Program.cs

using System;
namespace Acme.Collections
{
    public class Stack
    {
        Entry top;
        public void Push(object data) 
        {
            top = new Entry(top, data);
        }

        public object Pop() 
        {
            if (top == null)
            {
                throw new InvalidOperationException();
            }
            object result = top.data;
            top = top.next;
            return result;
        }
        
        class Entry
        {
            public Entry next;
            public object data;
            public Entry(Entry next, object data)
            {
                this.next = next;
                this.data = data;
            }
        }
    }
}

namespace StackExample
{
using System;
using Acme.Collections;
class Example
{
    static void Main() 
    {
        Stack s = new Stack();
        s.Push(1);
        s.Push(10);
        s.Push(100);
        Console.WriteLine(s.Pop());
        Console.WriteLine(s.Pop());
        Console.WriteLine(s.Pop());
    }
}
}