Variable initialized in another class returns null when accessed from another

Question

Please help, i need to use the Geoposition pos variable which is in class UserLocation in a class MainPage but it keeps returning null ,this method initializes pos,

public Geoposition pos;
    async public void preparePosition()
    {
        try
        {
            // Get the cancellation token.
            _cts = new CancellationTokenSource();
            CancellationToken token = _cts.Token;

            //MessageTextbox.Text = "Waiting for update...";

            // Get the position to derive the coordinates
            _geolocator.DesiredAccuracy = Windows.Devices.Geolocation.PositionAccuracy.High;
            pos = await _geolocator.GetGeopositionAsync().AsTask(token);

        }
        catch (System.UnauthorizedAccessException)
        {

        }
        catch (TaskCanceledException)
        {

        }
        finally
        {
            _cts = null;
        }

    }

then in class MainPage

UserLocation LocationData = new UserLocation();
 Geoposition p = LocationData.pos;

but it returns null, iv also tried using a return method

    public Geoposition getPosition()
    {
        preparePosition();
        return pos;
    }

//then

using p = LocationData.getPosition();

Answer 1

preparePosition() is async so it returns before the GPS hardware gets the position.

can i suggest

public async Task<GeoPosition> PreparePosition()
    {
        ...
        return pos;
    }

You then have to call it from an async proc

public async void MyButtonClick(sender, e){
  using (var p = await preparePosition()){
    ...
  }
}

Of course you can only say using (var p= ... if p implements IDisposable .

Answer 2

Problem is that your method preparePosition is async. It retrieves value of "pos" asynchronously but you are using it without waiting for its result.

First, change preparePosition signature to:

async public Task preparePosition()
{
...
}

Then it returns Task instead of void and you can wait on its completion.

Then change getPosition to:

public Geoposition getPosition()
{
    preparePosition().Wait();
    return pos;
}

I recommend reading some articles about how async/await work and how they should be used.

• http://msdn.microsoft.com/en-us/library/vstudio/hh156528.aspx
• http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx