WebClient, 以字符串的形式上传/下载数据, 以流的方式上传/下载数据(转)

<
UserControl 
x:Class
=”Silverlight20.Communication.WebClientDownload”

    xmlns
=”http://schemas.microsoft.com/winfx/2006/xaml/presentation”
 
    xmlns:x
=”http://schemas.microsoft.com/winfx/2006/xaml”
>

    
<
StackPanel 
HorizontalAlignment
=”Left”
 Orientation
=”Horizontal”
>

    

        
<
StackPanel 
Margin
=”5″
 Width
=”200″
>

            
<
TextBox 
x:Name
=”lblMsgString”
 Margin
=”5″
 
/>

            
<
ProgressBar 
x:Name
=”progressBarString”
 Height
=”20″
 Margin
=”5″
 Minimum
=”0″
 Maximum
=”100″
 
/>

        
</
StackPanel
>

        

        
<
StackPanel 
Margin
=”5″
 Width
=”200″
>

            
<
TextBox 
x:Name
=”lblMsgStream”
 Margin
=”5″
 
/>

            
<
ProgressBar 
x:Name
=”progressBarStream”
 Height
=”20″
 Margin
=”5″
 Minimum
=”0″
 Maximum
=”100″
 
/>

            
<
Image 
x:Name
=”img”
 Margin
=”5″
 
/>

        
</
StackPanel
>

        

    
</
StackPanel
>


</
UserControl
>

using System;


using System.Collections.Generic;


using System.Linq;


using System.Net;


using System.Windows;


using System.Windows.Controls;


using System.Windows.Documents;


using System.Windows.Input;


using System.Windows.Media;


using System.Windows.Media.Animation;


using System.Windows.Shapes;

using System.IO;

namespace Silverlight20.Communication

{

    
public 
partial 
class WebClientDownload : UserControl

    {

        
//
 用于演示以字符串的形式下载数据

        
string _urlString = 
“
http://localhost/Files/Demo.zip
“;

        
//
 用于演示以流的形式下载数据

        
string _urlStream = 
“
http://localhost/Files/Logo.png
“;

        
public WebClientDownload()

        {

            InitializeComponent();

            
//
 演示字符串式下载

            DownloadStringDemo();

            
//
 演示流式下载

            DownloadStreamDemo();

        }

        
/**/
///
 
<summary>

        
///
 演示字符串式下载
        
///
 
</summary>

        
void DownloadStringDemo()

        {

            Uri uri = 
new Uri(_urlString, UriKind.Absolute);

            
/**//*
 
             * WebClient – 将数据发送到指定的 URI，或者从指定的 URI 接收数据的类
             *     DownloadStringCompleted – 下载数据完毕后（包括取消操作及有错误发生时）所触发的事件
             *     DownloadProgressChanged – 下载数据过程中所触发的事件。正在下载或下载完全部数据后会触发
             *     DownloadStringAsync(Uri address, Object userToken) – 以字符串的形式下载指定的 URI 的资源
             *         Uri address – 需要下载的资源地址
             *         Object userToken – 用户标识
             
*/

            System.Net.WebClient clientDownloadString = 
new System.Net.WebClient();

            clientDownloadString.DownloadStringCompleted += 
new DownloadStringCompletedEventHandler(clientDownloadString_DownloadStringCompleted);

            clientDownloadString.DownloadProgressChanged += 
new DownloadProgressChangedEventHandler(clientDownloadString_DownloadProgressChanged);

            clientDownloadString.DownloadStringAsync(uri, 
“
userToken
“);

        }

        
void clientDownloadString_DownloadProgressChanged(
object sender, DownloadProgressChangedEventArgs e)

        {

            
/**//*

             * DownloadProgressChangedEventArgs.ProgressPercentage – 下载完成的百分比
             * DownloadProgressChangedEventArgs.BytesReceived – 当前收到的字节数
             * DownloadProgressChangedEventArgs.TotalBytesToReceive – 总共需要下载的字节数
             * DownloadProgressChangedEventArgs.UserState – 用户标识
             
*/

            lblMsgString.Text = 
string.Format(
“
下载完成的百分比：{0}\r\n当前收到的字节数：{1}\r\n总共需要下载的字节数：{2}\r\n
“,

                e.ProgressPercentage.ToString() + 
“
%
“,

                e.BytesReceived.ToString(),

                e.TotalBytesToReceive.ToString());

            progressBarString.Value = (
double)e.ProgressPercentage;

