Tuesday, February 8, 2011

Does Class need to implement IEnumerable to use Foreach

This is in C#, I have a class that I am using from some else's DLL. It does not implement IEnumerable but has 2 methods that pass back a IEnumerator. Is there a way I can use a foreach loop on these. The class I am using is sealed.

From stackoverflow Brian G

  • foreach does not require IEnumerable, contrary to popular belief. All it requires is a method GetEnumerator that returns any object that has the method MoveNext and the get-property Current with the appropriate signatures.

    /EDIT: In your case, however, you're out of luck. You can trivially wrap your object, however, to make it enumerable:

    class EnumerableWrapper {
    private readonly TheObjectType obj;

    public EnumerableWrapper(TheObjectType obj) {
        this.obj = obj;
    }

    public IEnumerator<YourType> GetEnumerator() {
        return obj.TheMethodReturningTheIEnumerator();
    }
}

// Called like this:

foreach (var xyz in new EnumerableWrapper(yourObj))
    …;

    /EDIT: The following method, proposed by several people, does not work if the method returns an IEnumerator:

    foreach (var yz in yourObj.MethodA())
    …;
    From Konrad Rudolph

  • You could always wrap it, and as an aside to be "foreachable" you only need to have a method called "GetEnumerator" with the proper signature. 

    
class EnumerableAdapter
{
  ExternalSillyClass _target;

  public EnumerableAdapter(ExternalSillyClass target)
  {
    _target = target;
  }

  public IEnumerable GetEnumerator(){ return _target.SomeMethodThatGivesAnEnumerator(); }

}
    From Torbjörn Gyllebring

  • Not strictly. As long as the class has the required GetEnumerator, MoveNext, Reset, and Current members, it will work with foreach

    From Bill the Lizard

  • If foreach doesn't require an explicit interface contract, does it find GetEnumerator using reflection?

    Yuck.

    David B : Not run-time reflection... The compiler does the finding.
    From FlySwat

  • Given class X with methods A and B that both return IEnumerable, you could use a foreach on the class like this:

    foreach (object y in X.A())
{
    //...
}

// or

foreach (object y in X.B())
{
   //...
}

    Presumably the meaning for the enumerables returned by A and B are well-defined.

    Konrad Rudolph : This isn't what was asked: the methods return `IEnumerator`, not `IEnumerable`.
    Joel Coehoorn : D'oh! Parser error :o
    From Joel Coehoorn

  • No, you don't and you don't even need an GetEnumerator method, e.g.:

    class Counter
{
    public IEnumerable<int> Count(int max)
    {
        int i = 0;
        while (i <= max)
        {
            yield return i;
            i++;
        }
        yield break;
    }
}

    which is called this way:

    Counter cnt = new Counter();

foreach (var i in cnt.Count(6))
{
    Console.WriteLine(i);
}
    Hallgrim : IEnumerable has a GetEnumerator method. This is the one that gets called when you do foreach.
    From pb

  • According to MSDN:

    foreach (type identifier in expression) statement

    where expression is:

    Object collection or array expression. The type of the collection element must be convertible to the identifier type. Do not 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, GetEnumerator should either return a type that implements IEnumerator or declares all the methods defined in IEnumerator.

    chris : RTFM! What a great concept. +1
    From Adam Hughes

  • Re: If foreach doesn't require an explicit interface contract, does it find GetEnumerator using reflection?

    (I can't comment since I don't have a high enough reputation.)

    If you're implying runtime reflection then no. It does it all compiletime, another lesser known fact is that it also check to see if the returned object that might Implement IEnumerator is disposable.

    To see this in action consider this (runnable) snippet.

