HttpClient上ObjectDisposedException

Question

我有一個Windows通用項目與多個API調用。 一種方法拒絕工作eventhought我的其他調用工作完全是這樣的。 我試過using關鍵字認爲它可以解決問題。

功能：

public async Task<User> GetNewUser(string user_guid, OAuthTokens OAuth) 
{ 
    String userguidJSON = VALIDJSON_BELIEVE_ME; 
    using (var httpClient = new HttpClient()) 
    { 
     httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json")); 
     httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Authorization", Encrypt(OAuth.Accesstoken)); 

     using (HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, BASE_URL + URL_USERS + "/data")) 
     { 
      req.Content = new StringContent(userguidJSON, Encoding.UTF8, "application/json"); 
      await httpClient.SendAsync(req).ContinueWith(respTask => 
      { 
       Debug.WriteLine(req.Content.ReadAsStringAsync()); //Error is thrown ono this line 
      }); 
      return null; 
     } 
    } 
} 

編輯

public async Task<User> GetNewUser(string user_guid, OAuthTokens OAuth) 
{ 
    String userguidJSON = VALIDJSON_BELIEVE_ME; 
    using (var httpClient = new HttpClient()) 
    { 
     httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json")); 
     httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Authorization", Encrypt(OAuth.Accesstoken)); 

     using (HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, BASE_URL + URL_USERS + "/data")) 
     { 
      req.Content = new StringContent(userguidJSON, Encoding.UTF8, "application/json"); 
      await httpClient.SendAsync(req); 
      var result = await req.Content.ReadAsStringAsync(); //Cannot access a disposed object. Object name: 'System.Net.Http.StringContent'. 
      Debug.WriteLine(result); 
      return null; 
     } 
    } 
} 

堆棧跟蹤

at System.Net.Http.HttpContent.CheckDisposed() 
    at System.Net.Http.HttpContent.ReadAsStringAsync() 
    at Roadsmart.Service.RoadsmartService.<GetNewUser>d__2e.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() 
    at Roadsmart.ViewModel.SettingsPageViewModel.<SetNewProfilePicture>d__1e.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__3(Object state) 
    at System.Threading.WinRTSynchronizationContext.Invoker.InvokeCore() 

Answer 1

ObjectDisposedException被拋出，因爲你在處理之前的HttpRequestMessage和HttpClientreq.Content.ReadAsStringAsync()完成。

請注意，req.Content.ReadAsStringAsync()是一種異步方法。在處理HttpClient之前，您需要等待它完成。

此外，你似乎要撥打ReadAsStringAsync在req.Content，應該不是response.Content？

using (HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, BASE_URL + URL_USERS + "/data")) 
{ 
    req.Content = new StringContent(userguidJSON, Encoding.UTF8, "application/json"); 
    var response = await httpClient.SendAsync(req); 
    var result = await response.Content.ReadAsStringAsync();//await it 
    Debug.WriteLine(result); 
    return null; 
} 

幾乎沒有理由在處理異步/等待時使用ContinueWith。所有這些都是由編譯器爲你完成的。

Answer 2

您正在訪問請求內容，而不是響應。

這

await httpClient.SendAsync(req); 
var result = await req.Content.ReadAsStringAsync(); //Cannot access a disposed object. Object name: 'System.Net.Http.StringContent'. 

應該

var response = httpClient.SendAsync(req); 
var result = await response.Content.ReadAsStringAsync(); 

Answer 3

實際原因ObjectDisposedException拋出是因爲HttpClient完成的請求後，立即部署了Content。看看docs。

所以，如果你需要閱讀Request的內容，例如在測試中，請確保調用前SendAsync閱讀它。