ASP MVC-基础连接已关闭：发送中发生意外错误

Question

当我要从URL获取流以下载图像时出现以下错误。

这是我的代码：

public async Task<Stream> GetStreamFromUrlAsync()
{
    byte[] imageData = null;
    using (var wc = new System.Net.WebClient())
        imageData = await wc.DownloadDataTaskAsync(url);
    return new MemoryStream(imageData);
}

在动作中使用上述方法

[HttpPost]
public async Task<ActionResult> CropImage(UploadImage data)
{
    // some code for getting path
    try
    {               
        Bitmap img = new Bitmap(await data.GetStreamFromUrlAsync());
        img.Save(path);
     }
     catch (Exception ex)
     {
         throw new Exception("Error After Crroping Image: " + ex.Message, ex);
     }
}

这是通过堆栈跟踪的完整错误消息：

ErrorMessage: Error After Crroping Image: The underlying connection was closed: An unexpected error occurred on a send.<br/>
InnerException: The underlying connection was closed: An unexpected error occurred on a send.
StackTrace:    at codetiMAit.Controllers.MyController.<CropImage>d__21.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.Web.Mvc.Async.TaskAsyncActionDescriptor.EndExecute(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass8_0.<BeginInvokeAsynchronousActionMethod>b__1(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__11_0()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass11_1.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass7_0.<BeginInvokeActionMethodWithFilters>b__1(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass3_6.<BeginInvokeAction>b__3()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass3_1.<BeginInvokeAction>b__5(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.<>c.<BeginExecuteCore>b__152_1(IAsyncResult asyncResult, ExecuteCoreState innerState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.<>c.<BeginExecute>b__151_2(IAsyncResult asyncResult, Controller controller)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult)
at System.Web.Mvc.MvcHandler.<>c.<BeginProcessRequest>b__20_1(IAsyncResult asyncResult, ProcessRequestState innerState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult)
at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result)
at System.Web.HttpApplication.CallHandlerExecutionStep.InvokeEndHandler(IAsyncResult ar)
at System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)
Date :7/19/2018 9:19:53 PM

----------------------------------

我在计算机和Windows Server 2012上使用.net Framework 4.7.1。 我找到了一些解决方案，但没有人为我工作，例如，我这样做：

static MyController()
{
     ServicePointManager.Expect100Continue = true;
     ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
}