发生一个或多个错误：PostAsJsonAsync

Question

这是我得到的错误：

One or more errors occurred.
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()
   at myProject.mymethod(Int32[] myIds) in path\MyClass.cs:line 758

这是那个方法：

private void mymethod(int[] myIds)
{
    var uri = new Uri(string.Format(UriPath, string.Format(MyPath)));
    var client = GetHttpClient(uri);

    var postModel = new PostModel
    {
        Ids = myIds,
        LastUpdate = NewItem ? null : _lastUpdated
    };

    if (client != null)
    {
        var response = client.PostAsJsonAsync(uri, postModel).Result;//this line crashes

        if (response.IsSuccessStatusCode)
        {
            //doSomething
        }
    }
}

我调用了很多这样的方法，除了这个之外，它们都可以工作。当它被击中时，它需要很长时间，然后抛出这个异常。使用所有其他方法都不会发生错误。

这是内部异常：

Inner.System.Threading.Tasks.TaskCanceledException: A task was canceled.

这是我的 GetCLient() 方法：

private HttpClient GetHttpClient(Uri uri)
{
    var handler = new HttpClientHandler
    {
        CookieContainer = CoockieContainer
    };

    return new HttpClient(handler)
    {
        BaseAddress = uri
    };
}

API 方法如下：

[HttpPost]
    public IList<MyModel> MyAPIMethod(PostModel model)
    {
        List<MyModel> myTranslations;
        using (var db = new myEntities(GetDbConnStrByUser(new GetCookies().GetUserName())))
        {
            myTranslations = db.tblTranslations.Where(it => model.Ids.Contains(it.id)
                                    && (!model.Update.HasValue || it.update > model.LastUpdate.Value))
                                    .Select(it => new MyModel
                                    {
                                        Id = it.id,
                                        Name = it.name,
                                        Description = it.desc,
                                        LanguageId = it.language_id                                            
                                    }).ToList();
        }

        return myTranslations.GroupBy(x => new { x.Id, x.LanguageId }).Select(x => x.First()).ToList();
    }

可能发生超时。

Fiddler 返回此错误：等待操作超时。

Answer 1

.Result 试图将 Task 变成 T。它同步阻塞等待任务完成，只有在没有异常发生时才会返回 T。您应该预料到的两种异常类型是 OperationCanceledExceptions/TaskCanceledExceptions（当 HTTP 请求超时或使用 CancellationToken 并且令牌被取消时——我认为后者在这里不适用）和一般/HTTP 相关异常。

您可能遇到了 HTTP 超时情况。抛出异常需要很长时间吗？如果没有，您的 GetHttpClient 方法是什么样的？是否明确设置超时或取消令牌？

如果你想坚持同步方法而不返回任务，你应该这样做：

try
{    
    var response = client.PostAsJsonAsync(uri, postModel).Result;//this line crashes
} 
catch (OperationCanceledException oce) 
{
    // Tell the user that the request timed our or you cancelled a CancellationToken
}
catch (Exception exc) 
{
    // Look at the HttpClient docs and try to catch something more specific.  You don't want
    // to swallow StackOverflowExceptions or OutOfMemoryExceptions.
}

如果您愿意尝试异步，这正是您想要的：

private async Task mymethodAsync(int[] myIds)
{
    var uri = new Uri(string.Format(UriPath, string.Format(MyPath)));
    var client = GetHttpClient(uri);

    var postModel = new PostModel { ... };

    if (client != null)
    {
        try 
        {
            var response = await client.PostAsJsonAsync(uri, postModel);

            if (response.IsSuccessStatusCode)
            {
                //doSomething
            }
        }
        catch (OperationCanceledException oce) 
        {
            // because timeouts are still possible
        }
        catch (Exception exc) {
            // Because other things can still go wrong too (HTTP 500, parsing errors)
        }
    }
}

有关 .Result 调用如何使用 HttpClient 的更多信息，请参见此处：What happens while waiting on a Task's Result?