhost/BAJSamples/ASPPlus/WebServices/GreetingService.asmx"
End Sub
Public Function
<System.Web.Services.Protocols.SoapMethodAttribute("http://tempuri.org/getMsg",
RequestElementName:="getMsg",
ResponseElementName:="getMsgResult")> GetMsg(ByVal id As
Integer) As String
Dim results As Object() = Me.Invoke("GetMsg", New Object()
{id})
return CType(results(0),String)
End Function
Public Function
BeginGetMsg(ByVal id As Integer, ByVal callback As
System.AsyncCallback, ByVal asyncState As Object) As
System.IAsyncResult
return Me.BeginInvoke("GetMsg", New Object() {id}, callback,
asyncState)
End Function
Public Function
EndGetMsg(ByVal asyncResult As System.IAsyncResult) As String
Dim results As Object() = Me.EndInvoke(asyncResult)
return CType(results(0),String)
End Function
End Class
End Namespace
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Do not bother about the code since you will not be generally
modifying it manually.
Compile VB or C# source code
Now we will compile the VB source code to get the final DLL.
Type in following command
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vbc
/out:GreetingService.dll
/t:library
/r:System.Xml.Serialization.dll
/r:System.Web.Services.dll Greetings.vb
pause
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If you are interested in what each of above switch means just
type vbc at the command prompt and find out yourself :-)
You finally got the GreetingService.DLL! Well done! What next?
Deploy the resulting DLL
In order to use the DLL you created you must copy it in the \bin
folder of your web application. If such folder is already not
there you need to create it.
You can now test the web service you created by using it in an
ASP.NET page. You might use following code:
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<%@ language="VB"%>
<script runat=server>
public sub showmsg(s as Object,e as EventArgs)
dim c as
GreetingService.Greetings
c=new
GreetingService.Greetings()
label1.text=c.getMsg(101)
end sub
</script>
<form runat=server>
<asp:label id=label1 runat=server text="no msg set
yet"/>
<asp:button id=btn1 runat=server onclick="showmsg"
text="Click" />
</form>
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You can create instances of the web service component just like
any other object.
Since the DLL file is already in the bin folder, our service is
already available to the ASP.NET page
Web Services provide "web callable" functions that you can call
from your .NET applications. Web services transfer data using SOAP.
The details about various web methods are provided by the SDL. To
use a web service you create a proxy for the web service and call
the methods on the proxy from your .NET application.
Please feel free to send your feedback and suggestions at bipinjoshi@yahoo.com or
visit his website at
http://www.bipinjoshi.com
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