How to use asp.net web service 3.5 (soap) on android,windows phone 8 apps or windows store 8 apps?

On windows phone app or windows store apps when I add service reference of my web service it add perfectly but when I create a new object of my service and calling helloWorld() method it show helloWorldAsync() no return type show, some await-able. How to get data from my helloWorld() method.

Now on android side I don't know how to use this service, I am working on Xamrine C# for android, I can't get proper reference or method or tutorial about using soap service on android platform.