当我意识到它缺失时，我试图将价值设置为PCL。这就是我的HttpWebRequest类的样子：
#region Assembly System.Net.Requests, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
// C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.5\Profile\Profile111\System.Net.Requests.dll
#endregion
using System.IO;
namespace System.Net
{
//
// Summary:
// Provides an HTTP-specific implementation of the System.Net.WebRequest class.
public class HttpWebRequest : WebRequest
{
//
// Summary:
// Gets or sets the value of the Accept HTTP header.
//
// Returns:
// The value of the Accept HTTP header. The default value is null.
public string Accept { get; set; }
//
// Summary:
// Gets or sets a value that indicates whether to buffer the received from the Internet
// resource.
//
// Returns:
// Returns System.Boolean.true to enable buffering of the data received from the
// Internet resource; false to disable buffering. The default is true.
public virtual bool AllowReadStreamBuffering { get; set; }
public override string ContentType { get; set; }
//
// Summary:
// Gets or sets a timeout, in milliseconds, to wait until the 100-Continue is received
// from the server.
//
// Returns:
// Returns System.Int32.The timeout, in milliseconds, to wait until the 100-Continue
// is received.
public int ContinueTimeout { get; set; }
//
// Summary:
// Gets or sets the cookies associated with the request.
//
// Returns:
// A System.Net.CookieContainer that contains the cookies associated with this request.
public virtual CookieContainer CookieContainer { get; set; }
//
// Summary:
// Gets or sets authentication information for the request.
//
// Returns:
// An System.Net.ICredentials that contains the authentication credentials associated
// with the request. The default is null.
public override ICredentials Credentials { get; set; }
//
// Summary:
// Gets a value that indicates whether a response has been received from an Internet
// resource.
//
// Returns:
// true if a response has been received; otherwise, false.
public virtual bool HaveResponse { get; }
//
// Summary:
// Specifies a collection of the name/value pairs that make up the HTTP headers.
//
// Returns:
// A System.Net.WebHeaderCollection that contains the name/value pairs that make
// up the headers for the HTTP request.
//
// Exceptions:
// T:System.InvalidOperationException:
// The request has been started by calling the System.Net.HttpWebRequest.GetRequestStream,
// System.Net.HttpWebRequest.BeginGetRequestStream(System.AsyncCallback,System.Object),
// System.Net.HttpWebRequest.GetResponse, or System.Net.HttpWebRequest.BeginGetResponse(System.AsyncCallback,System.Object)
// method.
public override WebHeaderCollection Headers { get; set; }
//
// Summary:
// Gets or sets the method for the request.
//
// Returns:
// The request method to use to contact the Internet resource. The default value
// is GET.
//
// Exceptions:
// T:System.ArgumentException:
// No method is supplied.-or- The method string contains invalid characters.
public override string Method { get; set; }
//
// Summary:
// Gets the original Uniform Resource Identifier (URI) of the request.
//
// Returns:
// A System.Uri that contains the URI of the Internet resource passed to the System.Net.WebRequest.Create(System.String)
// method.
public override Uri RequestUri { get; }
//
// Summary:
// Gets a value that indicates whether the request provides support for a System.Net.CookieContainer.
//
// Returns:
// Returns System.Boolean.true if a System.Net.CookieContainer is supported; otherwise,
// false.
public virtual bool SupportsCookieContainer { get; }
//
// Summary:
// Gets or sets a System.Boolean value that controls whether default credentials
// are sent with requests.
//
// Returns:
// true if the default credentials are used; otherwise false. The default value
// is false.
//
// Exceptions:
// T:System.InvalidOperationException:
// You attempted to set this property after the request was sent.
public override bool UseDefaultCredentials { get; set; }
//
// Summary:
// Cancels a request to an Internet resource.
public override void Abort();
//
// Summary:
// Begins an asynchronous request for a System.IO.Stream object to use to write
// data.
//
// Parameters:
// callback:
// The System.AsyncCallback delegate.
//
// state:
// The state object for this request.
