HttpClient上的ObjectDisposedException

Question

我有一个带有多个API调用的Windows Universal Project。 一种方法拒绝工作eventhought我的其他调用完全像这样工作。 我尝试过using关键字，认为它可以解决问题。

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;
        }
    }
}

修改

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;
        }
    }
}

stacktrace

 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因为您在HttpRequestMessage完成之前处置HttpClient和req.Content.ReadAsStringAsync()而被抛出。

请注意req.Content.ReadAsStringAsync()是一种异步方法。在处置HttpClient之前，您需要等待它完成。

此外，您似乎在ReadAsStringAsync中呼叫req.Content，那不应该是response.Content吗？

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;
}

在处理async / await时几乎没有理由使用ContinueWith。所有这些都由编译器为您完成。

Answer 2

引发ObjectDisposedException的实际原因是HttpClient在完成请求后立即处理Content。看看docs。

因此，如果您需要阅读Request的内容，例如在测试中，请务必在调用SendAsync之前阅读。

Answer 3

您正在访问请求内容，而不是响应。

此

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

应该是

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