为什么不能实现枚举接口

public class Program2 : IEnumerable
{
Stack stack;
public Program2()
{
stack = new Stack();
}
public void Push(T item) { stack.Push(item); }
public T Pop() { return (T)stack.Pop(); }
public IEnumerator GetEnumerator()
{
while (stack.Count > 0)
{
yield return (T)stack.Pop();
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerable();
}
}
class Test
{
static void Main()
{
Program2 p = new Program2();
for (int i = 0; i < 10; i++)
{
p.Push(i);
}
foreach (int m in p)
{
Console.Write(m + " ");
}
Console.ReadLine();
}
}

 public class Program2<T> : IEnumerable
    {
        Stack stack;
        public Program2()
        {
            stack = new Stack();
        }
        public void Push(T item) { stack.Push(item); }
        public T Pop() { return (T)stack.Pop(); }
        public IEnumerator<T> GetEnumerator()
        {
            while (stack.Count > 0)
            {
                yield return (T)stack.Pop();
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

     class Test
    {
        static void Main()
        {
            Program2<int> p = new Program2<int>();
            for (int i = 0; i < 10; i++)
            {
                p.Push(i);
            }
            foreach (int m in p)
            {
                Console.Write(m + " ");
            }
            Console.ReadLine();
        }
    }

最简单的

 using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class Program2<T> : IEnumerable<T>
{
    Stack<T> stack;
    public Program2()
    {
        stack = new Stack<T>();
    }
    public void Push(T item) { stack.Push(item); }
    public T Pop() { return stack.Pop(); }
    public IEnumerator<T> GetEnumerator() { return stack.GetEnumerator(); }
    IEnumerator IEnumerable.GetEnumerator() { return stack.GetEnumerator(); }
}

class Test
{
    static void Main()
        {
            Program2<int> p = new Program2<int>();
            for (int i = 0; i < 10; i++)
            {
                p.Push(i);
            }
            foreach (int m in p)
            {
                    Console.Write(m + " ");
            }
            Console.ReadLine();
        }
}