Asynchronous Web Services Invocation in. Net framework 0

Consume the Web Service Asynchronously from a Windows Forms Application

Now you are ready to consume the Web service. To start, import the Web service proxy namespace at the top of the Form1:

using AsyncWebServicesClient.HelloWorldServiceProxy;

To the form, add a command button named btnInvokeHelloWorld and modify the click event of the command button to look as follows:

private void btnInvokeHelloWorld_Click(object sender, EventArgs e)

{

HelloService service = new HelloService();

service.HelloWorldCompleted += new

HelloWorldCompletedEventHandler(this.HelloWorldCompleted);

service.HelloWorldAsync();

}

The above code shows the event handler HelloWorldCompleted (that is available through the proxy of the Web service) being hooked up to a local method named HelloWorldCompleted. The method named HelloWorldAsync provides a way to asynchronously invoke the HelloWorld method.

• 
  Create an event handler for the GetCategoriesCompleted method and hook it up to a local method that can process the results returned by the Web service.
  
• 
  Invoke the Web service method by calling the GetCategoriesAsync() method through the Web service proxy.
  

void HelloWorldCompleted(object sender,

HelloWorldCompletedEventArgs args)

{

//Display the return value

}

After the Web service has finished its execution and returned the results, the .NET Framework will automatically invoke the above HelloWorldCompleted method. As you can see, this method takes two arguments: an object of type "object" and a "HelloWorldCompletedEventArgs" object. You can capture the results returned by the Web service by using the Result property of the HelloWorldCompletedEventArgs object (as shown in the above code). In this example, the output the Web service returns is displayed in a message box.

Implement Data Binding with a Web Service

Apart from the new Web service asynchronous invocation framework, .NET Framework 2.0 also introduces a new feature through which you can perform data binding, with the results returned from the Web service in a seamless manner. This section demonstrates this feature with a practical example. It begins by creating the Web service.

Implementation of the Web service

To start, add a new Web service to the AsyncWebServices project by right-clicking on the project and selecting Add New Item from the context menu. In the Add New Item dialog box, select Web service from the list of templates and name the Web service CategoriesService.asmx. Modify the code-behind of the CategoriesService.asmx file to look as follows:

using System;

using System.Web;

using System.Collections;

using System.Web.Services;

using System.Web.Services.Protocols;

using System.Data;

using System.Data.SqlClient;

using System.Diagnostics;

[WebService(Namespace = "http://tempuri.org/")]

[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1,

EmitConformanceClaims = true)]

public class CategoriesService : System.Web.Services.WebService

{

[WebMethod]

public DataSet GetCategories()

{

try

{

using (SqlConnection conn = new SqlConnection())

{

string connectionString =

System.Configuration.ConfigurationManager.

ConnectionStrings["northwindConnectionString"].

ConnectionString;

conn.ConnectionString = connectionString;

SqlCommand command = new SqlCommand

("Select * from Categories", conn);

command.CommandType = CommandType.Text;

SqlDataAdapter adapter = new

SqlDataAdapter(command);

DataSet categories = new DataSet("Categories");

adapter.Fill(categories);

return categories;

}

}

catch (Exception ex)

{

EventLog.WriteEntry("Application", ex.Message);

throw ex;

}

}

}

Line-by-Line Code Breakdown

The above code first encloses the SqlConnection object within the scope of a using block. Then, it retrieves the connection string from the ConnectionStrings section of the web.config file:

string connectionString = System.Configuration.ConfigurationManager.

ConnectionStrings["northwindConnectionString"].

ConnectionString;

The code stores the connection string directly under the root element in the web.config as follows:

connectionString="server=localhost;uid=sa;pwd=thiru;

database=northwind"/>

Then, it sets the retrieved connection string to the ConnectionString property of the SqlConnection object. After that, the code creates an instance of the SqlCommand object, passing to its constructor the SQL statement to be executed and the SqlConnection object as arguments. Then, it sets the CommandType property of the SqlCommand object to CommandType.Text to indicate that it wants to execute a SQL statement through the SqlCommand object:

SqlCommand command = new SqlCommand("Select * from Categories", conn);

command.CommandType = CommandType.Text;

Next, it creates an instance of the SqlDataAdapter and passes in the SqlCommand object to its constructor:

SqlDataAdapter adapter = new SqlDataAdapter(command);

After that, the code creates a DataSet object and then supplies the DataSet object as an argument to the Fill method of the SqlDataAdapter object:

DataSet categories = new DataSet("Categories");

adapter.Fill(categories);

Finally, it returns the DataSet object back to the caller using the return statement:

return categories;

Now that you have seen the implementation of the Web service, take a look at the client application that will consume the Web service.

Consume the Web Service from the Client

Open the previously created AsyncWebServicesClient project from Visual Studio 2005. Add a Web reference to the CategoriesService by selecting Project->Add Web Reference from the menu. In the Add Web Reference dialog box, enter the location of the Web service and specify the name of the Web reference asCategoriesServiceProxy. Once the Web reference is created, you can import the namespace of the proxy using the following line:

using AsyncWebServicesClient.CategoriesServiceProxy;

Next, open Form1 and drag and drop a DataGrid control onto the Form designer. Add another command button named btnInvokeCategories to the form and modify the code in the Click event to look like the following:

private void btnInvokeCategories_Click(object sender, EventArgs e)

{

CategoriesService service = new CategoriesService();

service.GetCategoriesCompleted += new

GetCategoriesCompletedEventHandler

(this.GetCategoriesCompleted);

service.GetCategoriesAsync();

}

The implementation of the GetCategoriesCompleted method is as follows:

void GetCategoriesCompleted(object sender,

GetCategoriesCompletedEventArgs args)

{

//Bind returned results to the UI data grid

dataGrid1.DataSource = args.Result;

}

As you can see, the DataSet returned from the Web service is directly bound to a DataGrid control and displayed on the form. If you run the form and click on the Invoke Categories Service button, you will see the output shown in Figure 5.

(Full Size Image)

Figure 5. Run the Form and Click on the Invoke Categories Service Button

As Figure 5 shows, the categories information that the Web service returns is directly data bound to the DataGrid control.

Increase Developer Productivity, Enhance User Experience

The new asynchronous invocation framework that .NET Framework 2.0 introduced is an extremely useful feature that can go a long way toward increasing developer productivity. You can leverage the framework to increase the responsiveness of a .NET application, which greatly enhances the user experience as well.

This tutorial provided a thorough discussion of the asynchronous Web service invocation features of .NET Framework 2.0 and provided examples. It also showed how to perform data binding with the results returned from a Web service. Although the application you created was simple in functionality, it should provide a solid foundation for understanding how to easily implement the asynchronous Web service invocation feature in your .NET applications.

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