Question

I have a class

    public class Foo
    {
       public IList<Foo> Items { get; set; }
    }

    public class FooList
    {
       public IList<Foo> Items { get; set; }
    }

I want to be able to get all of the Foo objects in one list, instead of a hierarchy.

I have tried

    IEnumerable<Foo> result = Items.SelectMany(f =>  f.Items);

but this just gets me the items in that specific object – it doesn’t get all the items in all of the child objects.

I also tried

    IEnumerable<Foo> result = Items.SelectMany(t => t)

But I get the error:

The type arguments for method ‘System.Linq.Enumerable.SelectMany(System.Collections.Generic.IEnumerable, System.Func>)’ cannot be inferred from the usage. Try specifying the type arguments explicitly.

Answer 1

It’s fairly easy to knock together a “flatten-this-tree” LINQ-like function that you can use in the more general case:

public static IEnumerable<T> Flatten<T>(
    this IEnumerable<T> source,
    Func<T, IEnumerable<T>> childSelector)
{
    HashSet<T> added = new HashSet<T>();
    Queue<T> queue = new Queue<T>();
    foreach(T t in source)
        if (added.Add(t))
            queue.Enqueue(t);
    while (queue.Count > 0)
    {
        T current = queue.Dequeue();
        yield return current;
        if (current != null)
        {
            IEnumerable<T> children = childSelector(current);
            if (children != null)
                foreach(T t in childSelector(current))
                    if (added.Add(t))
                        queue.Enqueue(t);
        }
    }
}

Then if you have a FooList, you can use

var allFoos = myFooList.Items.Flatten(foo => foo.Items);

to get all the Foos in the list, along with all their children and all their children and…