我的桌面应用程序挂起了使用async和await的Web API服务调用

Question

在数据库中保存数据时,我遇到了很多延迟.我有一个exe(Deskptop应用程序)从串口读取数据并通过Web API服务将该条目推送到数据库,但我的应用程序在此行上挂起:

httpClient.PostAsync("api/MyController/Save", httpConent).Result;

这个exe负责调用我的Web API服务方法并将数据保存到我的数据库.

这是我的代码:

void _serialPort_DataReceived(object sender, SerialDataReceivedEventArgs e)
       {
           int dataLength = _serialPort.BytesToRead;
           byte[] data = new byte[dataLength];
           int nbrDataRead = _serialPort.Read(data, 0, dataLength);

           if (nbrDataRead == 0)
               return;

           // Send data to whom ever interested
           if (NewSerialDataRecieved != null)
           {
               NewSerialDataRecieved(this, new SerialDataEventArgs(data));
           }
       }

 void _spManager_NewSerialDataRecieved(object sender, SerialDataEventArgs e)
        {
            if (this.InvokeRequired)
            {
                // Using this.Invoke causes deadlock when closing serial port, and BeginInvoke is good practice anyway.
                //// Fired-off asynchronously; let the current thread continue.
                this.BeginInvoke(new EventHandler<SerialDataEventArgs>(_spManager_NewSerialDataRecieved), new object[] { sender, e });
                return;
            }
            //data is converted to text
            string str = Encoding.ASCII.GetString(e.Data);
            if (!string.IsNullOrEmpty(str))
            {
               CallWebservice(str)
            }
        }

public void CallWebservice(string xmlRequest)
        {
            using (var httpClient = new HttpClient())
            {
               httpClient.BaseAddress = new Uri("WebService Url");
                httpClient.DefaultRequestHeaders.Accept.Clear();
                httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                StringContent httpConent = new StringContent(xmlRequest, Encoding.UTF8);
                HttpResponseMessage responseMessage = null;
                try
                {
                    responseMessage = httpClient.PostAsync("api/MyController/Save", httpConent).Result;
                }
                catch (Exception ex)
                {
                    if (responseMessage == null)
                    {
                        responseMessage = new HttpResponseMessage();
                    }
                    responseMessage.StatusCode = HttpStatusCode.InternalServerError;
                    responseMessage.ReasonPhrase = string.Format("RestHttpClient.SendRequest failed: {0}", ex);
                }
            }
        }

我的web api方法:

 public async Task<HttpResponseMessage> Save(HttpRequestMessage request)
        {
               var requestdata = request.Content.ReadAsStringAsync().Result;//extract Users Id's from this
                var users=context.User.Where(t => (t.Stats == userId1) || (t.Stats == userId2)).ToList();
                var objUsersMapping= new UsersMapping();
                objUsersMapping.Work1 = users[0].Work1;
                objUsersMapping.Work2 = users[1].Work1;
                await this.SaveUsersMapping(objUsersMapping); 
        }

 public async Task<UsersMapping> SaveUsersMapping(UsersMapping objUsersMapping)
        {
            using (var context = new MyEntities())
            {
                try
                {
                    context.UsersMapping.Add(objUsersMapping);
                    await context.SaveChangesAsync();
                    return objUsersMapping;
                }
                catch (Exception foExe)
                {
                    return null;
                }
            }
        }

我没有在Windows应用程序上工作太多,所以我不明白为什么我的应用程序挂起.

注意:数据将不断进入我的串口,因此通过Web服务保存数据不应该干扰_serialPort_DataReceived事件.

Answer 1

这是我在OP问题下的评论摘要

您正在同步调用异步方法.这将导致当前线程阻塞.摆脱.Result并相应地改变代码的其余部分(如包括async与await有太多).

例如改变这一行

responseMessage = httpClient.PostAsync("api/MyController/Save", httpConent).Result;

...至:

 responseMessage = await httpClient.PostAsync("api/MyController/Save", httpConent);

您的方法签名需要更改如下:

public async Task CallWebservice(string xmlRequest)
{

}

任何调用它的方法也需要async使用await例如你的_spManager_NewSerialDataRecieved()方法.

请注意它已从更改void为async void.await之前也要注意CallWebservice().

async void _spManager_NewSerialDataRecieved(object sender, SerialDataEventArgs e)
    {
        if (this.InvokeRequired)
        {
            // Using this.Invoke causes deadlock when closing serial port, and BeginInvoke is good practice anyway.
            //// Fired-off asynchronously; let the current thread continue.
            this.BeginInvoke(new EventHandler<SerialDataEventArgs>(_spManager_NewSerialDataRecieved), new object[] { sender, e });
            return;
        }
        //data is converted to text
        string str = Encoding.ASCII.GetString(e.Data);
        if (!string.IsNullOrEmpty(str))
        {
           await CallWebservice(str)
        }
    }

关于异步void的注释

因为上面的方法是一个事件处理程序,所以方法就可以了async void.通常,您希望避免async void使用非事件处理程序代码.欲了解更多信息,请参阅Stephen Cleary先生的精彩文章.

先生,这是唯一的问题吗？

您应该async Save()在服务器上修复您的方法,因为它也有.Result().这将阻止服务器上的当前线程.用它作为前缀await.通常,您希望避免.Result 作为等待任务完成的手段.在等待它之后,可以安全地使用它作为获取结果的方法,但是有更优雅的方法可以等待并在单行代码中获得结果.例如x = await FooAsync();.