    
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication3
{
    class FakeIterator
    {
        int _count;

        public FakeIterator(int count)
        {
            _count = count;
        }
        public string Current { get { return "Hello World!"; } }
        public bool MoveNext()
        {
            if(_count-- > 0)
                return true;
            return false;
        }
    }

    class FakeCollection
    {
        public FakeIterator GetEnumerator() { return new FakeIterator(3); }
    }

    class Program
    {
        static void Main(string[] args)
        {
            foreach (string value in new FakeCollection())
                Console.WriteLine(value);
        }
    }
}
    From Torbjörn Gyllebring

  • Short answer:

    You need a class with a method named GetEnumerator, which returns the IEnumerator you already have. Achieve this with a simple wrapper:

    class ForeachWrapper
{
  private IEnumerator _enumerator;

  public ForeachWrapper(Func<IEnumerator> enumerator)
  {
    _enumerator = enumerator;
  }

  public IEnumerator GetEnumerator()
  {
    return _enumerator();
  }
}

    Usage:

    foreach (var element in new ForeachWrapper(x => myClass.MyEnumerator()))
{
  ...
}

    From the C# Language Specification:

    The compile-time processing of a foreach statement first determines the collection type, enumerator type and element type of the expression. This determination proceeds as follows:

    • If the type X of expression is an array type then there is an implicit reference conversion from X to the System.Collections.IEnumerable interface (since System.Array implements this interface). The collection type is the System.Collections.IEnumerable interface, the enumerator type is the System.Collections.IEnumerator interface and the element type is the element type of the array type X.

    • Otherwise, determine whether the type X has an appropriate GetEnumerator method:

      • Perform member lookup on the type X with identifier GetEnumerator and no type arguments. If the member lookup does not produce a match, or it produces an ambiguity, or produces a match that is not a method group, check for an enumerable interface as described below. It is recommended that a warning be issued if member lookup produces anything except a method group or no match.

      • Perform overload resolution using the resulting method group and an empty argument list. If overload resolution results in no applicable methods, results in an ambiguity, or results in a single best method but that method is either static or not public, check for an enumerable interface as described below. It is recommended that a warning be issued if overload resolution produces anything except an unambiguous public instance method or no applicable methods.

      • If the return type E of the GetEnumerator method is not a class, struct or interface type, an error is produced and no further steps are taken.

      • Member lookup is performed on E with the identifier Current and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a public instance property that permits reading, an error is produced and no further steps are taken.

      • Member lookup is performed on E with the identifier MoveNext and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a method group, an error is produced and no further steps are taken.

      • Overload resolution is performed on the method group with an empty argument list. If overload resolution results in no applicable methods, results in an ambiguity, or results in a single best method but that method is either static or not public, or its return type is not bool, an error is produced and no further steps are taken.

      • The collection type is X, the enumerator type is E, and the element type is the type of the Current property.

    • Otherwise, check for an enumerable interface:

      • If there is exactly one type T such that there is an implicit conversion from X to the interface System.Collections.Generic.IEnumerable, then the collection type is this interface, the enumerator type is the interface System.Collections.Generic.IEnumerator, and the element type is T.

      • Otherwise, if there is more than one such type T, then an error is produced and no further steps are taken.

      • Otherwise, if there is an implicit conversion from X to the System.Collections.IEnumerable interface, then the collection type is this interface, the enumerator type is the interface System.Collections.IEnumerator, and the element type is object.

      • Otherwise, an error is produced and no further steps are taken.

    From VVS

  • @Brian: Not sure you try to loop over the value return from method call or the class itself, If what you want is the class then by make it an array you can use with foreach.

    From c.sokun

  • For a class to be usable with foeach all it needs to do is have a public method that returns and IEnumerator named GetEnumerator(), that's it:

    Take the following class, it doesn't implement IEnumerable or IEnumerator :

    public class Foo
{
    private int[] _someInts = { 1, 2, 3, 4, 5, 6 };
    public IEnumerator GetEnumerator()
    {
        foreach (var item in _someInts)
        {
            yield return item;
        }
    }
}

    alternatively the GetEnumerator() method could be written:

        public IEnumerator GetEnumerator()
    {
        return _someInts.GetEnumerator();
    }

    When used in a foreach ( Note that the no wrapper is used, just a class instance ):

        foreach (int item in new Foo())
    {
        Console.Write("{0,2}",item);
    }

    prints:

    1 2 3 4 5 6

    Jon Skeet : It doesn't need to return an IEnumerator - just a type with the right members.
    From Pop Catalin
Posted by Mu Cat at 12:58 AM

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