在vb.net中的回调中处理错误

新的异步调用，在这种情况下使用回调 . 以下代码有效 .

代码调用了一个非常基本的Web服务，它返回xml . 如果服务返回xml，代码工作正常，msresponse用xml填充，函数“GetWebserviceData”能够通过函数“CreateDT_Implantable_Device”向调用者返回解析的msresponse . 只要返回xml，这就可以毫无障碍地工作 .

如果服务没有返回任何xml，则respcallback中的此行会抛出一个错误：

“Dim resp As HttpWebResponse = _ CType（req.EndGetResponse（ar），HttpWebResponse）”

抛出的错误是“400-bad request）”

似乎如果服务失败，则回调的线程无法正确完成，整个应用程序在此时锁定 . 它返回到名为“GetWebserviceData”的vb.net表单，并且没有任何控件工作：表单被冻结 .

我已经尝试将try tryches放在回调函数中的所有代码（如下所示），“respcallback”和“GetWebserviceData”中的所有代码（不在下面的代码中） . 正如你在“respcallback”中的try catch中看到的，如果出现此错误，我试图结束回调线程并填充msResponse，以便后面的“GetWebserviceData”中的循环得到满足并将结束 . 这是循环 .

Do while msResponse.Length = 0'等待循环

完整的代码如下：任何帮助将不胜感激 . 我正在尝试允许Web服务失败，让代码继续并提出一个消息框说服务失败 . 显然，我的尝试捕获不能像“respcallback”中看到的那样工作;代码只是锁定 .

代码如下 . 任何帮助，将不胜感激：

公共函数GetWebserviceData（psUDI As String）As DataTable FillGS1Delimeters（）miUDI = psUDI ' Create the request object. '尝试Dim wreq As HttpWebRequest = _ CType（WebRequest.Create（“https://accessgudid.nlm.nih.gov/api/v1/devices/lookup.xml?udi=”＆psUDI），HttpWebRequest）

' Create the state object.
    Dim rs As RequestState = New RequestState()

    ' Put the request into the state so it can be passed around...
    rs.Request = wreq


    ' Issue the async request..
    Dim r As IAsyncResult = _
       CType(wreq.BeginGetResponse( _
       New AsyncCallback(AddressOf RespCallback), rs), IAsyncResult)

    ' Wait until the ManualResetEvent is set so that the application
    ' does not exit until after the callback is called.
    allDone.WaitOne()


    Do While msResponse.Length = 0
        'wait
    Loop

    'Catch ex As Exception
    'msResponse = "Error"
    'MessageBox.Show("an error")

    'End Try
    Return CreateDT_Implantable_Device(msResponse)
End Function

私有函数CreateDT_Implantable_Device（wsResponse As String）Dim dtDevice As DataTable = New DataTable（）Dim dtPI As DataTable = New DataTable（）Dim ds As DataSet = New DataSet

createEmptyDeviceTable(dtDevice)
    createEmptyPITable(dtPI)
    ds = StoreXMLAsText(msResponse)
    dtPI = FillRecord(dtDevice, ds)
    Return dtPI
End Function

Shared Sub RespCallback（ar As IAsyncResult）尝试'从异步结果中获取RequestState对象 . Dim rs As RequestState = CType（ar.AsyncState，RequestState）

' Get the HttpWebRequest from RequestState..
        Dim req As HttpWebRequest = rs.Request

        ' Call EndGetResponse, which returns the HttpWebResponse object
        ' that came from the request issued above.
        Dim resp As HttpWebResponse = _
            CType(req.EndGetResponse(ar), HttpWebResponse)

        ' Start reading data from the respons stream.
        Dim ResponseStream As Stream = resp.GetResponseStream()

        ' Store the reponse stream in RequestState to read
        ' the stream asynchronously.
        rs.ResponseStream = ResponseStream

        ' Pass rs.BufferRead to BeginRead. Read data into rs.BufferRead.
        Dim iarRead As IAsyncResult = _
           ResponseStream.BeginRead(rs.BufferRead, 0, BUFFER_SIZE, _
           New AsyncCallback(AddressOf ReadCallBack), rs)
    Catch ex As Exception
        msResponse = "Invalid UDI"
        MessageBox.Show(ex.Message)
        msResponse = "Error"
    End Try

End Sub

Shared Sub ReadCallBack（asAsResult As IAsyncResult）'从AsyncResult获取RequestState对象 . Dim rs As RequestState = CType（asyncResult.AsyncState，RequestState）

' Retrieve the ResponseStream that was set in RespCallback.
    Dim responseStream As Stream = rs.ResponseStream

    ' Read rs.BufferRead to verify that it contains data.
    Dim read As Integer = responseStream.EndRead(asyncResult)
    If read > 0 Then
        ' Prepare a Char array buffer for converting to Unicode.
        Dim charBuffer(1024) As Char

        ' Convert byte stream to Char array and then String.
        ' len contains the number of characters converted to Unicode.
        Dim len As Integer = _
          rs.StreamDecode.GetChars(rs.BufferRead, 0, read, charBuffer, 0)
        Dim str As String = New String(charBuffer, 0, len)

        ' Append the recently read data to the RequestData stringbuilder 
        ' object contained in RequestState.
        rs.RequestData.Append( _
           Encoding.ASCII.GetString(rs.BufferRead, 0, read))

        ' Continue reading data until responseStream.EndRead
        ' returns –1.
        Dim ar As IAsyncResult = _
           responseStream.BeginRead(rs.BufferRead, 0, BUFFER_SIZE, _
           New AsyncCallback(AddressOf ReadCallBack), rs)
    Else
        If rs.RequestData.Length > 1 Then
            ' Display data to the console.
            Dim strContent As String
            strContent = rs.RequestData.ToString()
            msResponse = strContent
            Console.WriteLine(strContent)
        End If

        ' Close down the response stream.
        responseStream.Close()

        ' Set the ManualResetEvent so the main thread can exit.
        allDone.Set()
    End If

    Return
End Sub