Hierarchical structure iteration and LINQ

Question

Hello!

Assume that we have class

public class RMenuItem
{
    public List<RMenuItem> ChildrenItems { get; }
    public decimal OperationID { get; }
    public string Name { get; }
}

as you can see - each menuitem could have children items - as usual in menu. My task is to iterate through each items of this list and apply some action to it. Classical decision is to write recursive iteration. But I'm interesting if LINQ could make my task easier? For example, I suppose that we can write query that can get flat list of objects, which i can iterate simply with foreach. But my attempts in this way weren't successful yet. So any help appreciated!

Answer 1

It's possible:

    public void PrintAllNames(RMenuItem rootItem)
    {
        Action<RMenuItem> print = null;
        print = m =>
            {
                Console.WriteLine(m.Name);
                m.ChildrenItems.ForEach(print);
            };
        print(rootItem);
    }

Notice how it's necessary to declare print so that print can use itself. This is directly comparable to a recursive method, which I'd rather use:

    public void PrintAllNames(RMenuItem rootItem)
    {
        Console.WriteLine(rootItem.Name);
        rootItem.ChildrenItems.ForEach(PrintAllNames);
    }

(although for a more complex situation, maybe the functional solution would make the most sense)

Answer 2

Indeed you can do that using LINQ, SelectMany flats out the list, just some example

menuItemsList.SelectMany(x => x.ChildrenItems).Where(c => c.someChildProperty);

Thanks

Edit:

In response to the comments, I was just giving an example of SelectMany previously. Thanks for pointing out.

menuItemsList.SelectMany(x => x.ChildrenItems.Select(p => p)).Where(c => c.someChildProperty);

OR something like this

menuItemsList.SelectMany(x => x.ChildrenItems).Select(p => p).Where(c => c.someChildProperty);

Edit2

Ahh .. now I understood what you want ..

We can just slightly modify my above query to do what you want

menuItemsList
.SelectMany(x => { //do something with x like printing it  
                    x.ChildrenItems 
                 })
.Select(p => { // do something with p like printing it 
                  p 
             });

Basically you can do what you want the element inside the {}

Thanks

Answer 3

I suggest 2 ways of achieving this. You can opt with an utility method to get all the items or you can implement the Visitor Pattern, though it implies changing the RMenuItem class.

Utility method:

    static IEnumerable<RMenuItem> GetAllMenuItems(IList<RMenuItem> items)
    {
        if (items == null)
            throw new ArgumentNullException("items");

        Queue<RMenuItem> queue = new Queue<RMenuItem>(items);

        while (queue.Count > 0)
        {
            var item = queue.Dequeue();
            if (item.ChildrenItems != null)
            {
                foreach (var child in item.ChildrenItems)
                {
                    queue.Enqueue(child);
                }
            }
            yield return item;
        }
    }

I prefer an imperative way to a recursive because we can use iterator blocks.

Visitor Pattern:

    public interface IRMenuItemVisitor
    {
        void Visit(RMenuItem item);
    }

    public class PrintRMenuItemVisitor : IRMenuItemVisitor
    {
        public void Visit(RMenuItem item)
        {
            Console.WriteLine(item);
        }
    }

    public interface IRMenuItem
    {
        void Accept(IRMenuItemVisitor visitor);
    }

    public class RMenuItem : IRMenuItem
    {
        // ...

        public void Accept(IRMenuItemVisitor visitor)
        {
            visitor.Visit(this);
            if (ChildrenItems != null)
            {
                foreach (var item in ChildrenItems)
                {
                    item.Accept(visitor);
                }
            }
        }
    }

Usage:

        RMenuItem m1 = new RMenuItem
        {
            Name = "M1",
            ChildrenItems = new List<RMenuItem> { 
                new RMenuItem { Name = "M11" }, 
                new RMenuItem { 
                    Name = "M12", 
                    ChildrenItems = new List<RMenuItem> {
                        new RMenuItem { Name = "M121" },
                        new RMenuItem { Name = "M122" }
                    }
                } 
            }
        };

        RMenuItem m2 = new RMenuItem
        {
            Name = "M2",
            ChildrenItems = new List<RMenuItem> { 
                new RMenuItem { Name = "M21" }, 
                new RMenuItem { Name = "M22" }, 
                new RMenuItem { Name = "M23" } 
            }
        };

        IList<RMenuItem> menus = new List<RMenuItem> { m1, m2 };
        foreach (var menu in GetAllMenuItems(menus))
        {
            Console.WriteLine(menu);
        }

        // or

        IList<RMenuItem> menus = new List<RMenuItem> { m1, m2 };
        foreach (var menu in menus)
        {
            menu.Accept(new PrintRMenuItemVisitor());
        }

Answer 4

You could difine a Flatten method in your class (or as an extension if you prefer) like this

public IEnumerable<RMenuItem> Flatten()
{
    foreach (var item in ChildrenItems)
    {
        yield return item;
    }
    return ChildrenItems.SelectMany(item => item.Flatten());
}

then doing somthing with each elements will be as simple as

RMenuItem rootItem ;

    // do somthing with the root item
    foreach (var item  in rootItem.Flatten())
    {
        // do somthing
    }