        }

        
void clientDownloadString_DownloadStringCompleted(
object sender, DownloadStringCompletedEventArgs e)

        {

            
/**//*

             * DownloadStringCompletedEventArgs.Error – 该异步操作期间是否发生了错误
             * DownloadStringCompletedEventArgs.Cancelled – 该异步操作是否已被取消
             * DownloadStringCompletedEventArgs.Result – 下载后的字符串类型的数据
             * DownloadStringCompletedEventArgs.UserState – 用户标识
             
*/

            
if (e.Error != 
null)

            {

                lblMsgString.Text += e.Error.ToString();

                
return;

            }

            
if (e.Cancelled != 
true)

            {

                lblMsgString.Text += 
string.Format(
“
用户标识：{0}
“, e.UserState.ToString());

            }

        }

        
/**/
///
 
<summary>

        
///
 演示流式下载
        
///
 
</summary>

        
void DownloadStreamDemo()

        {

            Uri uri = 
new Uri(_urlStream, UriKind.Absolute);

            
/**//*
 
             * WebClient – 将数据发送到指定的 URI，或者从指定的 URI 接收数据的类
             *     IsBusy – 指定的web请求是否正在进行中
             *     CancelAsync() – 取消指定的异步操作 
             *     OpenReadCompleted – 数据读取完毕后（包括取消操作及有错误发生时）所触发的事件。流的方式
             *     DownloadProgressChanged – 下载数据过程中所触发的事件。正在下载或下载完全部数据后会触发
             *     OpenReadAsync(Uri address, Object userToken) – 以流的形式下载指定的 URI 的资源
             *         Uri address – 需要下载的资源地址
             *         Object userToken – 用户标识
             
*/

            System.Net.WebClient clientDownloadStream = 
new System.Net.WebClient();

            
if (clientDownloadStream.IsBusy)

                clientDownloadStream.CancelAsync();

            clientDownloadStream.OpenReadCompleted += 
new OpenReadCompletedEventHandler(clientDownloadStream_OpenReadCompleted);

            clientDownloadStream.DownloadProgressChanged += 
new DownloadProgressChangedEventHandler(clientDownloadStream_DownloadProgressChanged);

            clientDownloadStream.OpenReadAsync(uri);

        }

        
void clientDownloadStream_DownloadProgressChanged(
object sender, DownloadProgressChangedEventArgs e)

        {

            
/**//*

             * DownloadProgressChangedEventArgs.ProgressPercentage – 下载完成的百分比
             * DownloadProgressChangedEventArgs.BytesReceived – 当前收到的字节数
             * DownloadProgressChangedEventArgs.TotalBytesToReceive – 总共需要下载的字节数
             * DownloadProgressChangedEventArgs.UserState – 用户标识
             
*/

            lblMsgString.Text = 
string.Format(
“
下载完成的百分比：{0}\r\n当前收到的字节数：{1}\r\n总共需要下载的字节数：{2}\r\n
“,

                e.ProgressPercentage.ToString() + 
“
%
“,

                e.BytesReceived.ToString(),

                e.TotalBytesToReceive.ToString());

            progressBarStream.Value = (
double)e.ProgressPercentage;

        }

        
void clientDownloadStream_OpenReadCompleted(
object sender, OpenReadCompletedEventArgs e)

        {

            
/**//*

             * OpenReadCompletedEventArgs.Error – 该异步操作期间是否发生了错误
             * OpenReadCompletedEventArgs.Cancelled – 该异步操作是否已被取消
             * OpenReadCompletedEventArgs.Result – 下载后的 Stream 类型的数据
             * OpenReadCompletedEventArgs.UserState – 用户标识
             
*/

            
if (e.Error != 
null)

            {

                lblMsgStream.Text += e.Error.ToString();

                
return;

            }

            
if (e.Cancelled != 
true)

            {

                System.Windows.Media.Imaging.BitmapImage imageSource = 
new System.Windows.Media.Imaging.BitmapImage();

                imageSource.SetSource(e.Result);

                img.Source = imageSource;

            }

        }

    }

}

using System;


using System.Linq;


using System.Runtime.Serialization;


using System.ServiceModel;


using System.ServiceModel.Activation;

using System.ServiceModel.Web;


using System.Collections.Generic;


using System.Text;


using System.IO;

