using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using StackDataStructure;

namespace StackTesting
{
    [TestClass]
    public class Tests
    {
        [TestMethod]
        public void TestEmptyStackSize()
        {
            var stack = new Stack();
            Assert.IsTrue(stack.IsEmpty());
        }

        [TestMethod]
        [ExpectedException(typeof(InvalidOperationException))]
        public void EmptyStackPopException()
        {
            var stack = new Stack();
            stack.Pop();
        }

        [TestMethod]
        public void PushingAndPoppintItemsToStack()
        {
            var stack = new Stack();
            stack.Push(1);
            Assert.AreEqual(1, stack.Size);
            stack.Push(2);
            Assert.AreEqual(2, stack.Size);
            Assert.AreEqual(2, stack.Pop());
            Assert.AreEqual(1, stack.Size);
            Assert.AreEqual(1, stack.Pop());
            Assert.IsTrue(stack.IsEmpty());
            Assert.IsTrue(stack.Size == 0);
            stack.Push(10);
            Assert.IsTrue(stack.Size == 1);
            Assert.IsTrue(10 == stack.Pop());
            Assert.IsTrue(stack.IsEmpty());
        }
    }
}

using System;

namespace StackDataStructure
{
    internal class Node
    {
        internal int value;
        internal Node underTop;
    }

    public class Stack
    {
        private int size;
        private Node top;

        public int Size { get { return size; } }

        public bool IsEmpty()
        {
            return top == null;
        }

        public int Pop()
        {
            if (IsEmpty())
                throw new InvalidOperationException("Stack is empty");

            int value = top.value;
            top = top.underTop;
            size--;
            return value;
        }

        public void Push(int v)
        {
            top = new Node{
                value = v,
                underTop = top
            };
            size++;
        }
    }
}