同步调用异步方法时出错

Question

I'm writing code in a desktop application that will be used in police cars to use the new Windows Geolocation API. I've written an event listener for the Geolocator.PositionChanged event, but it's not getting called. In my reading of the documentation, it seems that the Gelocator only raises this event when the position changes.

I figure the first thing my program should do after it finished setting up the event listener is to call the Geolocator.GetPositionAsync method to get the current position and then let the position get updated as they happen.

I need to call the Geolocator.GetPositionAsync method synchronously. To that end, I wrote the following method:

private async Task<Geoposition> GetCurrentPosition() {
    await Geolocator.GetGeopositionAsync();
}

However, I get the following compiler error on the line with await in it:

'await' requires that the type 'Windows.Foundation.IAsyncOperation' have a suitable GetAwaiter method. Are you missing a using directive for 'System'?

Here are the using statements at the top of the file:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Globalization;
using System.Runtime.Serialization;
using System.Threading;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Serialization;
using Windows.Devices.Geolocation;

How do I fix this compiler error?

Edit

After I added a reference to System.Runtime.WindowsRuntime.dll the project, the error went away. However, now I'm getting a new error:

'System.Runtime.CompilerServices.TaskAwaiter`1<Windows.Devices.Geolocation.Geoposition>' does not contain a definition for 'IsCompleted'

How do I fix this one?

Edit

Here's the code that calls the method:

protected override void Initialize() {
    // Make sure we "forget" any previous Gelocator device
    if ( Geolocator != null ) {
        Geolocator.PositionChanged -= PositionChanged;
        Geolocator.StatusChanged   -= StatusChanged;
        Geolocator                  = null;
    }

    CurrentPosition = new GpsInformation() { TimeStamp = DateTime.Now };
    Geolocator = new Geolocator();
    Geolocator.DesiredAccuracy   = PositionAccuracy.High;    // Sends position updates with the highest accuracy possible.
    Geolocator.MovementThreshold = 0;                        // Sends position updates no matter how far the vehicle has moved since the last update.
    Geolocator.ReportInterval    = 0;                        // Sends position updates as they become availble.
    Geolocator.PositionChanged  += PositionChanged;          // Sends position updates to this method.
    Geolocator.StatusChanged    += StatusChanged;            // Sends status changed updates to this method.
    switch ( Geolocator.LocationStatus ) {
        case PositionStatus.Disabled:
        case PositionStatus.NotAvailable: 
            // The Geolocator device is disabled.  We won't get any updates.  Throw an exception.
            throw new Exception( "LPRCore does not have permission to use Windows Geolocation services or Geolocation services are not working." );
    }

    Task<Geoposition> getPositionTask = GetCurrentPosition();
    positionRecord = getPositionTask.Result;
}

Answer 1

Thank you Scott for your help. I was able to figure out which DLLs to add references to from your suggestions & then hit-or-miss trying things.

To fix the problem, I needed to add references to:

• System.Threading.dll and
• System.Threading.Tasks.dll

Once I did this, the error went away.

I wish MS would add information about which DLLs need to be referenced to use the Windows 8 APIs from VS2012 to their documentation.

Answer 2

可能存在重复问题，但如果尚未添加，则还应添加对System.Runtime.WindowsRuntime.dll的引用。

Answer 3

I think you ran into this problem while trying to resolve the wrong issue.

While investigation to write my Exposing the Geolocator as a Reactive Service on the Nokia Developer Wiki I came across the same problem (no change events) and tried the same solution (calling Geolocator.GetGeopositionAsync ).

It turns out that you need to set some properties ( Geolocator.MovementThreshold , Geolocator.DesiredAccuracy , Geolocator.ReportInterval ) with some values.

If you are not familiar with the Reactive Extensions (Rx) , you should try it. You can get my implementation from my CodeProject Workspace .