In C# 8.0, we can write a function that calculates the sum of a tree as follows:

tree.cs

using System;

namespace hoge {
    class Program {
        static void Main() {
            var tree =
                new Node(
                    new Leaf(1),
                    new Node(
                        new Leaf(2),
                        new Leaf(3)
                    ));

            Console.WriteLine(Sum(tree));
        }

        static int Sum(Tree tree) {
            switch (tree) {
                case Leaf leaf:
                    return leaf.Value;
                case Node node:
                    (var l, var r) = node;
                    return Sum(l) + Sum(r);
                default:
                    throw new InvalidOperationException();
            }
        }

/*
        // in C# 8.0, Sum can be written as follows:
        static int Sum(Tree tree) =>
            t switch {
                Leaf(n) => n,
                Node(l, r) => Sum(l) + Sum(r)
            };
*/

    }

    abstract class Tree {}

    class Leaf : Tree {
        public int Value { get; }
        public Leaf(int value) => Value = value;
        public void Deconstruct(out int value) => value = Value;
    }

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