Background Service error while using as async Task

Question

I have a background service in Xamarin Android but the problem is I cannot use it as Async method, it gives an error.

return type must be 'StartCommandResult' to match overridden member 'Service.OnStartCommand(Intent, StartCommandFlags, int)

The actual overridden method is void but to use await I have to change it to async Task<T> and then it produces the error. I searched everywhere but couldn't find a proper solution on this.

The code snippet for entire class is here:

{
 [Service(IsolatedProcess = false, Enabled = true, Label = "BackgroundNotificationService")]
class BackgroundNotificationService : Service
{
    static readonly string TAG = "X:" + typeof(BackgroundNotificationService).Name;
    static readonly int TimerWait = 3600000;
    Timer timer;
    DateTime startTime;
    bool isStarted = false;

    public override IBinder OnBind(Intent intent)
    {
        return null;
    }

    public override void OnCreate()
    {
        base.OnCreate();
    }

    public override void OnDestroy()
    {
        timer.Dispose();
        timer = null;
        isStarted = false;

        TimeSpan runtime = DateTime.UtcNow.Subtract(startTime);
        Log.Info(TAG, $"Simple Service destroyed at {DateTime.UtcNow} after running for {runtime:c}.");
        base.OnDestroy();
    }

    public override async Task<StartCommandResult> OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
    {
        StartCommandResult a = StartCommandResult.NotSticky;
        Log.Debug(TAG, $"OnStartCommand called at {startTime}, flags={flags}, startid={startId}");
        if (isStarted)
        {
            TimeSpan runtime = DateTime.UtcNow.Subtract(startTime);
            Log.Info(TAG, $"This service was already started, it's been running for {runtime:c}.");
            await UpdateData();
        }
        else
        {
            startTime = DateTime.UtcNow;
            Log.Info(TAG, $"Starting the service, at {startTime}.");
            timer = new Timer(HandleTimerCallback, startTime, 0, TimerWait);
            isStarted = true;
            await UpdateData();
        }
        return a;
    }

    void HandleTimerCallback(object state)
    {
        TimeSpan runTime = DateTime.UtcNow.Subtract(startTime);
        Log.Info(TAG, $"This service has been running for {runTime:c} (since ${state}).");
    }


}
}

As far as I know it should work because I am returning StartCommandResult in override method. But if I am wrong then is there any method that I am missing? Maybe there is another way to do this and I don't know that.

Update and Solution

I ended up implementing another new IntentService class to do all the background updating work while the above service only handles the calling of the IntentService. The service is called with an explicit intent as any other service.

 public class CustomIntentService : IntentService
{
    public CustomIntentService() : base("CustomIntentService")
    {
    }

    protected override async void OnHandleIntent(Intent intent)
    {
        // do your work
        string servicename = typeof(CustomIntentService).Name;
        Log.Info(servicename, "Starting background work");
        try
        {
            await UpdateData();
        }
        catch (Exception ex)
        {
            Log.Info(servicename, "Background work returned with errors.\nError:" + ex.Message);
            return;
        }
        Log.Info(servicename, "Background work completed without errors.");
    }
 }

Unfortunately, you're asking for a change in reality, which we can't really deliver. You have a method that is expected to return with a `StartCommandResult`. You want to provide a method that will return and that will *at some later point in time* produce a `StartCommandResult`. — Damien_The_Unbeliever, Jan 19 '17 at 07:24
I have implemented the method with two separate services. One with `Service` as above and the other which actually does the all work in the background is `IntentService`. — Ahmar, Jan 19 '17 at 09:57

Ehsan Sajjad · Accepted Answer · 2021-03-03T12:16:12.593

This is because the method OnStartCommand defined in the base class Service has return type StartCommandResult not Task<StartCommandResult>, so you cannot change the return type of a method when overriding in the child class.

You can create a proxy method if you want to make it async like:

public override StartCommandResult OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
{
        return OnStartCommandAsync(intent,flags,startId).Result;
}

and the async method would be like:

private async Task<StartCommandResult> OnStartCommandAsync(Intent intent, StartCommandFlags flags, int startId)
{
  StartCommandResult a = StartCommandResult.NotSticky;
        Log.Debug(TAG, $"OnStartCommand called at {startTime}, flags={flags}, startid={startId}");
        if (isStarted)
        {
            TimeSpan runtime = DateTime.UtcNow.Subtract(startTime);
            Log.Info(TAG, $"This service was already started, it's been running for {runtime:c}.");
            await UpdateData();
        }
        else
        {
            startTime = DateTime.UtcNow;
            Log.Info(TAG, $"Starting the service, at {startTime}.");
            timer = new Timer(HandleTimerCallback, startTime, 0, TimerWait);
            isStarted = true;
            await UpdateData();
        }
        return a;
}

But note that it wouldn't serve you the purpose you want, because .Result will block the thread until the async method returns the result, so it is basically the async method synchronously

For reference see this post.

So how can I use `await` method in this service? For me this is not an answer, it is just half of it. — Ahmar, Jan 19 '17 at 06:48
Still it's not helping. You need to have `async` in the overridden method to use `await` and then its return type will be `Task`. So we are back at where we started from. — Ahmar, Jan 19 '17 at 07:01

Background Service error while using as async Task

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