TaskCompletionSource demo

async Task Main()
{
    var service = new RandomNumberService();

    Console.WriteLine("Requesting number via event...");
    service.RandomNumberProduced += RandomNumberProduced;
    service.RequestRandomNumber();
    Console.WriteLine("Press any key after you get your number");
    Console.ReadLine();
    service.RandomNumberProduced -= RandomNumberProduced;



    Console.WriteLine("Awaiting random number");
    var helper = new RandomHelper(service);
    var result = await helper.RequestRandomNumberAsyc();
    Console.WriteLine(result);
}

public void RandomNumberProduced(object sender, RandomNumberProducedEventArgs e)
{
    Console.WriteLine(e.Number.ToString());
}

public class RandomHelper
{
    private RandomNumberService _service;

    public RandomHelper(RandomNumberService service)
    {
        _service = service;
    }

    public Task<int> RequestRandomNumberAsyc()
    {
        var tcs = new TaskCompletionSource<int>();
        _service.RandomNumberProduced += RandomNumberProduced;

        _service.RequestRandomNumber();
        return tcs.Task;

        void RandomNumberProduced(object sender, RandomNumberProducedEventArgs e)
        {
            _service.RandomNumberProduced -= RandomNumberProduced;
            tcs.SetResult(e.Number);
        }
    }
}

// You can define other methods, fields, classes and namespaces here

public class RandomNumberService
{
    private Random _rnd = new Random();

    public event EventHandler<RandomNumberProducedEventArgs>? RandomNumberProduced;

    public void RequestRandomNumber()
    {
        Task.Run(() =>
        {
            Thread.Sleep(5000);
            RandomNumberProduced?.Invoke(this, new RandomNumberProducedEventArgs { Number = _rnd.Next() });
        });
    }

}

public class RandomNumberProducedEventArgs : EventArgs
{
    public int Number { get; set; }
}