Windows Phone 8 - Getting Multiple Street Names

Question

I created for Windows Phone 8.0 Silverlight App an async method GetStreetName

string streetname;

private async Task<string> GetStreetName(int i)
{
    MapAddress address;
    ReverseGeocodeQuery query = new ReverseGeocodeQuery();
    query.GeoCoordinate = Route[i].Item1;
    query.QueryCompleted += (s, e) =>
    {
        if (e.Error != null)
            return;

        address = e.Result[0].Information.Address;
        streetname = address.Street;
};
query.QueryAsync();
return streetname;
}

and I call it using the await operator inside of a for loop:

for (int i = 0; i < Route.Count; i++)
{
       ListBox.Items.Add(await GetStreetName(i));
}

but I always get only the street name of the first loaded geoposition back and I have no idea why (I thought the await operator is waiting until the async method is finished).

Additional info: i just saw that this is not 100% clear at this short snippet, streetname and Route are global "variables", Route is a tuple list where the first item is a geocoordinate.

How can I fix this issue?

Answer 1

You are returning from GetStreetName before the results are ready becayse query.QueryAsync(); just starts the query and doesn't wait for it to be complete.

On top of that, you're writing all results to the same global streetname.

You need to use a TaskCompletionSource.

Try something like this:

private async Task<string> GetStreetNameAsync(int i)
{
    var tcs = new TaskCompletionSource<IEnumerable<string>>();

    EventHandler<QueryCompletedEventArgs<IList<MapLocation>>> handler = (s, e) =>
        {
            if (e.Error != null)
            {
                tcs.TrySetException(e.Error);
                return;
            }

            if (e.Cancelled)
            {
                tcs.TrySetCanceled();
                return;
            }

            tcs.TrySetResult(e.Result[0].Information.Address.Street);
        };

    var query = new ReverseGeocodeQuery();
    query.GeoCoordinate = Route[i].Item1;

    try
    {
        query.QueryCompleted += handler;

        query.QueryAsync();

        return await tcs.Task;
    }
    finally
    {
        query.QueryCompleted -= handler;
    }
}