http请求与响应
若要从 Internet 请求数据并读取响应，请完成以下步骤
如果要访问像 Web 页这样的资源，请用要使用的资源的 URI 调用 WebRequest.Create 来创建 WebRequest，如下例所示。
[C#]
WebRequest wReq = WebRequest.Create("http://www.contoso.com/");
[Visual Basic]
Dim wReq as WebRequest = WebRequest.Create("http://www.contoso.com/")
注意 .NET Framework 为以“http:”、“https:”和“file:”开头的 URI 提供特定于协议的 WebRequest 和 WebResponse 子代。若要访问其他协议，必须实现协议特定的 WebRequest 和 WebResponse 子代。有关更多信息，请参见编写可插接协议。
在 WebRequest 中设置任何所需的属性值。例如，若要支持身份验证，请将 Credentials 属性设置为 NetworkCredential 类的实例，如下例所示。
[C#]
wReq.Credentials =
new NetworkCredential(UserName, SecurelyStoredPassword);
[Visual Basic]
wReq.Credentials = _
New NetworkCredential(UserName, SecurelyStoredPassword)
大多数情况下，WebRequest 本身就已足够发送和接收数据。但是，如果需要设置特定于协议的属性，则应将 WebRequest 转换为特定于协议的实例。仅当处理 WebRequest 的 WebRequest 子代正确时，此类型转换才有效。例如，若要访问 HttpWebRequest 的 HTTP 特定的属性，请将 WebRequest 转换为 HttpWebRequest。下面的代码实例显示如何设置 HTTP 特定的 UserAgent 属性。
[C#]
if (wReq is HttpWebRequest)
{
((HttpWebRequest)wReq).UserAgent = ".NET Framework Example Client";
}
[Visual Basic]
If TypeOf wReq is HttpWebRequest Then
Ctype(wReq,HttpWebRequest).UserAgent = _
".NET Framework Example Client"
End If
若要从 Internet 下载资源，请调用 WebRequest 的 GetResponse 方法。
若要将数据发送或上载到资源，请调用 WebRequest 的 GetRequestStream 方法，并使用结果 Stream 对象编写数据。完成上载后，必须使用 Stream.Close 方法关闭请求流。关闭流后，可以调用 GetResponse 以确保服务器已正确接收到数据。为 WebResponse 实例返回的实际类由所请求的 URI 方案决定。下面的代码示例显示如何使用 GetResponse 方法创建 WebResponse。
[C#]
WebResponse wResp = wReq.GetResponse();
[Visual Basic]
Dim wResp As WebResponse = wReq.GetResponse()
注意 调用 WebResponse 后，必须使用 WebResponse.Close 或 Stream.Close 关闭响应。如果不关闭每个响应，应用程序将用完与服务器的连接，而无法处理其他请求。
使用 WebResponse 的 GetResponseStream 方法从网络资源中获取包含响应数据的流。还可以访问 WebResponse 的属性或将 WebResponse 转换为特定于协议的实例以读取特定于协议的属性。例如，若要访问 HttpWebResponse 的 HTTP 特定的属性，请将 WebResponse 转换为 HttpWebResponse。下面的代码示例显示如何访问 HTTP 特定的属性和读取响应流。
[C#]
// Read an HTTP-specific property.
if (wResp is HttpWebResponse)
{
DateTime Updated = ((HttpWebResponse)wResp).LastModified;
}
// Get the response stream.
Stream respStream = wResp.GetResponseStream();
// This example uses a StreamReader to read the entire response
// into a string and then writes the string to the console.
StreamReader reader = new StreamReader(respStream, Encoding.ASCII);
String respHTML = reader.ReadToEnd();
Console.WriteLine(respHTML);
// Close the response stream.
respStream.Close();
[Visual Basic]
' Read an HTTP-specific property.
If TypeOf wResp Is HttpWebResponse Then
Dim updated As DateTime = Ctype(wResp,HttpWebResponse).LastModified
End If
' Get the response stream.
Dim respStream As Stream = wResp.GetResponseStream()
' This example uses a StreamReader to read the entire response
' into a string and then writes the string to the console.
