我正在使用我为HttpClient制作的包装类。当我调用SendAsync时，它会抛出一个异常，说明“任务被取消”。
我不明白为什么会发生这种情况，我没有传递一个取消令牌，我正在使用Await在返回之前等待结果。有人可以帮忙吗？
编辑1：我已经读过HttpClient和.NET 4.0中的使用问题，如果编码不正确的话。 http://forums.asp.net/t/1863442.aspx?HttpClient+Error+A+Task+was+canceled+。但是，我使用的是.NET 4.5和Await关键字，所以我不应该遇到这个问题。此外，当我在断点期间将HttpClient对象鼠标悬停在抛出异常时，它并不表示该对象已被丢弃。
来自HttpClientWrapper Class的代码片段，其中发生异常;
Try
Return Await Client.SendAsync(Message, HttpCompletionOption.ResponseHeadersRead)
Catch ex As Exception
If DebugMode Then Output(OutputBufferKey, Method.ToString & " " & Url & " " & ex.Message, OutputType.Error)
End Try
自定义HttpClientWrapper类;
Public Class HttpClientWrapper
Implements IDisposable
Protected Handler As New HttpClientHandler
Protected Client As HttpClient = Nothing
Public Property OutputBufferKey As String = Nothing
Public Property CancellationToken As CancellationToken? = Nothing
Public Sub New(Optional Proxy As WebProxy = Nothing, Optional Cookies As CookieContainer = Nothing, Optional UserAgent As String = Nothing, _
Optional OutputBufferKey As String = Nothing, Optional CancellationToken As CancellationToken? = Nothing)
Me.OutputBufferKey = OutputBufferKey
Me.CancellationToken = CancellationToken
If UserAgent Is Nothing Then UserAgent = RandomString(UserAgents)
SetCookies(Cookies)
SetProxy(Proxy)
Handler.AutomaticDecompression = DecompressionMethods.GZip Or DecompressionMethods.Deflate
Client = New HttpClient(Handler, True)
Client.Timeout = New TimeSpan(DefaultTimeout)
Client.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate")
Client.DefaultRequestHeaders.Add("Accept-Language", "en-US,en;q=0.5")
Client.DefaultRequestHeaders.Add("DNT", "1")
Client.DefaultRequestHeaders.Add("User-Agent", UserAgent)
End Sub
Public Sub New(User As User, Optional OutputBufferKey As String = Nothing, Optional CancellationToken As CancellationToken? = Nothing)
Me.New(User.GetProxy, User.Cookies, User.UserAgent, OutputBufferKey, CancellationToken)
End Sub
Public Sub New(User As User, Task As Task)
Me.New(User.GetProxy, User.Cookies, User.UserAgent, Task.OutputBufferKey, Task.CancellationToken.Token)
End Sub
Public Sub SetTimeout(Value As Integer)
Client.Timeout = New TimeSpan(Value)
End Sub
Private Sub SetCookies(Cookies As CookieContainer)
If Cookies Is Nothing Then
Handler.UseCookies = False
Else
Handler.UseCookies = True
Handler.CookieContainer = Cookies
End If
End Sub
Private Sub SetProxy(Proxy As WebProxy)
If Proxy Is Nothing Then
Handler.UseProxy = False
Else
Handler.UseProxy = True
Handler.Proxy = Proxy
End If
End Sub
Public Async Function WebRequestByteAsync(Url As String, Optional RequestMethod As RequestMethod = RequestMethod.GET, Optional Content As Object = Nothing, _
Optional ContentType As ContentType = ContentType.Default, Optional Accept As String = DefaultAcceptString, _
Optional AdditionalHeaders As NameValueCollection = Nothing, Optional Referer As String = Nothing, Optional NoCache As Boolean = False) _
As Tasks.Task(Of Byte())
Using Response As HttpResponseMessage = Await WebRequestAsync(Url, RequestMethod, Content, ContentType, Accept, AdditionalHeaders, Referer, NoCache)
If Response IsNot Nothing Then
If Response.StatusCode = HttpStatusCode.OK Then
Return Await GetResponseByteAsync(Response)
Else
Output(OutputBufferKey, Url & " did not return OK (200)", OutputType.Error)
End If
End If
Return Nothing
End Using
End Function
Public Async Function WebRequestStringAsync(Url As String, Optional RequestMethod As RequestMethod = RequestMethod.GET, Optional Content As Object = Nothing, _
Optional ContentType As ContentType = ContentType.Default, Optional Accept As String = DefaultAcceptString, _
Optional AdditionalHeaders As NameValueCollection = Nothing, Optional Referer As String = Nothing, Optional NoCache As Boolean = False) _
As Tasks.Task(Of String)
Using Response As HttpResponseMessage = Await WebRequestAsync(Url, RequestMethod, Content, ContentType, Accept, AdditionalHeaders, Referer, NoCache)
If Response IsNot Nothing Then
If Response.StatusCode = HttpStatusCode.OK Then
Return Await GetResponseStringAsync(Response)
Else
Output(OutputBufferKey, Url & " did not return OK (200)", OutputType.Error)
End If
End If
Return Nothing
End Using
End Function
Public Async Function WebRequestAsync(Url As String, Optional RequestMethod As RequestMethod = RequestMethod.GET, Optional Content As Object = Nothing, _
Optional ContentType As ContentType = ContentType.Default, Optional Accept As String = DefaultAcceptString, _
