if C# 8.0 released

tree.cs

using System;

namespace tree {
    static class Program {

        static void Main() =>
            Console.WriteLine(MyTree.Sum());
        
        static int Sum(this Tree<int> t) =>
            t switch {
                Leaf<int> leaf => leaf.Value,
                Node<int>(var l, var r) => l.Sum() + r.Sum()
            };

        static Tree<int> MyTree =
            new Node<int>(
                new Node<int>(
                    new Leaf<int>(1),
                    new Node<int>(
                        new Leaf<int>(2),
                        new Leaf<int>(3))),
                new Node<int>(
                    new Node<int>(
                        new Leaf<int>(4),
                        new Leaf<int>(5)),
                    new Leaf<int>(6)));
    }

    abstract class Tree<T> {}

    class Leaf<T> : Tree<T> {
        public readonly T Value;
        public Leaf(T value) => Value = value;
        public void Deconstruct(out T value) => value = Value;
    }

    class Node<T> : Tree<T> {
        public readonly Tree<T> Left;
        public readonly Tree<T> Right;
        public Node(Tree<T> left, Tree<T> right) => (Left, Right) = (left, right);
        public void Deconstruct(out Tree<T> left, out Tree<T> right) => (left, right) = (Left, Right);
    }
}