How to map the custom object with corresponding Task in the following scenario?

Question

I am developing a console application in which I have a third party rest client (wordpress rest client to be precise), which has some methods with return Task objects. An example method signature can be like:

public Task<bool> Delete(int id);

I have a list of Post to be deleted. I could do something simply like:

public void DeleteGivenPosts(List<Post> posts) {
  posts.ForEach(async post => await wpRestClient.Delete(post.Id));
}

In this case the deletion is fire and forget. It would be better if I log the information about the post which is deleted. A log statement like:

logger.Log($"A post with {post.Id} is deleted");

So, I decided to project the Tasks.

public async Task DeleteGivenPosts(List<Post> posts) {
  var postDeletionTasks = posts.Select(post => wpRestClient.Delete(post.Id));

  foreach (var deletionTask in TaskExtensionUtil.GetTasksInCompletingOrder(postsDeletionTasks)) {
    bool deletionResult = await deletionTask;

    if(deletionResult) {
      //i want to log success of deletion here
    } else {
      //i want to log the failure of deletion here
    }
  }
}

Here TaskExtensionUtil.GetTasksInCompletingOrder() is a helper method which returns the tasks in the order they complete. Code for this is:

public static List<Task<T>> GetTasksInCompletingOrder<T>(IEnumerable<Task<T>> sourceTasks) {
  var sourceTasksArr = sourceTasks.ToArray();
  var taskCompletionSourceArr = new TaskCompletionSource<T>[sourceTasksArr.Length];

  var currentSlot = -1;
  for (int i = 0; i < sourceTasksArr.Length; i++) {
    taskCompletionSourceArr[i] = new TaskCompletionSource<T>();
    sourceTasksArr[i].ContinueWith(prev => {
      int indexToSet = Interlocked.Increment(ref currentSlot);
      taskCompletionSourceArr[indexToSet].SetResult(prev.Result);
    });
  }

  return taskCompletionSourceArr.Select(i => i.Task).ToList();
}

The problem is that the deletionResult is a bool. In order to log information about which post is deleted I need to get the Post object associated with the deletion task.

I was thinking to create a dictionary that maps the deletion task to the corresponding Post by doing something like:

posts.Select(post => new { deletionTask = wpRestClient.Delete(post.Id), post})
     .ToDictionary(i => i.deletionTask, i => i.post);

But this will not work because in the GetTasksInCompletingOrder the original deletion tasks are translated to TaskCompletionSource tasks. So I will always get an exception that the key is not present in the dictionary. Also I am not sure how will a dictionary behave when it has Task objects as keys.

At this point I have no clue how to achieve the logging. I will appreciate any help.

Answer 1

how about to write the logstatement after deletion?

public async Task DeleteGivenPosts(List<Post> posts)
{
    await Task.WhenAll(
    posts.Select(async post =>
    {
        bool res = await wpRestClient.Delete(post.Id);
        string message = res ? $"Post {post.Id} is deleted" : $"Post {post.Id} survived!";
        logger.Log(message);
    }));
}

Here is a small LinqPad Programm to examplify the workings of the method:

async void Main()
{
    List<Post> postList = Enumerable.Range(1, 12).Select(id => new Post {Id = id}).ToList();
    Console.WriteLine("Start Deletion");
    await DeleteGivenPosts(postList);
    Console.WriteLine("Finished Deletion");
}

public static MyRestClient wpRestClient = new MyRestClient();
// Define other methods and classes here
public async Task DeleteGivenPosts(List<Post> posts)
{
    await Task.WhenAll(
    posts.Select(async post =>
    {
        bool res = await wpRestClient.Delete(post.Id);
        string message = res ? $"Post {post.Id} is deleted" : $"Post {post.Id} survived!";
        Console.WriteLine(message);
    }));
}

public static Random rand = new Random(DateTime.Now.Millisecond);

public class MyRestClient
{
    public async Task<bool> Delete(int i)
    {
        return await Task<bool>.Run(() => { Thread.Sleep(400); return rand.Next(1,4) == 1;});
    }
}

public class Post
{
    public int Id { get; set; }
}

Output:

Start Deletion
Post 1 is deleted
Post 3 survived!
Post 5 survived!
Post 6 survived!
Post 2 survived!
Post 4 survived!
Post 8 survived!
Post 7 survived!
Post 9 survived!
Post 11 survived!
Post 10 is deleted
Post 12 is deleted
Finished Deletion

Answer 2

You could wrap it with your own method too, something like this:

IEnumerable<Task<DeletionResult>> postDeletionTasks = posts.Select(post => DeletePost(post.Id));

Where your method and result class might look something like:

private async Task<DeletionResult> DeletePost(int postId)
{
    bool result = await wpRestClient.Delete(postId);
    return new DeletionResult(result, postId);
}

And

public class DeletionResult
{
    public DeletionResult(bool result, int postId)
    {
        Result = result;
        PostId = postId;
    }

    public bool Result { get; }
    public int PostId { get; }
}

That way you'll have a list of tasks which result contains the PostId it affected.