無法在 Xamarin Android 8.1 API 27 上獲取位置

Question

嘗試在 Android 8.1（API 27）上獲取位置時遇到問題。 Android 7.0(API 7.0), Android 9.0 and Android 10+ works properly but on Android 8.1(API 27) I get this error(debug or release mode):

Unable to get location: System.NullReferenceException: Object reference not set to an instance of an object
  at Project.Permissions.GetCurrentPosition () [0x000a9] in <f8798290f683423180c09ee7a2b0a8fa>:0

我的代碼的相關部分：

Project.Permissions class：

public class Permissions
{

  public async Task<bool> LocationPermission()
        {
            try
            {
                PermissionStatus Location = await new Permissions.LocationWhenInUse().CheckStatusAsync();
                if (Location == PermissionStatus.Granted)
                    return true;
                else
                {
                    PermissionStatus status = await new Permissions.LocationWhenInUse().RequestAsync();
                    if (status == PermissionStatus.Granted)
                        return true;
                    else
                        return false;
                }                
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error =>" + ex.Message);
                return false;
            }
        }

public async Task<Position> GetCurrentPosition()
{
            Position position;
            try
            {

                var request = new GeolocationRequest(GeolocationAccuracy.Best);

                Location locator = await Geolocation.GetLastKnownLocationAsync();
                locator.Accuracy = 50;

                Location localNow = await Geolocation.GetLocationAsync(request);
                localNow.Accuracy = 50;

                if (localNow?.Latitude != null && localNow?.Longitude!=null)
                    position = new Position(localNow.Latitude, localNow.Longitude);
                else
                    position = new Position(locator.Latitude, locator.Longitude);
            }
            catch (FeatureNotSupportedException fnsEx)
            {
                position = new Position(0, 0);
                // Handle not supported on device exception
                Debug.WriteLine("Feature not supported => " +fnsEx);
            }
            catch (PermissionException pEx)
            {
                position = new Position(0, 0);
                // Handle permission exception
                Debug.WriteLine("Error => "+pEx);
            }
            catch (Exception ex)
            {
                position = new Position(0,0);
                Console.WriteLine("Unable to get location => " + ex);
            }
            return position;
     }
}

安卓清單：

<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

主要活動：

public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
{
    Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    PermissionsImplementation.Current.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
}

我做錯了什么？

Answer 1

問題出在Location locator = await Geolocation.GetLastKnownLocationAsync();

它試圖獲取最后知道的位置，但由於第一次執行模擬器設備，所以它不存在，所以它返回 NULL 並導致 System.NullReferenceException。