Creating Web Services in .NET
Web services are one of the building blocks of overall .NET architecture.
This article explains the basics of ASP.NET web services along with
What are Web Services?
To understand the meaning of term "Web Services" we will
take analogous example form COM world. If you are programmer working with
Microsoft Technologies, by this time you must have used third party components
or at least components developed for your own applications. The COM components
you developed 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 need same business logic at many
places such component will be of great use. Components also isolate your business
logic from the rest of the application. Web services offer similar
functionality. Web services offer "services" over "web". The communication
between your application and web service is via HTTP protocol using SOAP
standards (Simple Object Access Protocol).
How web services work?
In simple terms it works as follows :
- Your application requests a service (or a method) from a Web
- Your request is converted into SOAP format (which is nothing but XML) and routed to web
service over web
- The web service processes your request and sends back the result again in SOAP
For routing your method calls to actual web service a special proxy (which is nothing but a dll) is used. Following figure will make
it clear :
Following points need to be noted about
web services :
- Web services are written as plain text files with
- You generate a proxy source code for this .asmx file first into VB or
C# source code using a utility provided with .NET (more on that later)
- You then compile the source
code into a DLL file (actual proxy dll) which makes the "Web Service" component
- You can consume a web service from any type of application that supports HTTP e.g. Console, WinForms and WebForms or even plain old
Web Service Description Language
Web Service description language or WSDL 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.
Creating Your First Web Service
In this section we will create a simple web service called GreetingService. This web
service will provide various greeting messages. You will need to pass an integer
indicating the type of message you want e.g. Diwali 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 to obtain a
- Deploy the resulting DLL to the client applications bin folder
Create GreetingService.asmx file containing source code for the service
<%@ webservice language= "VB" class= "Greetings"%>Imports
Class Greetings <WebMethod()>Public
Function GetMsg(id as integer) As String
Select case id
GetMsg="Happy New Year"
Let us dissect the code
- Web service files have extention .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 which constitutes the web service.
One .asmx file can have multiple class files (may be supporting classes). Out
of available classes the class specified in the class attribute is considered
for creating web service.
- <WebMethod()>public function GetMsg(id as integer) as string
This line marks GetMsg() method as "web callable" method using special attribute <WebMethod()>. This method returns
the greeting message based on id we pass.
You can add as many web methods you want to the web
service. You can also add normal methods i.e. methods without
<WebMethod()> attribute. Such methods will not be exposed over web but can
be used internal to the web
service. You can also have more than one class per ASMX file. Typically, such classes will
act as support class for the main class.
Convert .asmx file into VB or C# source code file
Now that we have our web service source code file
ready, we need to convert it into VB source code. To do this .NET comes with a utility called
WSDL.EXE. Type in the following command at the DOS prompt :
WSDL http://localhost/mywebapp/GreetingService.asmx?wsdl /l:VB /n:GreetingService
- http://localhost/mywebapp/GreetingService.asmx?wsdl We need to supply the WSDL of the web service. This can be achieved by
appending WSDL 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 the source code for the proxy dll. Do not bother much about the code contained in this file since you will not be
generally modifying it manually.
Compile VB or C# source code to obtain proxy DLL
Now we will compile the VB source code to get the proxy DLL.
Type in following Command
You will get GreetingService.DLL.
Deploy the resulting DLL
In order to use the DLL you created you must copy it in the \bin folder of your web
If such folder is already not there you need to create it
Note : Here, we are assuming that we want to consume
our web service from a web client developed in ASP.NET
Creating proxy dll using visual studio.nET
VS.NET provides very easy way to create this proxy dll. It almost hides all
the complexities of the process. In your project you can simply right click on
references and select "Add a web reference..." . Now select the web service and
that's it ! VS.NET will automatically create a proxy for you and place in your
web application's bin directory.
Testing our service
You can now test the web service you created by using it in an ASP.NET page. You might use
following code :
public sub showmsg(s as Object,e as EventArgs)
dim c as GreetingService.Greetings
<asp:label id=label1 runat=server text="no msg set yet"/>
<asp:button id=btn1 runat=server onclick="showmsg" text="Click" />
You can create instances of the web service component just like any
Web services and
Just like any ASP.NET application web services can hold session or application state. However, there are some special things to
Your web service class must derive from WebService. Our class used above is not derived from WebService class and hence can not use
You need to mark each method that wants to access
session state with following attribute
EnableSessionState parameter tells the web service that we want to access session state inside