HttpClient 上的 ObjectDisposedException

Question

我有一个包含多个 API 调用的 Windows 通用项目。 一种方法拒绝工作，即使我的其他调用完全像这样工作。 我已经尝试了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;
        }
    }
}

编辑

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;
        }
    }
}

堆栈跟踪

 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 被抛出，因为您要在 req.Content.ReadAsStringAsync() 完成之前处理 HttpRequestMessage 和 HttpClient。

请注意，req.Content.ReadAsStringAsync() 是一种异步方法。您需要等待它完成才能处理HttpClient。

另外，你好像在req.Content中调用ReadAsStringAsync，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

ObjectDisposedException 被抛出的实际原因是因为HttpClient 在完成请求后立即释放Content。看看docs。

因此，如果您需要阅读Request 的内容，例如在测试中，请确保在调用SendAsync 之前阅读它。

Answer 3

您正在访问请求内容，而不是响应。

这个

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();