NullReferenceException for DependencyService on Android

Question

I am attempting to get the phone service information on a Xamarin project.

When I call var carrierHelper = DependencyService.Get<ICarrierHelper>();, I get a System.NullReferenceException: Object reference is not set to an instance of an object.

ICarrierHelper.cs

using System;
namespace MyProject
{
    public interface ICarrierHelper
    {
        string GetPhoneCarrier();
    }
}

MyProject.Droid/Helpers/CarrierHelper.cs

using MyProject.Droid;
using Android.Content;
using Android.Telephony;
using Xamarin.Forms;

[assembly: Dependency(typeof(CarrierHelper))]
namespace MyProject.Droid
{
    public class CarrierHelper : ICarrierHelper
    {
        public string GetPhoneCarrier()
        {
            Console.WriteLine("****************************** MAIN ACTIVITY " + MainActivity.Current.ToString());

            TelephonyManager mgr = MainActivity.Current.GetSystemService(Context.TelephonyService) as TelephonyManager;

            Console.WriteLine("****************************** TELEPHONY MANAGER " + mgr.ToString());

            return mgr.NetworkOperatorName;

            //return mgr.SimOperatorName;
        }
    }
}

I have breakpoints in the Droid CarrierHelper.cs that never catch. So, I can only assume that the DependencyService can't find my file at all.

UPDATE (Adding more information and console logs.)

MyDevice.cs

using System;
using Plugin.DeviceInfo;
using Xamarin.Forms;

namespace MyProject
{
public class MyDevice
{
    private static MyDevice current;
    public static MyDevice Current
    {
        get
        {
            if (current == null)
                current = new MyDevice();

            return current;
        }
    }

    public string platform { get; set; }
    public string manufacturer { get; set; }
    public string model { get; set; }
    public string name { get; set; }
    public string os_version { get; set; }
    public string carrier { get; set; }
    public string app_version { get; set; }
    public string app_build { get; set; }

    public MyDevice()
    {
        switch (Device.RuntimePlatform)
        {
            case Device.iOS:
                platform = "iOS";
                break;
            case Device.Android:
                platform = "Android";
                break;
            default:
                platform = "UNKNOWN";
                break;
        }

        manufacturer = CrossDeviceInfo.Current.Manufacturer;
        model = CrossDeviceInfo.Current.Model;
        name = CrossDeviceInfo.Current.DeviceName;
        os_version = CrossDeviceInfo.Current.Version;
        app_version = CrossDeviceInfo.Current.AppVersion;
        app_build = CrossDeviceInfo.Current.AppBuild;

        var carrierHelper = DependencyService.Get<ICarrierHelper>();

        Console.WriteLine("----- CARRIER HELPER " + carrierHelper);

        carrier = DependencyService.Get<ICarrierHelper>().GetPhoneCarrier();

        Console.WriteLine("----- CARRIER: " + carrier);
    }
}
}

The following Console logs print:

----- CARRIER HELPER MyProject.Droid.CarrierHelper

UPDATE (Adding stack trace and info.)

Here is the stack trace:

at MyProject.Droid.CarrierHelper.GetPhoneCarrier () [0x00001] in /MyProject/Helpers/CarrierHelper.cs:14 at MyProject.MyDevice..ctor () [0x000d2] in /MyProject.Core/Models/MyDevice.cs:57 at MyProject.MyDevice.get_Current () [0x0000d] in /MyProject.Core/Models/MyDevice.cs:15 at MyProject.UserDataStore+d__16.MoveNext () [0x00057] in /MyProject.Core/Services/UserDataStore.cs:197

UserDataStore.cs

public async Task UpdateLocation(double lat, double lng)
    {
        try
        {
            if (!CrossConnectivity.Current.IsConnected)
                throw new Exception("Not currently connected to the internet.");

            var model = new
            {
                access_token = $"{Settings.AuthToken}",
                latitude = lat,
                longitude = lng,
                device = MyDevice.Current,
                format = "json"
            };

            var json = JsonConvert.SerializeObject(model);

            var location_content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");

            var response = await client.PostAsync(users_url + location_url, location_content);

            if (!response.IsSuccessStatusCode)
                throw new Exception(response.StatusCode.ToString() + " " + response.ReasonPhrase);
        }
        catch (Exception e)
        {
            Console.WriteLine("ERROR: " + e.Message); // this is where the stack trace is generated

            await LocationDatabase.Connection.SaveLocationAsync(new Location(lat, lng));

            await AppEventDatabase.Connection.SaveEventAsync(new AppEvent("location", e.Message, lat, lng));
        }
    }

This is the error message from the file above:

ERROR: Object reference not set to an instance of an object.

Answer 1

You have a catch-22 situation. GetPhoneCarrier() requires MyDevice.Current which is implemented via a lazy-loaded constructor for MyDevice. That constructor resolves ICarrierHelper and calls GetPhoneCarrier(), which requires MyDevice.Current...

I'm assuming that the resolution works out that it has already tried resolving that, and it hasn't completed, so it returns null.

I would suggest that you do less in the MyDevice constructor. Avoid resolving that dependency until you actually need it. That way, the constructor can complete, avoiding the catch-22.

Also beware that your approach to lazy loading isn't thread safe. Consider using Lazy<T> which nicely encapsulates double-checked thread-safe locking.