using System;

using System.Collections;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var categoryTree = new List<Category>();

        categoryTree.Add(new Category { Name = "Parent1"});

        categoryTree.Last().Children.Add(new Category { Name = "Child1OfParent1" });

        categoryTree.Last().Children.Add(new Category { Name = "Child2OfParent1" });

        categoryTree.Last().Children.Add(new Category { Name = "Child3OfParentz" });

        Console.WriteLine("source list of children: " + string.Join(",", categoryTree.Last().Children.Select(x => x.Name)));

        Console.WriteLine();

        //var changed = ModifyChildren(categoryTree);

        var changed = RemoveMatchingChildren(categoryTree, "Parent1");

        Console.WriteLine();

        Console.WriteLine("after modification");

        Console.WriteLine("source list of children: " + string.Join(",", categoryTree.Last().Children.Select(x => x.Name)));        

    }

    public static List<Category> RemoveMatchingChildren(List<Category> categories, string nameParam)

    {

        Console.WriteLine("~~removing matching child categories~~");

        var filtered = categories;

        filtered.ForEach(parent =>

        {

            for(var i = parent.Children.Count; i > 0; i--)

            {

                var childIndex = i-1;

                if(parent.Children[childIndex].Name.Contains(nameParam))

                {

                    parent.Children.RemoveAt(childIndex);

                }               

            }           

        });

        Console.WriteLine("list of children after modification: " + string.Join(",", filtered.Last().Children.Select(x => x.Name)));        

        return filtered;

    }

    public static List<Category> ModifyChildren(List<Category> categories)

    {

        Console.WriteLine("~~modifying categories~~");

        var filtered = categories;

        filtered.ForEach(category =>

        {

            category.Children = new List<Category>();   

            category.Children.Add(new Category { Name = "I'm the destructor of the world" });

        });

        Console.WriteLine("list of children after modification: " + string.Join(",", filtered.Last().Children.Select(x => x.Name)));        

        return filtered;

    }

    public class Category

    {

        public string Name {get;set;}

        public List<Category> Children {get;set;} = new List<Category>();

    }

}