foo.cs

// BynaryTree.cs
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


// all code copy pasted in haste
namespace ConsoleApplication1 {
    // inherrit from interface
    public class BinaryTree : ISet {

        private class node {
            public int value;
            public node left;
            public node right;
            
            public node(int val) {
                left = null;
                right = null;
                value = val;
            }

        }

        private node root;

        public BinaryTree() {
            root = null;
        }

        public void insert(int item) {
            root = insert(root, item);
        }

        /**
         * recursively insert an item into tree 
         */
        private static node insert(node curr, int val) {
            // base case if we are at a leaf
            if(curr == null) {
                // create new node
                curr = new node(val);
            }else if (curr.value > val) {
                curr.left = insert(curr.left, val);
            } else if (curr.value < val) {
                curr.right = insert(curr.right, val);
            }
            return curr;
        }

        public bool contains(int item) {
            return contains(root, item);
        }

        private bool contains(node curr, int val) {
            if(curr == null) { return false; } // reached end without finding anything
            
            if(curr.value == val) { return true; }

            if(curr.value > val) { return contains(curr.left, val); }

            if(curr.value < val) { return contains(curr.right, val); }

            return false;
        }
    }

    public class LazyImplement : ISet {
        private List<int> myList = new List<int>();

        public bool contains(int item) {
            return myList.Contains(item);
        }

        public void insert(int item) {
            myList.Add(item);
        }
    }

    // here is our interface
    public interface ISet {
        void insert(int item);

        bool contains(int item);
    }
}

// Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1 {
    class Program {
        public static bool hasItem(ISet container, int value) {
            return container.contains(value);
        }

        static void Main(string[] args) {
            BinaryTree myTree = new BinaryTree();
            LazyImplement anotherSet = new LazyImplement();

            myTree.insert(42);
            myTree.insert(20);
            myTree.insert(10);
            myTree.insert(15);
            myTree.insert(5);
            myTree.insert(100);
            myTree.insert(1337);

            anotherSet.insert(21);

            Console.WriteLine(myTree.contains(5));
            Console.WriteLine(myTree.contains(23));

            Console.WriteLine(hasItem(myTree, 5));
            Console.WriteLine(hasItem(myTree, 21));

            Console.WriteLine(hasItem(anotherSet, 21));
            Console.WriteLine(hasItem(anotherSet, 4));


            Console.ReadLine();
        }
    }
}