//
// Returns:
// An System.IAsyncResult that references the asynchronous request.
//
// Exceptions:
// T:System.Net.ProtocolViolationException:
// The System.Net.HttpWebRequest.Method property is GET or HEAD.-or- System.Net.HttpWebRequest.KeepAlive
// is true, System.Net.HttpWebRequest.AllowWriteStreamBuffering is false, System.Net.HttpWebRequest.ContentLength
// is -1, System.Net.HttpWebRequest.SendChunked is false, and System.Net.HttpWebRequest.Method
// is POST or PUT.
//
// T:System.InvalidOperationException:
// The stream is being used by a previous call to System.Net.HttpWebRequest.BeginGetRequestStream(System.AsyncCallback,System.Object)-or-
// System.Net.HttpWebRequest.TransferEncoding is set to a value and System.Net.HttpWebRequest.SendChunked
// is false.-or- The thread pool is running out of threads.
//
// T:System.NotSupportedException:
// The request cache validator indicated that the response for this request can
// be served from the cache; however, requests that write data must not use the
// cache. This exception can occur if you are using a custom cache validator that
// is incorrectly implemented.
//
// T:System.Net.WebException:
// System.Net.HttpWebRequest.Abort was previously called.
//
// T:System.ObjectDisposedException:
// In a .NET Compact Framework application, a request stream with zero content length
// was not obtained and closed correctly. For more information about handling zero
// content length requests, see Network Programming in the .NET Compact Framework.
public override IAsyncResult BeginGetRequestStream(AsyncCallback callback, object state);
//
// Summary:
// Begins an asynchronous request to an Internet resource.
//
// Parameters:
// callback:
// The System.AsyncCallback delegate
//
// state:
// The state object for this request.
//
// Returns:
// An System.IAsyncResult that references the asynchronous request for a response.
//
// Exceptions:
// T:System.InvalidOperationException:
// The stream is already in use by a previous call to System.Net.HttpWebRequest.BeginGetResponse(System.AsyncCallback,System.Object)-or-
// System.Net.HttpWebRequest.TransferEncoding is set to a value and System.Net.HttpWebRequest.SendChunked
// is false.-or- The thread pool is running out of threads.
//
// T:System.Net.ProtocolViolationException:
// System.Net.HttpWebRequest.Method is GET or HEAD, and either System.Net.HttpWebRequest.ContentLength
// is greater than zero or System.Net.HttpWebRequest.SendChunked is true.-or- System.Net.HttpWebRequest.KeepAlive
// is true, System.Net.HttpWebRequest.AllowWriteStreamBuffering is false, and either
// System.Net.HttpWebRequest.ContentLength is -1, System.Net.HttpWebRequest.SendChunked
// is false and System.Net.HttpWebRequest.Method is POST or PUT.-or- The System.Net.HttpWebRequest
// has an entity body but the System.Net.HttpWebRequest.BeginGetResponse(System.AsyncCallback,System.Object)
// method is called without calling the System.Net.HttpWebRequest.BeginGetRequestStream(System.AsyncCallback,System.Object)
// method. -or- The System.Net.HttpWebRequest.ContentLength is greater than zero,
// but the application does not write all of the promised data.
//
// T:System.Net.WebException:
// System.Net.HttpWebRequest.Abort was previously called.
public override IAsyncResult BeginGetResponse(AsyncCallback callback, object state);
//
// Summary:
// Ends an asynchronous request for a System.IO.Stream object to use to write data.
//
// Parameters:
// asyncResult:
// The pending request for a stream.
//
// Returns:
// A System.IO.Stream to use to write request data.
//
// Exceptions:
// T:System.ArgumentNullException:
// asyncResult is null.
//
// T:System.IO.IOException:
// The request did not complete, and no stream is available.
//
// T:System.ArgumentException:
// asyncResult was not returned by the current instance from a call to System.Net.HttpWebRequest.BeginGetRequestStream(System.AsyncCallback,System.Object).
//
// T:System.InvalidOperationException:
// This method was called previously using asyncResult.