Optional AdditionalHeaders As NameValueCollection = Nothing, Optional Referer As String = Nothing, Optional NoCache As Boolean = False, _
Optional CanBeCancelled As Boolean = True) _
As Tasks.Task(Of HttpResponseMessage)
Dim Method As HttpMethod = Nothing
Select Case RequestMethod
Case Variables.RequestMethod.DELETE : Method = HttpMethod.Delete
Case Variables.RequestMethod.GET : Method = HttpMethod.Get
Case Variables.RequestMethod.OPTIONS : Method = HttpMethod.Options
Case Variables.RequestMethod.POST : Method = HttpMethod.Post
Case Variables.RequestMethod.PUT : Method = HttpMethod.Put
End Select
'prepare message
Dim Message As New HttpRequestMessage(Method, Url)
Message.Headers.ExpectContinue = False
Message.Headers.TryAddWithoutValidation("Accept", Accept)
If Referer IsNot Nothing Then Message.Headers.Add("Referer", Referer)
If NoCache Then
Message.Headers.Add("Pragma", "no-cache")
Message.Headers.Add("Cache-Control", "no-cache")
End If
If AdditionalHeaders IsNot Nothing Then
For Each Key In AdditionalHeaders.AllKeys
Message.Headers.TryAddWithoutValidation(Key, AdditionalHeaders(Key))
Next
End If
'set content
If Content IsNot Nothing Then
Dim ContentTypeString As String = GetEnumDescription(ContentType)
Dim ContentBytes As Byte() = Nothing
If TypeOf Content Is String Then
ContentBytes = Encoding.UTF8.GetBytes(CType(Content, String))
ElseIf TypeOf Content Is Byte() Then
ContentBytes = CType(Content, Byte())
ElseIf TypeOf Content Is MultiPartPostData Then
Dim MultiPartPostData As MultiPartPostData = CType(Content, MultiPartPostData)
ContentBytes = MultiPartPostData.Bytes
ContentTypeString += "; boundary=" & MultiPartPostData.Boundary
End If
Dim ByteArrayContent As New ByteArrayContent(ContentBytes)
ByteArrayContent.Headers.Add("Content-Type", ContentTypeString)
Message.Content = ByteArrayContent
End If
'get response
Output(OutputBufferKey, RequestMethod.ToString & " " & Url, OutputType.Debug)
Try
Return Await Client.SendAsync(Message, HttpCompletionOption.ResponseHeadersRead)
Catch ex As Exception
If DebugMode Then Output(OutputBufferKey, Method.ToString & " " & Url & " " & ex.Message, OutputType.Error)
End Try
Return Nothing
End Function
Public Async Function GetResponseByteAsync(Response As HttpResponseMessage) As Tasks.Task(Of Byte())
If Response Is Nothing Then
If DebugMode Then Output(OutputBufferKey, "Unable read content from response message: Is nothing", OutputType.Error)
Else
Try
Return Await Response.Content.ReadAsByteArrayAsync
Catch ex As Exception
If DebugMode Then Output(OutputBufferKey, "Unable read content from " & Response.RequestMessage.RequestUri.ToString & ": " & ex.Message, OutputType.Error)
End Try
End If
Return Nothing
End Function
Public Async Function GetResponseStringAsync(Response As HttpResponseMessage) As Tasks.Task(Of String)
If Response Is Nothing Then
If DebugMode Then Output(OutputBufferKey, "Unable read content from response message: Is nothing", OutputType.Error)
Else
Try
Return Await Response.Content.ReadAsStringAsync
Catch ex As Exception
If DebugMode Then Output(OutputBufferKey, "Unable read content from " & Response.RequestMessage.RequestUri.ToString & ": " & ex.Message, OutputType.Error)
End Try
End If
Return Nothing
End Function
#Region "IDisposable Support"
Private disposedValue As Boolean ' To detect redundant calls
' IDisposable
Protected Overridable Sub Dispose(disposing As Boolean)
If Not Me.disposedValue Then
If disposing Then
If Client IsNot Nothing Then Client.Dispose()
End If
End If
Me.disposedValue = True
End Sub
' This code added by Visual Basic to correctly implement the disposable pattern.
Public Sub Dispose() Implements IDisposable.Dispose
' Do not change this code. Put cleanup code in Dispose(disposing As Boolean) above.
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
#End Region
End Class
主要代码;
Module Main
Dim ProxyUrl As String = "proxies.txt"
Sub main()
DebugMode = True
OutputToConsole = True
Dim Proxies As New ProxyList(ProxyUrl)
Try
Run(Proxies).GetAwaiter.GetResult()
Catch ex As Exception
MsgBox(ex.Message)
End Try
Console.ReadLine()
End Sub
Public Async Function Run(Proxies As ProxyList) As Tasks.Task
Dim Proxy As WebProxy = Await Proxies.GetProxy
Using Client As New HttpClientWrapper(Proxy)
Dim Response As String = Await Client.WebRequestStringAsync("http://www.google.com")
If Response IsNot Nothing Then Console.WriteLine(Response)
End Using
End Function
End Module
答案 0 :(得分：4)
HTTPClient表示在达到超时时已取消任务：您确定您有大量的超时/网络服务及时响应吗？