/**/
///
 
<summary>


///
 提供 REST 服务的类

///
 注：Silverlight只支持 GET 和 POST

///
 
</summary>

[ServiceContract]

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]


public 
class REST

{

    
/**/
///
 
<summary>

    
///
 用于演示返回 JSON（对象） 的 REST 服务
    
///
 
</summary>

    
///
 
<param name=”name”></param>

    
///
 
<returns></returns>

    [OperationContract]

    [WebGet(UriTemplate = 
“
User/{name}/json
“, ResponseFormat = WebMessageFormat.Json)]

    
public User HelloJson(
string name)

    {

        
return 
new User { Name = name, DayOfBirth = 
new DateTime(
1980, 
2, 
14) };

    }

    
/**/
///
 
<summary>

    
///
 用于演示返回 JSON（集合） 的 REST 服务
    
///
 
</summary>

    
///
 
<returns></returns>

    [OperationContract]

    [WebGet(UriTemplate = 
“
Users/json
“, ResponseFormat = WebMessageFormat.Json)]

    
public List<User> HelloJson2()

    {

        
return 
new List<User> 

        { 

            
new User(){ Name = 
“
webabcd01
“, DayOfBirth = 
new DateTime(
1980, 
1, 
1) },

            
new User(){ Name = 
“
webabcd02
“, DayOfBirth = 
new DateTime(
1980, 
2, 
2) },

            
new User(){ Name = 
“
webabcd03
“, DayOfBirth = 
new DateTime(
1980, 
3, 
3) },

        };

    }

    
/**/
///
 
<summary>

    
///
 用于演示返回 XML（对象） 的 REST 服务
    
///
 
</summary>

    
///
 
<param name=”name”></param>

    
///
 
<returns></returns>

    [OperationContract]

    [WebGet(UriTemplate = 
“
User/{name}/xml
“, ResponseFormat = WebMessageFormat.Xml)]

    
public User HelloXml(
string name)

    {

        
return 
new User { Name = name, DayOfBirth = 
new DateTime(
1980, 
2, 
14) };

    }

    
/**/
///
 
<summary>

    
///
 用于演示返回 XML（集合） 的 REST 服务
    
///
 
</summary>

    
///
 
<returns></returns>

    [OperationContract]

    [WebGet(UriTemplate = 
“
Users/xml
“, ResponseFormat = WebMessageFormat.Xml)]

    
public List<User> HelloXml2()

    {

        
return 
new List<User> 

        { 

            
new User(){ Name = 
“
webabcd01
“, DayOfBirth = 
new DateTime(
1980, 
1, 
1) },

            
new User(){ Name = 
“
webabcd02
“, DayOfBirth = 
new DateTime(
1980, 
2, 
2) },

            
new User(){ Name = 
“
webabcd03
“, DayOfBirth = 
new DateTime(
1980, 
3, 
3) },

        };

    }

    
/**/
///
 
<summary>

    
///
 用于演示以字符串的形式上传数据的 REST 服务
    
///
 
</summary>

    
///
 
<param name=”fileName”>
上传的文件名
</param>

    
///
 
<param name=”stream”>
POST 过来的数据
</param>

    
///
 
<returns></returns>

    [OperationContract]

    [WebInvoke(UriTemplate = 
“
UploadString/?fileName={fileName}
“, Method = 
“
POST
“, ResponseFormat = WebMessageFormat.Json)]

    
public 
bool UploadString(
string fileName, Stream stream)

    {

        
//
 文件的服务端保存路径

        
string path = Path.Combine(
“
C:\\
“, fileName);

        
try

        {

            
using (StreamReader sr = 
new StreamReader(stream))

            {

                
//
 将 POST 过来的被 Base64 编码过字符串传换成 byte[]

                
byte[] buffer = Convert.FromBase64String(sr.ReadToEnd());

                
using (FileStream fs = 
new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.None))

                {

                    
//
 将文件写入到服务端

                    fs.Write(buffer, 
0, buffer.Length);

                }

            }

            
return 
true;

        }

        
catch

        {

            
return 
false;

