class E
    {
        public IEnumerable MyGetEnumerator1()
        {
            yield return 1;
        }
        public IEnumerable<int> MyGetEnumerator2()
        {
            yield return 1;
            yield return 2;
        }
        public IEnumerator MyGetEnumerator3()
        {
            yield return 1;
            yield return 2;
            yield return 3;
        }
        public IEnumerator<int> MyGetEnumerator4()
        {
            yield return 1;
            yield return 2;
            yield return 3;
            yield return 4;
        }      
        public IEnumerator GetEnumerator()
        {
            yield return 1;
            yield return 2;
            yield return 3;
            yield return 4;
        }
    }


E e = new E();
foreach (var item in e.MyGetEnumerator1())            
    Console.WriteLine(item);

foreach (var item in e.MyGetEnumerator2())
    Console.WriteLine(item);

//報錯：foreach statement cannot operate on variables of type 'IEnumerator' because 'IEnumerator' does not contain a public instance or extension definition for 'GetEnumerator'
foreach (var item in e.MyGetEnumerator3())  
    Console.WriteLine(item);

//報錯：同上
foreach (var item in e.MyGetEnumerator4())  
    Console.WriteLine(item);

//當(dāng)GetEnumerator()的返回類型為IEnumerator或IEnumerator<int>時都能正常運行，當(dāng)為IEnumerable或IEnumerable<int>時，則報錯：1、'IEnumerable' does not contain a definition for 'Current'，2、foreach requires that the return type 'IEnumerable' of 'E.GetEnumerator()' must have a suitable public 'MoveNext' method and public 'Current' property
foreach (var item in e)  
     Console.WriteLine(item);

   foreach (var item in e.MyGetEnumerator1())            
       Console.WriteLine(item);

foreach (var item in e)  
    Console.WriteLine(item);