LISTENABLE WORKER

Question

LISTENABLE WORKER

Giorgio Sfiligoi 656

I am using OnSleep to launch an asynchronous background worker that saves certain files as follows:

using Android.Content;
using AndroidX.Work;
using Google.Common.Util.Concurrent;

namespace Draft4.Platforms.Android.Workers;

public class OnSleepAsyncWorker : ListenableWorker
{
    public const string TAG = "OnSleepAsyncWorker";

    public OnSleepAsyncWorker(Context context, WorkerParameters workerParams)
        : base(context, workerParams) { }

    public override IListenableFuture StartWork()
    {
        var taskCompletionSource = new TaskCompletionSource<Result>();
        Task.Run(async () =>
        {
            try
            {
                // save files: upload to cloud storage
                await Draft4.Services.Document.OnSleep();
                Debug.WriteLine("OnSleepAsyncWorker succeeded");
                taskCompletionSource.SetResult(Result.InvokeSuccess()!);
            }
            catch (Exception)
            {
                Debug.WriteLine("OnSleepAsyncWorker failed");
                taskCompletionSource.SetResult(Result.InvokeFailure()!);
            }
        });
        return (IListenableFuture)taskCompletionSource.Task;
    }
}

(adding ... Task.Result; makes no difference)

Running in the Emulator in Debug mode, I get the following message in output:

04-29 20:08:44.119 E/WM-WorkerWrapper(29856): Work [ id=cf526e62-7af7-4840-85e4-5fb9259ae836, tags={ crc64620a83f8248190ee.OnSleepAsyncWorker,OnSleepAsyncWorker } ] failed because it threw an exception/error
04-29 20:08:44.119 E/WM-WorkerWrapper(29856): android.runtime.JavaProxyThrowable: [System.InvalidCastException]: Specified cast is not valid.
...

04-29 20:31:39.104 I/WM-WorkerWrapper(30601): Worker result FAILURE for Work [ id=626111ed-e983-4e42-89bf-76d38c8ed937, tags={ crc64620a83f8248190ee.OnSleepAsyncWorker,OnSleepAsyncWorker } ]

On the other hand at the end it reports:

OnSleepAsyncWorker succeeded

(but this is just the output from Debug.WriteLine...)

Also note that the cast (IListenableFuture) is required by the compiler.

I am puzzled: is this really an error? How can I fix it?

Giorgio Sfiligoi 656 Reputation points

2026-04-29T18:52:09.9466667+00:00
I tried the code provided by AI, but the compiler does not like

new CallbackToFutureAdapter.IResolver()

CS0144: Cannot create an instance of the interface ...

Also, I do not understand:

Complete that future (completer.Set(...)) when the .NET async work finishes.

Answer accepted by question author

1 additional answer

Your answer

Giorgio Sfiligoi 656 Reputation points

2026-04-29T18:52:09.9466667+00:00

I tried the code provided by AI, but the compiler does not like

new CallbackToFutureAdapter.IResolver()

CS0144: Cannot create an instance of the interface ...

Also, I do not understand:

Complete that future (completer.Set(...)) when the .NET async work finishes.

Answer 1

Hello @Giorgio Sfiligoi ,

Thanks for your question.

A .NET Task<Result> and an Android IListenableFuture are completely different types. You cannot cast one to the other, even though the compiler allows it, Android throws InvalidCastException at runtime.

It is highly likely that the error is coming from this line:

return (IListenableFuture)taskCompletionSource.Task;

I recommend using Simple Worker instead. Just switch from ListenableWorker to the regular Worker class and block the async call:

using Android.Content;
using AndroidX.Work;
using System.Diagnostics;

namespace Draft4.Platforms.Android.Workers
{
    public class OnSleepWorker : Worker
    {
        public const string TAG = "OnSleepWorker";

        public OnSleepWorker(Context context, WorkerParameters workerParams)
            : base(context, workerParams) { }

        public override Result DoWork()
        {
            try
            {
                Draft4.Services.Document.OnSleep()
                    .GetAwaiter()
                    .GetResult();

                Debug.WriteLine("OnSleepWorker succeeded");
                return Result.InvokeSuccess()!;
            }
            catch (Exception ex)
            {
                Debug.WriteLine($"OnSleepWorker failed: {ex.Message}");
                return Result.InvokeFailure()!;
            }
        }
    }
}

I hope this addresses your question. If this response was helpful, please consider following the guidance to provide feedback.

Giorgio Sfiligoi 656 Reputation points

2026-04-30T05:01:22.7366667+00:00

Thank you.

I had tried to use ListenableWorker because the documentation says that async code should be avoided in the sync Worker.
Nancy Vo (WICLOUD CORPORATION) 3,550 Reputation points Microsoft External Staff Moderator

2026-04-30T06:13:17.52+00:00

Thanks for your feedback. Please feel free to reach out if you have any questions.

Answer 2

ListenableWorker.StartWork must return a real IListenableFuture implementation from the Guava / WorkManager side, not a .NET Task cast to IListenableFuture. The invalid cast is what causes the System.InvalidCastException and the WorkerWrapper error, even though the inner async code runs and logs "OnSleepAsyncWorker succeeded".

The correct pattern is:

Use the CallbackToFutureAdapter (from AndroidX.Work / Guava) to create an IListenableFuture.
Complete that future from the .NET async code.

Conceptually, the flow should be:

public override IListenableFuture StartWork()
{
    // Create the Java-side future
    var future = CallbackToFutureAdapter.GetFuture(new CallbackToFutureAdapter.IResolver()
    {
        public Java.Lang.Object AttachCompleter(CallbackToFutureAdapter.Completer completer)
        {
            // Run .NET async work
            Task.Run(async () =>
            {
                try
                {
                    await Draft4.Services.Document.OnSleep();
                    Debug.WriteLine("OnSleepAsyncWorker succeeded");
                    completer.Set(Result.InvokeSuccess());
                }
                catch (Exception)
                {
                    Debug.WriteLine("OnSleepAsyncWorker failed");
                    completer.Set(Result.InvokeFailure());
                }
            });

            return "OnSleepAsyncWorker"; // debug tag
        }
    });

    return future;
}

Key points:

Do not cast Task/TaskCompletionSource.Task to IListenableFuture; they are unrelated types.
Let StartWork return the IListenableFuture created by the Java/AndroidX APIs.
Complete that future (completer.Set(...)) when the .NET async work finishes.

Once StartWork returns a valid IListenableFuture, the InvalidCastException and WM-WorkerWrapper error will stop, and WorkManager will correctly see SUCCESS/FAILURE based on the result.

References:

Share via

LISTENABLE WORKER

1 additional answer

Your answer