//
// T:System.Net.WebException:
// System.Net.HttpWebRequest.Abort was previously called.-or- An error occurred
// while processing the request.
public override Stream EndGetRequestStream(IAsyncResult asyncResult);
//
// Summary:
// Ends an asynchronous request to an Internet resource.
//
// Parameters:
// asyncResult:
// The pending request for a response.
//
// Returns:
// A System.Net.WebResponse that contains the response from the Internet resource.
//
// Exceptions:
// T:System.ArgumentNullException:
// asyncResult is null.
//
// T:System.InvalidOperationException:
// This method was called previously using asyncResult.-or- The System.Net.HttpWebRequest.ContentLength
// property is greater than 0 but the data has not been written to the request stream.
//
// T:System.Net.WebException:
// System.Net.HttpWebRequest.Abort was previously called.-or- An error occurred
// while processing the request.
//
// T:System.ArgumentException:
// asyncResult was not returned by the current instance from a call to System.Net.HttpWebRequest.BeginGetResponse(System.AsyncCallback,System.Object).
public override WebResponse EndGetResponse(IAsyncResult asyncResult);
}
}
我甚至不知道要在问题中添加什么细节。请告诉我，我可以。
答案 0 :(得分：12)
根据MSDN上有关HttpWebRequest.UserAgent Property
的文档这是版本信息
您会注意到，与HttpWebRequest Class
不同，没有提及可移植类库（PCL）所以这意味着PCL中无法访问HttpWebRequest.UserAgent Property
现在虽然可移植类库中有HttpWebRequest.Headers Property，但您应该注意以下来自MSDN的评论
Headers集合包含与之关联的协议标头 请求。下表列出了不是的HTTP标头 存储在Headers集合中，但由系统或 由属性或方法设置。
然后他们继续在该列表中包含User-Agent标题。
如果尝试设置其中一个，则Add方法抛出ArgumentException 这些受保护的标题 ...
你不应该假设标题值保持不变， 因为Web服务器和缓存可能会更改或添加标头到Web 请求。
最有可能是由PCL正在执行的系统设置。
Xamarin documentation表明该属性应该存在但我相信这些信息可能不正确。
System.Net.HttpWebRequest.UserAgent Property
Gets or sets the value of the User-agent HTTP header.
Syntax
public String UserAgent { get; set; }
Value
A String containing the value of the HTTP User-agent header. The default value is null.
Remarks
Note: For additional information see section 14.43 of IETF RFC 2616 - HTTP/1.1.
Requirements
Namespace: System.Net
Assembly: System (in System.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0
您应该确认可移植类库所针对的平台，以确保不存在会删除UserAgent属性的冲突。
PCL：支持的类型和成员
可移植类库项目中可用的类型和成员受到多个兼容性因素的约束：
必须在您选择的目标之间共享它们。
这些目标必须表现得相似。
他们不能成为弃用的候选人。
它们必须在便携式环境中有意义，特别是当支持成员不可携带时。
<强>更新
我可以使用HttpClient API设置User-Agent并发出Get请求。
public async Task GetURL(string url) {
var handler = new HttpClientHandler();
var httpClient = new HttpClient(handler);
httpClient.DefaultRequestHeaders.Add("User-Agent", "My Custom User Agent");
var request = new HttpRequestMessage(HttpMethod.Get, url);
var response = await httpClient.SendAsync(request);
//...other code removed for brevity
}
答案 1 :(得分：6)
我不清楚你是想添加它还是检索它，所以我会在你指定你正在做的事情后更新我的答案。
用户代理将包含在Http Header中。您包含的类具有名为Headers的属性，该属性应该在其中找到/设置。
添加：类型WebHeaderCollection有一个字符串索引器，它将标题字段作为键。您应该能够添加它或从那里检索它。
request.Headers["User-Agent"] = "your user agent string";
对于检索，您可以使用字符串索引器。
var userAgent = request.Headers["User-Agent"];
您可以在此处找到WebHeaderCollection
类型的源代码