public class MyCoolViewModel: ReactiveObject
{
    // Create a Property to store the results
    ObservableAsPropertyHelper<byte[]> _BytesWeHaveRead;
    public byte[] BytesWeHaveRead {
        get { return _BytesWeHaveRead.Value; }
    }

    // The command we'll be testing
    ReactiveAsyncCommand ReadBytesCommand { get; private set; }

    public MyCoolViewModel(Func<IObservable<byte[]>> readBytesFunc)
    {
        ReadBytesCommand = new ReactiveAsyncCommand();

        // Take our Command, send it through the readBytesFunc function, then
        // pipe the results to the BytesWeHaveRead property
        _BytesWeHaveRead = ReadBytesCommand
            .RegisterAsyncObservable(_ => readBytesFunc())
            .ToProperty(this, x => x.BytesWeHaveRead);
    }
}

IObservable<byte[]> mockReadBytesAsync()
{
    // Wait ten seconds, then return the byte array
    return Observable.Return(new byte[] {1,2,3}).Delay(TimeSpan.FromSeconds(10), RxApp.TaskpoolScheduler);
}

[Fact]
public void ReadBytesAsyncCommandTest()
{
    // Replace all schedulers with the TestScheduler
    (new TestScheduler()).With(sched => {
        var fixture = new MyCoolViewModel(mockReadBytesAsync);

        // Execute the command - remember that it should take 10 seconds to
        // execute before returning
        fixture.ReadBytesCommand.Execute(null);

        // 1 second in, it should still be running, the results should be empty
        sched.RunToMilliseconds(1000)
        Assert.False(fixture.ReadBytesCommand.CanExecute(null));
        Assert.Null(fixture.BytesWeHaveRead);

        // 9 seconds in, same deal. Remember though, this doesn't take 9 seconds
        // of actual wall time to execute, this entire test is finished instantly
        sched.RunToMilliseconds(9000)
        Assert.False(fixture.ReadBytesCommand.CanExecute(null));
        Assert.Null(fixture.BytesWeHaveRead);

        // 11 seconds in, it should be complete - we should be able to read
        // another block since we're done with the first one.
        sched.RunToMilliseconds(11000)
        Assert.True(fixture.ReadBytesCommand.CanExecute(null));
        Assert.Equal(1, fixture.BytesWeHaveRead[0]);
        Assert.Equal(2, fixture.BytesWeHaveRead[1]);
        Assert.Equal(3, fixture.BytesWeHaveRead[2]);
    });
}