Dim reader As StreamReader = _
New StreamReader(respStream, Encoding.ASCII)
Dim respHTML as String = reader.ReadToEnd()
Console.WriteLine(respHTML)
' Close the response stream
respStream.Close()
如果应用程序只需要 WebResponse 中返回的标头信息并忽略任何返回的数据，则不必获取响应流。下面的代码示例显示如何从 Internet 主机返回服务器标头信息。
[C#]
WebRequest wReq = WebRequest.Create("http://www.contoso.com");
WebResponse wResp = wReq.GetResponse();
string server = wResp.Headers["Server"];
[Visual Basic]
Dim wReq As WebRequest = WebRequest.Create("http://www.contoso.com")
Dim wResp As WebResponse = wReq.GetResponse()
Dim server As String = wResp.Headers("Server")
读取响应中的数据后，必须使用 Stream.Close 方法关闭所有打开的流或使用 WebResponse.Close 方法关闭响应。
[C#]
wResp.Close();
[Visual Basic]
wResp.Close()
不必在响应流和 WebResponse 上都调用 Close 方法，但这样做并没有什么害处。WebResponse.Close 在关闭响应时调用 Stream.Close。
以下示例应用程序说明如何使用 WebRequest 和 WebResponse 类。
[C#]
using System;
using System.Net;
using System.Text;
using System.IO;
class ClientGet {
public static void Main(string[] args)
{
if (args.Length < 1)
{
showusage();
return;
}
// Get the URI from the command line.
Uri site = new Uri(args[0]);
// Create the request instance.
WebRequest wReq = WebRequest.Create(site);
// Set the HTTP-specific UserAgent property
if (wReq is HttpWebRequest)
{
((HttpWebRequest)wReq).UserAgent =
".NET Framework Example Client";
}
// Get the response instance
WebResponse wResp = wReq.GetResponse();
// Read an HTTP-specific property.
if (wResp is HttpWebResponse)
{
DateTime updated = ((HttpWebResponse)wResp).LastModified;
}
// Get the response stream.
Stream respStream = wResp.GetResponseStream();
// This example uses a StreamReader to read the entire response
// into a string and then writes the string to the console.
StreamReader reader =
new StreamReader(respStream, Encoding.ASCII);
String respHTML = reader.ReadToEnd();
Console.WriteLine(respHTML);
// Close the response and response stream.
wResp.Close();
}
public static void showusage()
{
Console.WriteLine("Attempts to GET a URI.");
Console.WriteLine("\r\nUsage:");
Console.WriteLine(" ClientGet URI");
Console.WriteLine("Example:");
Console.WriteLine(" ClientGet http://www.contoso.com/");
}
}
[Visual Basic]
Imports System
Imports System.Net
Imports System.IO
Imports System.Text
Imports Microsoft.VisualBasic
Class ClientGet
Public Shared Sub Main()
Dim Args() As String = System.Environment.GetCommandLineArgs()
If Args.Length < 2 Then
ShowUsage
Return
End If
' Get the URI from the command line.
Dim site As Uri = New Uri(Args(1))
' Create the request instance.
Dim wReq as WebRequest = WebRequest.Create(site)
' Set the HTTP-specific UserAgent property.
If TypeOf wReq Is HttpWebRequest Then
CType(wReq, HttpWebRequest).UserAgent = _
".NET Framework Example Client"
End If
' Get the response instance.
Dim wResp As WebResponse = wReq.GetResponse()
' Read an HTTP-specific property
If TypeOf wResp is HttpWebResponse Then
Dim updated As DateTime = CType(wResp,HttpWebResponse).LastModified
End If
' Get the response stream.
Dim respStream As Stream = wResp.GetResponseStream()
' Use the Stream. This example reads the entire response with a
' StreamReader into a string, and then writes the string to the
' console.
Dim reader As StreamReader = _
New StreamReader(respStream, Encoding.ASCII)
Dim respHTML As String = reader.ReadToEnd()
Console.WriteLine(respHTML)
' Close the response and response stream.
wResp.Close()
End Sub
Public Shared Sub ShowUsage
Console.WriteLine("Attempts to GET a URI")
Console.WriteLine(Constants.vbCrLf & "Usage")
Console.WriteLine(" ClientGet URI")
Console.WriteLine("Example")
Console.WriteLIne(" ClientGet http://www.contoso.com")
End Sub
End Class
