当我意识到它丢失时，我试图将值设置为来自 PCL 的值。这是我的 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);
    }
}

我什至不知道要在问题中添加什么细节。请告诉我，这样我就可以了。

最佳答案

根据 MSDN 上关于 HttpWebRequest.UserAgent Property 的文档

这是版本信息

Version Information

• .NET 框架 从 1.1 开始可用
• 银光 从 5.0 开始可用
• Windows Phone Silverlight 从 7.0 开始可用

您会注意到，与 HttpWebRequest Class 不同，这里没有提到可移植类库 (PCL)。

Version Information

• 通用 Windows 平台 自 4.5 开始可用
• .NET 框架 从 1.1 开始可用
• 可移植类库 支持于:可移植 .NET 平台
• 银光 从 2.0 开始可用
• Windows Phone Silverlight 从 7.0 开始可用
• Windows 手机 自 8.1 起可用

所以这意味着访问 HttpWebRequest.UserAgent Property在 PCL 中不可用

现在虽然有一个HttpWebRequest.Headers Property在可移植类库中可用，您应该注意 MSDN 的以下注释

Remarks

The Headers collection contains the protocol headers associated with the request. The following table lists the HTTP headers that are not stored in the Headers collection but are either set by the system or set by properties or methods.

然后他们继续在该列表中包含 User-Agent header 。

The Add method throws an ArgumentException if you try to set one of these protected headers.
...
You should not assume that the header values will remain unchanged, because Web servers and caches may change or add headers to a Web request.

它很可能是由执行 PCL 的系统设置的。

Xamarin documentation显示该属性应该存在，但我认为此信息可能不正确。

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面向的平台，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几个兼容性因素的限制:

• 它们必须在您选择的目标之间共享。

• 必须在这些目标中表现相似。

• 它们不能成为弃用的候选对象。

• 它们必须在可移植环境中有意义，尤其是当支持成员不可移植时。

这是一些 documentation from Xamarin

更新

我能够使用 HttpClient用于设置 User-Agent 并发出 Get 请求的 API。

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
}

关于c# - 为什么我的 HttpWebRequest 类中缺少 UserAgent 属性？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/38593271/