How to use LINQ to select all descendants of a compound object

Question

How to do ComponentTraversal.GetDescendants()better with LINQ?

Question

public static class ComponentTraversal
{
    public static IEnumerable<Component> GetDescendants(this Composite composite)
    {
        //How can I do this better using LINQ?
        IList<Component> descendants = new Component[]{};
        foreach(var child in composite.Children)
        {
            descendants.Add(child);
            if(child is Composite)
            {
                descendants.AddRange((child as Composite).GetDescendants());
            }
        }
        return descendants;
    }
}
public class Component
{
    public string Name { get; set; }
}
public class Composite: Component
{
    public IEnumerable<Component> Children { get; set; }
}
public class Leaf: Component
{
    public object Value { get; set; }
}

Answer

I edited Chris's answer to provide a common extension method that I added to my shared library. I see that this is useful for other people, so here it is:

    public static IEnumerable<T> GetDescendants<T>(this T component, Func<T,bool> isComposite, Func<T,IEnumerable<T>> getCompositeChildren)
    {
        var children = getCompositeChildren(component);
        return children
            .Where(isComposite)
            .SelectMany(x => x.GetDescendants(isComposite, getCompositeChildren))
            .Concat(children);
    }

Thanks to Chris!

Besides,

Take a look at LukeH's answer at http://blogs.msdn.com/b/wesdyer/archive/2007/03/23/all-about-iterators.aspx . His answer provides a better way to approach this problem as a whole, but I did not choose it because it was not a direct answer to my question.

Answer 1

var result = composite.Children.OfType<Composite>().SelectMany(child => child.GetDescendants()).Concat(composite.Children);
return result.ToList();

LINQ . :

• . , , LINQ.
• ,
• "if" . "Where" , . LINQ "OfType" .
• GetDescendants . , - , Select, SelectMany. , SelectMany.
• , . , .

Answer 2

, (1) , (2) (3) . :

public static IEnumerable<Component> GetDescendants(this Composite composite)
{
    var stack = new Stack<Component>();
    do
    {
        if (composite != null)
        {
            // this will currently yield the children in reverse order
            // use "composite.Children.Reverse()" to maintain original order
            foreach (var child in composite.Children)
            {
                stack.Push(child);
            }
        }

        if (stack.Count == 0)
            break;

        Component component = stack.Pop();
        yield return component;

        composite = component as Composite;
    } while (true);
}

:

public static IEnumerable<T> GetDescendants<T>(this T component,
    Func<T, bool> hasChildren, Func<T, IEnumerable<T>> getChildren)
{
    var stack = new Stack<T>();
    do
    {
        if (hasChildren(component))
        {
            // this will currently yield the children in reverse order
            // use "composite.Children.Reverse()" to maintain original order
            // or let the "getChildren" delegate handle the ordering
            foreach (var child in getChildren(component))
            {
                stack.Push(child);
            }
        }

        if (stack.Count == 0)
            break;

        component = stack.Pop();
        yield return component;
    } while (true);
}

Answer 3

, , :

public static IEnumerable<Component> GetDescendants(this Composite composite)
{
    return composite.Children
                .Concat(composite.Children
                            .Where(x => x is Composite)
                            .SelectMany(x => x.GetDescendants())
                );
}

, , . , , , , .

Answer 4

This is a good example of when you want to implement an iterator. This has the advantage of lazy evaluation in a slightly more readable syntax. In addition, if you need to add additional user logic, then this form is more extensible.

 public static IEnumerable<Component> GetDescendants(this Composite composite)
    {
        foreach(var child in composite.Children)
        {
            yield return child;
            if(!(child is Composite))
               continue;

            foreach (var subChild in ((Composite)child).GetDescendants())
               yield return subChild;
        }
    }