        }

    }

    
/**/
///
 
<summary>

    
///
 用于演示以流的形式上传数据的 REST 服务
    
///
 
</summary>

    
///
 
<param name=”fileName”>
上传的文件名
</param>

    
///
 
<param name=”stream”>
POST 过来的数据（流的方式）
</param>

    
///
 
<returns></returns>

    [OperationContract]

    [WebInvoke(UriTemplate = 
“
UploadStream/?fileName={fileName}
“, Method = 
“
POST
“, ResponseFormat = WebMessageFormat.Json)]

    
public 
bool UploadStream(
string fileName, Stream stream)

    {

        
//
 文件的服务端保存路径

        
string path = Path.Combine(
“
C:\\
“, fileName);

        
try

        {

            
using (FileStream fs = 
new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.None))

            {

                
byte[] buffer = 
new 
byte[
4096];

                
int count = 
0;

                
//
 每 POST 过来 4096 字节的数据，往服务端写一次

                
while ((count = stream.Read(buffer, 
0, buffer.Length)) > 
0)

                {

                    fs.Write(buffer, 
0, count);

                }

            }

            
return 
true;

        }

        
catch

        {

            
return 
false;

        }

    }

}

<
UserControl 
x:Class
=”Silverlight20.Communication.WebClientUpload”

    xmlns
=”http://schemas.microsoft.com/winfx/2006/xaml/presentation”
 
    xmlns:x
=”http://schemas.microsoft.com/winfx/2006/xaml”
>

    
<
StackPanel 
HorizontalAlignment
=”Left”
 Orientation
=”Horizontal”
>      

      

        
<
StackPanel 
Margin
=”5″
 Width
=”200″
>

            
<
TextBox 
x:Name
=”lblMsgString”
 Margin
=”5″
 
/>

            
<
ProgressBar 
x:Name
=”progressBarString”
 Height
=”20″
 Margin
=”5″
 Minimum
=”0″
 Maximum
=”100″
 
/>

            
<
Button 
x:Name
=”btnString”
 Content
=”上传文件（字符串的方式）”
 Margin
=”5″
 Click
=”btnString_Click”
 
/>

        
</
StackPanel
>

        

        
<
StackPanel 
Margin
=”5″
 Width
=”200″
>

            
<
TextBox 
x:Name
=”lblMsgStream”
 Margin
=”5″
 
/>

            
<
ProgressBar 
x:Name
=”progressBarStream”
 Height
=”20″
 Margin
=”5″
 Minimum
=”0″
 Maximum
=”100″
 
/>

            
<
Button 
x:Name
=”btnStream”
 Content
=”上传文件（流的方式）”
 Margin
=”5″
 Click
=”btnStream_Click”
 
/>

        
</
StackPanel
>

        

    
</
StackPanel
>


</
UserControl
>

using System;


using System.Collections.Generic;


using System.Linq;


using System.Net;


using System.Windows;


using System.Windows.Controls;


using System.Windows.Documents;


using System.Windows.Input;


using System.Windows.Media;


using System.Windows.Media.Animation;


using System.Windows.Shapes;

using System.IO;


using System.Windows.Resources;


using System.ComponentModel;


using System.Windows.Browser;

namespace Silverlight20.Communication

{

    
public 
partial 
class WebClientUpload : UserControl

    {

        
//
 用于演示以字符串的形式上传数据

        
string _urlString = 
“
http://localhost:3036/REST.svc/UploadString/?fileName=
“;

        
//
 用于演示以流的形式上传数据

        
string _urlStream = 
“
http://localhost:3036/REST.svc/UploadStream/?fileName=
“;

        
public WebClientUpload()

        {

            InitializeComponent();

        }

        
/**/
///
 
<summary>

        
///
 演示字符串式上传
        
///
 
</summary>

        
///
 
<param name=”sender”></param>

        
///
 
<param name=”e”></param>

        
private 
void btnString_Click(
object sender, RoutedEventArgs e)

        {

            
string data = 
“”;

            
/**//*

             * OpenFileDialog – 文件对话框
             *     ShowDialog() – 显示文件对话框。在文件对话框中单击“确定”则返回true，反之则返回false
             *     File – 所选文件的 FileInfo 对象
             
*/

            OpenFileDialog dialog = 
new OpenFileDialog();

            
if (dialog.ShowDialog() == 
true)

            {

                
using (FileStream fs = dialog.File.OpenRead())

                {

                    
byte[] buffer = 
new 
byte[fs.Length];

                    fs.Read(buffer, 
0, buffer.Length);

