foreach with classes that do not 'implement' IEnumerable / IEnumerator

According to foreach, in (C #) :

  

An expression with a collection or arrangement. The element type of the collection must be convertible to the identifier type. Do not use an expression that evaluates to null .
  Evaluate a type that implements IEnumerable or a type that declares a GetEnumerator method. In the latter case, you must return either a type that implements IEnumerator or that declares all methods defined in IEnumerator .

The code you mentioned complies with the last case. The method Reset() does not < em> needs to be implemented necessarily .

In fact, a statement foreach of the form: 1

foreach (V v in x) embedded-statement

is then expanded to:

{
    E e = ((C)(x)).GetEnumerator();
    try {
       V v;
       while (e.MoveNext()) {
           v = (V)(T)e.Current;
           embedded-statement
       }
    }
    finally {
       ... // Dispose of e
    }
}

The variable e is not visible or accessible from the original code. The variable v is read-only in the embedded statement ( embedded-statement ).

Notes

Here

Iterators (C # and Visual Basic)

is mentioned

  

When creating an iterator for a class or struct, the complete IEnumerator interface does not have to be implemented. When the compiler detects the iterator, it automatically generates the Current, MoveNext, and Dispose methods of the IEnumerator or IEnumerator interface.

I understand that the foreach is making use of Current , MoveNext() and GetEnumerator() but with this is enough, it does not validate that this interface IEnumerable and IEnumerator are declared, since you are equal expose the functionality that the iteration needs

How to make a Visual C # class usable in a foreach statement

The correct thing would be to define the interfaces that ensure that the class is iterable

using System;
using System.Collections;

public class Program
{
    public static void Main()
    {
        var test = new Test();

        foreach (var x in test)
            Console.WriteLine(x);

    }
}

public class TestEnumerator : IEnumerator 
{
    private int _veces;
    public TestEnumerator(int veces) { _veces = veces; }
    public object Current  { get { return "Hola " + _veces; } }
    public bool MoveNext() { return _veces-- > 0; }

    public void Reset(){ _veces = 0; }
}

public class Test : IEnumerable
{
    public IEnumerator GetEnumerator() { 
        return (IEnumerator)new TestEnumerator(10); 
    }
}