c# – 为什么我的HttpWebRequest类中缺少UserAgent属性？

#region Assembly System.Net.Requests,Version=4.0.0.0,Culture=neutral,PublicKeyToken=b03f5f7f11d50a3a
// C:Program Files (x86)Reference AssembliesMicrosoftFramework.NETPortablev4.5ProfileProfile111System.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,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,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);
    }
}

我甚至不知道添加什么细节的问题.请告诉我,我可以.