                    
//
 将指定的 byte[] 转换为字符串（使用Base64编码）

                    data = Convert.ToBase64String(buffer);

                }

                
/**//*

                 * WebClient – 将数据发送到指定的 URI，或者从指定的 URI 接收数据的类
                 *     UploadStringCompleted – 上传数据完毕后（包括取消操作及有错误发生时）所触发的事件
                 *     UploadProgressChanged – 上传数据过程中所触发的事件。正在上传或上传完全部数据后会触发
                 *     Headers – 与请求相关的的标头的 key/value 对集合
                 *     UploadStringAsync(Uri address, string data) – 以字符串的形式上传数据到指定的 URI。所使用的 HTTP 方法默认为 POST
                 *         Uri address – 接收上传数据的 URI
                 *         string data – 需要上传的数据
                 
*/

                System.Net.WebClient clientUploadString = 
new System.Net.WebClient();

                clientUploadString.UploadStringCompleted += 
new UploadStringCompletedEventHandler(clientUploadString_UploadStringCompleted);

                clientUploadString.UploadProgressChanged += 
new UploadProgressChangedEventHandler(clientUploadString_UploadProgressChanged);

                Uri uri = 
new Uri(_urlString + dialog.File.Name, UriKind.Absolute);

                clientUploadString.Headers[
“
Content-Type
“] = 
“
application/x-www-form-urlencoded
“;

                clientUploadString.UploadStringAsync(uri, data);

            }

        }

        
void clientUploadString_UploadProgressChanged(
object sender, UploadProgressChangedEventArgs e)

        {

            
/**//*

             * UploadProgressChangedEventArgs.ProgressPercentage – 上传完成的百分比
             * UploadProgressChangedEventArgs.BytesSent – 当前发送的字节数
             * UploadProgressChangedEventArgs.TotalBytesToSend – 总共需要发送的字节数
             * UploadProgressChangedEventArgs.BytesReceived – 当前接收的字节数
             * UploadProgressChangedEventArgs.TotalBytesToReceive – 总共需要接收的字节数
             * UploadProgressChangedEventArgs.UserState – 用户标识
             
*/

            lblMsgString.Text = 
string.Format(
“
上传完成的百分比：{0}\r\n当前发送的字节数：{1}\r\n总共需要发送的字节数：{2}\r\n当前接收的字节数：{3}\r\n总共需要接收的字节数：{4}\r\n
“,

                e.ProgressPercentage.ToString(),

                e.BytesSent.ToString(),

                e.TotalBytesToSend.ToString(),

                e.BytesReceived.ToString(),

                e.TotalBytesToReceive.ToString());

            progressBarString.Value = (
double)e.ProgressPercentage;

        }

        
void clientUploadString_UploadStringCompleted(
object sender, UploadStringCompletedEventArgs e)

        {

            
/**//*

             * UploadStringCompletedEventArgs.Error – 该异步操作期间是否发生了错误
             * UploadStringCompletedEventArgs.Cancelled – 该异步操作是否已被取消
             * UploadStringCompletedEventArgs.Result – 服务端返回的数据（字符串类型）
             * UploadStringCompletedEventArgs.UserState – 用户标识
             
*/

            
if (e.Error != 
null)

            {

                lblMsgString.Text += e.Error.ToString();

                
return;

            }

            
if (e.Cancelled != 
true)

            {

                var jsonObject = System.Json.JsonObject.Parse(e.Result);

                

                lblMsgString.Text += 
string.Format(
“
是否上传成功：{0}
“,

                    (
bool)jsonObject);

            }

        }

        
/**/
///
 
<summary>

        
///
 演示流式上传
        
///
 
</summary>

        
///
 
<param name=”sender”></param>

        
///
 
<param name=”e”></param>

        
private 
void btnStream_Click(
object sender, RoutedEventArgs e)

        {

            FileStream fs = 
null;

            OpenFileDialog dialog = 
new OpenFileDialog();

            
if (dialog.ShowDialog() == 
true)

            {

                fs = dialog.File.OpenRead();

