从异步方法返回值

Question

I have asmx webservice and I am using in it in PCL, so all of us know that now PCL no longer supports asmx webservice, it supports wcf webservices. I somehow manage to call asmx webservice through PCL by using silverlight plugin. But it creates async method and converts WSDL to wcf.

Now problem is I need to return some values from webmethods which are now async method. If anybody know solution for this please help me.

I am using following code,

public class PerformLogIn
{
     string temp;
     public string checkTemp(string code)
     {
         ServiceReference1.WeatherSoapClient obj = new ServiceReference1.WeatherSoapClient(
                                    new BasicHttpBinding(),
                                    new EndpointAddress("http://wsf.cdyne.com/WeatherWS/Weather.asmx"));

         obj.GetCityForecastByZIPCompleted+=getResult;
         obj.GetCityForecastByZIPAsync(code);

         return temp;
     }
     void getResult(Object sender,GetCityForecastByZIPCompletedEventArgs e)
     {
         string error = null;

         if (e.Error != null)
             error = e.Error.Message;
         else if (e.Cancelled)
             error = "cancelled";
         var result = e.Result; 
         temp=result.temprature;
     }

}

So when I run it, value of temp is null. When I debugged it, I found that when getResult occures, it enters into void getResult by that time it has reached to return statement.

The problem here is, when GetCityForecastByZIPAsync method which is async gets called, it doesn't wait for others' operation to complete, it just finishes its execution and returns. Now being having a webservice which gets invoked only on occurrence of event getResult (in this case), invocation of event getResult (in this case) is based on network, server response which will have a little delay that we can't control. So till that time GetCityForecastByZIPAsync method finishes its execution. And after some time when getResult event occurs, it executes but does not return to the calling method viz. checkTemp method.

So how do I get data from getResult event and put it back to checkTemp method so that I can use it for my purpose.

If anybody knows, please help me out.

Answer 1

FYI: the entire first paragraph of your question is pretty much gobbledygook to most of your audience, and does not seem relevant to the question (or at least, would not be relevant if you'd provided the actual important details in a more broadly-comprehensible way).

As far as the question goes…

The two most obvious problems with your code are that a) you are subscribing your getResult event handler after you initiate the asynchronous operation, which means the operation could complete before you are set up to be notified of the completion, and b) your checkTemp method doesn't wait for the completion before it returns the result.

The first issue is easy to fix. Just reverse the order of the two statements, so that they look like this:

     obj.GetCityForecastByZIPCompleted += getResult;
     obj.GetCityForecastByZIPAsync(code);

Fixing the second part is harder to comment on, mainly because you haven't provided enough context in your question. If the GetCityForecastByZIPAsync method is actually a C# async method, then the best approach is to get rid of the getResult method altogether (as well as subscribing it to the ...Completed event), and await the call to GetCityForecastByZIPAsync . Otherwise, the best approach is for checkTemp to not actually return the value, but rather for that method to be void and for your getResult method to handle whatever actually needs to happen when the operation completes (eg by invoking code that displays the result in the UI).

Without more context, it's impossible to say. See https://stackoverflow.com/help/mcve for information on how to post a better question.