非同期操作で再入可能でタスクのシーケンスを維持するにはどうすればよいですか?

class AsyncOp<T>
{
    Task<T> _pending = Task.FromResult(default(T));

    public Task<T> CurrentTask => _pending;

    public Task<T> RunAsync(Func<Task<T>> handler, bool useSynchronizationContext = false)
    {
        var pending = _pending;
        var wrapper = async () =>
        {
            var prevResult = await pending;
            Console.WriteLine($"\nprev task result:  {prevResult}");
            return await handler();
        };

        var task = new Task<Task<T>>(wrapper);
        var inner = task.Unwrap();
        _pending = inner;

        task.RunSynchronously(useSynchronizationContext ?
            TaskScheduler.FromCurrentSynchronizationContext() :
            TaskScheduler.Current);

        return inner;
    }
}

Test #1...

prev task result:  0
this task arg: 1000

prev task result:  1000
this task arg: 900

prev task result:  900
this task arg: 800

Press any key to continue to test #2...


prev task result:  800
this task arg: 100

prev task result:  100
this task arg: 200