Best practice recursive flattening of linked list in c#

Question

I built a data structure where the main class, TaskHolder, contains a List<Task> and Task contains another List<Task> and Name. is there a LINQ one line code that can retrieve all nested Names into a flattened string array?

this is the basic structure:

Public class TaskHolder
  List<Task>

Public class Task
  String Name
  List<Task>

This is only one example of a duplicate. The "LINQ recusive one-liner" is asked at least once every two weeks. — Gert Arnold, Jan 24 '15 at 22:10

ie. · Answer 1 · 2015-01-24T11:10:59.213

3

It looks like you can't achieve it without recursion. It can be something like this:

    static IEnumerable<string> Flatten(Task task)
    {
        return new[] {task.Name}.Concat(task.Tasks.SelectMany(Flatten));
    }

    //.. and then
    var allNames = Flatten(taskHolder.Task);

edited Jan 24 '15 at 11:10

answered Jan 24 '15 at 11:02

ie.

5,982
1
29
44

1

@MartinLiversage and why shouldn't recursion look recursive? ;) – ie. Jan 24 '15 at 11:12
Works like a charm. I encapsulated this Helper with a for each loop to iterate all tasks in the list and concatanated it all into a IEnumerable. – yaron hochman Jan 24 '15 at 11:28

score 0 · Answer 2 · answered Jan 24 '15 at 13:06

You can also write your own LINQ extension method to do this kind of thing, and use something like a stack to avoid explicit recursion (better if the nesting is very deep):

  public static class LinqExtensions
  {
    public static IEnumerable<TP> FlattenProperties<T, TP>(this IEnumerable<T> outers,
        Func<T, TP> propertySelector, Func<T, IEnumerable<T>> innersSelector)
    {
      Stack<T> stack = new Stack<T>(outers);
      while (stack.Any())
      {
        T outer = stack.Pop();

        TP prop = propertySelector(outer);
        yield return prop;
        if (innersSelector(outer) != null)
        {
          foreach (var inner in innersSelector(outer))
            stack.Push(inner);
        }
      }
    }
  }

You can then write something like:

TaskHolder th = new TaskHolder() { ... };
var names = th.Tasks.FlattenProperties(x => x.Name, x => x.Tasks).ToList();

Best practice recursive flattening of linked list in c#

2 Answers2