using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<TreeNode> tree = new List<TreeNode> { new TreeNode {

            Id = 1,

            Children = new List<TreeNode> { 

                new TreeNode {

                    Id = 11,

                    Children = new List<TreeNode> { 

                        new TreeNode {

                            Id = 111    

                        }

                    }

                },

                new TreeNode {

                    Id = 12,

                    Children = new List<TreeNode> { 

                        new TreeNode {

                            Id = 121    

                        }

                    }

                }

            }

        }

        };

        IEnumerable<TreeNode> result = GetAllNodes(tree.First());

        string test = string.Join("; ", result.Select(r => r.Id));

        Console.WriteLine(string.Format("Hello World - {0}", test));

    }

    public static IEnumerable<TreeNode> GetAllNodes(TreeNode node) {

        if(node == null)

            throw new ArgumentNullException(nameof(node));

        List<TreeNode> result = new List<TreeNode>();

        result.Add(node);

        if(node.Children != null)

        foreach(TreeNode child in node.Children)

        {

            result.AddRange(GetAllNodes(child));

        }

        return result;

    }

}

​

public class TreeNode {

    public int Id;

    public IEnumerable<TreeNode> Children { get; set; }

}