Silverlight用WebClient and HttpWebRequest两种方式来调用WebService

今天面试了一家公司，是关于Silverlight的。上来就问我，WebClient 与HttpWebRequest调用WebService的区别，一问我我就蒙了。因为平时做项目的话，对这两种概念不用太了解也可以开发。就是用VS.NET自带的一个功能，参照WebService就可以自动接口类，帮我们都做好了。

具体图解请参照：http://blog.csdn.net/ourmessage/archive/2008/03/27/2223497.aspx

微软帮我们做好了以后，容易让我们这些开发者在不知道怎么回事的情况下就可以开发，但这也许会害了我们。比如说我的面试。

然后我就在老外的网站上找到了如何直接调用WebService的例子，为了跟大家分享，也为了自己以后参照用，所以就转一下他的帖子。我也不翻译了。大家自己看吧。英语很简单。以下为转载内容。

The WebClient

and HttpWebRequest
classes can both be used for retrieving data from a Web service. WebClient
is most useful for one-time retrieval of data from a Web service, or
downloading remote resources (for more information on this see, "Downloading Content on Demand"
) and I think, is somewhat easier to use. HttpWebRequest
is perhaps a bit less user-friendly, but will enable you to set headers
and generally offers more control over a Web service request.

The
following code demonstrates how to make the same request using both
classes. I’ve set up a simple Silverlight control that contains two
buttons, a text box to capture user input, and a text block to display
the content returned from the Web service. The first example
demonstrates how to make a request using WebClient
, the second demonstrates the same request using HttpWebRequest
. Both requests are asynchronous.

First,
I show you the simple UI. It’s some stack panels, two buttons, a text
box that accepts user and a text block to display the output from the
Web service.

<StackPanel Background="White">
<StackPanel Orientation="Horizontal">
<Button Width="250" Content="Click for request with
HttpWebRequest" Click="Button1_Click"  />
<Button Width="250" Content="Click for request with WebClient"
Click="Button2_Click" />
</StackPanel>
<StackPanel Orientation="Horizontal">
<TextBlock  Text="Enter a number between 1-5: " />
<TextBox  Text="1" x:Name="numTextBox" Width="20" />
</StackPanel>
<TextBlock x:Name="resultBlock" />
</StackPanel>

The
request will go to a Digg Web service that returns the last photos
uploaded to its gallery service. This service that allows you to use the
URL to specify the expected response type and the number of photos to
return, which I will do. In addition, I must specify an AppKey, which is
required by the Digg Web service. It’s important to notice that Digg
has the required cross domain policy in place to allow calls from my
Silverlight application.

First I declare the base URL used by both WebClient
and HttpWebRequest
:

string
baseUri = http://services.digg.com/galleryphotos?count

;

I’ll use the same URL with both WebClient
and HttpWebRequest
,
but since I'll append the number of photos to return, I’ll build the
complete URL for each request using the specified number and the AppKey.
In addition, I'll specify an xml response by appending &type=xml
to the end of the URL.

To call the service by using WebClient
, declare a WebClient
object, and in the Page
constructor associate its DownloadStringCompleted
event with a handler.

WebClient client = new WebClient();

client.DownloadStringCompleted +=
new DownloadStringCompletedEventHandler(
client_DownloadStringCompleted);

In Button1_Click, I call DownloadStringAsync, passing the complete URL that includes the number specified by the user.

private void Button1_Click(object sender, RoutedEventArgs e)
{
client.DownloadStringAsync
(new Uri(baseUri + numTextBox.Text +
"&appkey=http%3A%2F%2Fsilverlight.net&type=xml"));
}
In the DownloadStringCompleted event-handler, I check that the Error property of the event args is null, and either output the response or the error message to the text block.

void client_DownloadStringCompleted(object sender,
DownloadStringCompletedEventArgs e)
{
if (e.Error == null)
resultBlock.Text = "Using WebClient: "+  e.Result;

else
resultBlock.Text = e.Error.Message;
}

Next, I’ll made the same call using HttpWebRequest
. In
Button2_Click
, I create the HttpWebRequest
by calling HttpWebRequest.Create
, passing the URL built from the
baseUr
l, the user-specified photo count, and AppKey. I call BeginGetResponse
on the request, passing the callback method as a new AsyncCallback
object and the request object as the state
parameter. I’ll use the request object in the callback to retrieve the response.

private void Button2_Click(object sender, RoutedEventArgs e)
{
HttpWebRequest request =
(HttpWebRequest)HttpWebRequest.Create(new Uri(baseUri +
numTextBox.Text + "&appkey=http%3A%2F%2Fsilverlight.net&type=xml"));
request.BeginGetResponse(new AsyncCallback(ReadCallback),
request);
}

In the callback method,
ReadCallback
, I retrieve the request with the AsyncState
property of the asynchronousResult
parameter, and get the response by calling EndGetResponse
on the request. Finally I use a StreamReader
to read the results and output them to the TextBlock.

private void ReadCallback(IAsyncResult asynchronousResult)
{
HttpWebRequest request =
(HttpWebRequest)asynchronousResult.AsyncState;
HttpWebResponse response =
(HttpWebResponse)request.EndGetResponse(asynchronousResult);
using (StreamReader streamReader1 =
new StreamReader(response.GetResponseStream()))
{
string resultString = streamReader1.ReadToEnd();
resultBlock.Text = "Using HttpWebRequest: " + resultString;
}
}

The
results are essentially the same either way, but for a more complex
request, if I need to specify headers, content type or need more control
over my request, I’ll need to use HttpWebRequest
.

Hopefully this helps you get started with your own Web service calls. If you need more help, see my previous post
, which also points to two other great blog posts on the topic.

原文地址：http://blogs.msdn.com/b/silverlight_sdk/archive/2008/04/01/using-webclient-and-httpwebrequest.aspx