public class Logger
{
    private Task previousTask = Task.Run(() => Task.FromResult(true));
    private readonly object key = new object();

    public void Log(string message)
    {
        lock (key)
        {
            previousTask = previousTask.ContinueWith(t => ActuallyLog(message)
                , CancellationToken.None
                , TaskContinuationOptions.None
                , TaskScheduler.Default);
        }
    }

    private async Task ActuallyLog(string message)
    {
        System.Diagnostics.Debug.Print("Logger: thread {0}, {1}", Thread.CurrentThread.ManagedThreadId, message);
        await Task.Yield();  // SaveChangesAsync();
    }
}