哈玛林.安卓权限.CheckSelfPermission() exception Java.Lang.NullPoint

Question

当我调用方法CheckSelfPermission(permission)时，我得到异常Java.Lang.NullPointerException(尝试在空对象引用上调用虚拟方法'int android.content.Context.checkSelfPermission(java.lang.String)')。 为什么？ 请帮助我

public void GetLocationPermission()
{
string permission = Manifest.Permission.AccessFineLocation;
if (CheckSelfPermission(permission) == (int)Permission.Granted)
{               
GetLocation();
return;
}
RequestPermissions(PermissionsLocation, RequestLocationId);
}
readonly string[] PermissionsLocation =
{
Manifest.Permission.AccessCoarseLocation,
Manifest.Permission.AccessFineLocation
};
const int RequestLocationId = 0;

Answer 1

根据我的经验，在这种情况下，NullPointerException源于Activity设置不正确。

确保扩展Activity并按如下所示添加属性：

[Activity(Label = "MyClass")]
class MyClass: Activity`

此外，使用 Xamarin.Essentials 1.4.0 时，请求权限变得更加容易。

你现在要做什么：

[Activity(Label = "MyClass")]
class MyClass: Activity
{
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
Platform.Init(this, savedInstanceState);
}
public override void OnRequestPermissionsResult(int requestCode, string[] permissions, Android.Content.PM.Permission[] grantResults)
{
Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
}
public async Task<PermissionStatus> CheckAndRequestLocationPermission()
{
var status = await Permissions.CheckStatusAsync<Permissions.LocationWhenInUse>();
if (status != PermissionStatus.Granted)
{
status = await Permissions.RequestAsync<Permissions.LocationWhenInUse>();
}
// Additionally could prompt the user to turn on in settings
return status;
}
}

您还需要确保您的MainActivity具有相同的OnRequestPermissionsResult覆盖，如果您没有手动创建项目，则默认情况下应该具有该覆盖。

延伸阅读：Xamarin.Essentials：权限