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Asked: 2020-07-21 06:28:51 +0800 CST 2020-07-21 06:28:51 +0800 CST

foreach with classes that don't 'implement' IEnumerable/IEnumerator

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I saw Equiso 's comment in one of my answers with an interesting detail, and that is that he implemented an interface (or part of it) without using the operator :to the right of the object name.

Provided the following code:

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

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

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

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

Indeed the output is as expected:

Hola 9
Hola 8
Hola 7
Hola 6
Hola 5
Hola 4
Hola 3
Hola 2
Hola 1
Hola 0

Which is equivalent to implementing an IEnumerable<T>e IEnumeratorin their respective classes. What is this type of implementation called? Since it's a class with an enumerator, but it's not a collection, I'd say it's a half-baked implementation of IEnumerable.

c#

2 Answers

  1. Best Answer

    According to foreach, in(C#) :

    An expression with a collection or array. The element type of the collection must be convertible to the type of the identifier . Don't use an expression that evaluates to null .
    Evaluates to a type that implements IEnumerable or a type that declares a GetEnumerator method . In the latter case, it must return either a type that implements IEnumerator or that declares all methods defined in IEnumerator .

    The code you mentioned satisfies the latter case. The method Reset()does not necessarily need to be implemented .

    In fact, a sentence foreachof the form: 1

    foreach (V v in x) embedded-statement

    is then expanded to:

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

    The variable eis not visible or accessible from the original code. The variable vis read-only in the embedded statement ( embedded-statement ).

    Grades

    1. See The C# Programming Language (Covering C# 4.0) by Anders Hejlsberg, Mads Torgersen, Scott Wiltamuth, Peter Golde, p. 519.
    • 4

  2. Here

    Iterators (C# and Visual Basic)

    It is mentioned

    When creating an iterator for a class or struct, you don't have to implement the full IEnumerator interface. When the compiler detects the iterator, it automatically generates the Current, MoveNext, and Dispose methods of the IEnumerator or IEnumerator interface.

    I understand that he foreachis making use of Current, MoveNext()and GetEnumerator()but this is enough for him, it does not validate that this is the interface IEnumerableand IEnumeratorthat they are declared, since these still 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); 
    }
}
    • 2