                
/**//*

                 * WebClient – 将数据发送到指定的 URI，或者从指定的 URI 接收数据的类
                 *     OpenWriteCompleted – 在打开用于上传的流完成时（包括取消操作及有错误发生时）所触发的事件
                 *     WriteStreamClosed – 在写入数据流的异步操作完成时（包括取消操作及有错误发生时）所触发的事件
                 *     UploadProgressChanged – 上传数据过程中所触发的事件。如果调用 OpenWriteAsync() 则不会触发此事件
                 *     Headers – 与请求相关的的标头的 key/value 对集合
                 *     OpenWriteAsync(Uri address, string method, Object userToken) – 打开流以使用指定的方法向指定的 URI 写入数据
                 *         Uri address – 接收上传数据的 URI
                 *         string method – 所使用的 HTTP 方法（POST 或 GET）
                 *         Object userToken – 需要上传的数据流
                 
*/

                System.Net.WebClient clientUploadStream = 
new System.Net.WebClient();

                clientUploadStream.OpenWriteCompleted += 
new OpenWriteCompletedEventHandler(clientUploadStream_OpenWriteCompleted);

                clientUploadStream.UploadProgressChanged += 
new UploadProgressChangedEventHandler(clientUploadStream_UploadProgressChanged);

                clientUploadStream.WriteStreamClosed += 
new WriteStreamClosedEventHandler(clientUploadStream_WriteStreamClosed);

                Uri uri = 
new Uri(_urlStream + dialog.File.Name, UriKind.Absolute);

                clientUploadStream.Headers[
“
Content-Type
“] = 
“
multipart/form-data
“;

                clientUploadStream.OpenWriteAsync(uri, 
“
POST
“, fs);

            }

        }

        
void clientUploadStream_UploadProgressChanged(
object sender, UploadProgressChangedEventArgs e)

        {

            
//
 因为是调用 OpenWriteAsync()，所以不会触发 UploadProgressChanged 事件



            lblMsgString.Text = 
string.Format(
“
上传完成的百分比：{0}\r\n当前发送的字节数：{1}\r\n总共需要发送的字节数：{2}\r\n当前接收的字节数：{3}\r\n总共需要接收的字节数：{4}\r\n
“,

                e.ProgressPercentage.ToString(),

                e.BytesSent.ToString(),

                e.TotalBytesToSend.ToString(),

                e.BytesReceived.ToString(),

                e.TotalBytesToReceive.ToString());

            progressBarStream.Value = (
double)e.ProgressPercentage;

        }

        
void clientUploadStream_OpenWriteCompleted(
object sender, OpenWriteCompletedEventArgs e)

        {

            System.Net.WebClient client = sender 
as System.Net.WebClient;

            
if (e.Error != 
null)

            {

                lblMsgStream.Text += e.Error.ToString();

                
return;

            }

            
if (e.Cancelled != 
true)

            {

                
//
 e.UserState – 需要上传的流（客户端流）

                Stream clientStream = e.UserState 
as Stream;

                
//
 e.Result – 目标地址的流（服务端流）

                Stream serverStream = e.Result;

                
byte[] buffer = 
new 
byte[
4096];

                
int count = 
0;

                
//
 clientStream.Read – 将需要上传的流读取到指定的字节数组中

                
while ((count = clientStream.Read(buffer, 
0, buffer.Length)) > 
0)

                {

                    
//
 serverStream.Write – 将指定的字节数组写入到目标地址的流

                    serverStream.Write(buffer, 
0, count);

                }

                serverStream.Close();

                clientStream.Close();

            }

        }

        
void clientUploadStream_WriteStreamClosed(
object sender, WriteStreamClosedEventArgs e)

        {

            
if (e.Error != 
null)

            {

                lblMsgStream.Text += e.Error.ToString();

                
return;

            }

            
else

            {

                lblMsgStream.Text += 
“
上传完成
“;

            }

        }

    }

}

/**//*

 * 其他：
 * 1、WebClient 对象一次只能启动一个请求。如果在一个请求完成（包括出错和取消）前，即IsBusy为true时，进行第二个请求，则第二个请求将会抛出 NotSupportedException 类型的异常
 * 2、如果 WebClient 对象的 BaseAddress 属性不为空，则 BaseAddress 与 URI（相对地址） 组合在一起构成绝对 URI
 * 3、WebClient 类的 AllowReadStreamBuffering 属性：是否对从 Internet 资源接收的数据做缓冲处理。默认值为true，将数据缓存在客户端内存中，以便随时被应用程序读取
 
*/