Does Class need to implement IEnumerable to use Foreach

Question

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 u

Answer 1

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(); }

}

Answer 2

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<T>, then the collection type is this interface, the enumerator type is the interface System.Collections.Generic.IEnumerator<T>, 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.

Answer 3

@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.

Answer 4

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

Answer 5

The type only requires to have a public/non-static/non-generic/parameterless method named GetEnumerator which should return something that has a public MoveNext method and a public Current property. As I recollect Mr Eric Lippert somewhere, this was designed so as to accommodate pre generic era for both type safety and boxing related performance issues in case of value types.

For instance this works:

class Test
{
    public SomethingEnumerator GetEnumerator()
    {

    }
}

class SomethingEnumerator
{
    public Something Current //could return anything
    {
        get { }
    }

    public bool MoveNext()
    {

    }
}

//now you can call
foreach (Something thing in new Test()) //type safe
{

}

This is then translated by the compiler to:

var enumerator = new Test().GetEnumerator();
try {
   Something element; //pre C# 5
   while (enumerator.MoveNext()) {
      Something element; //post C# 5
      element = (Something)enumerator.Current; //the cast!
      statement;
   }
}
finally {
   IDisposable disposable = enumerator as System.IDisposable;
   if (disposable != null) disposable.Dispose();
}

From 8.8.4 section of the spec.

---

Something worth noting is the enumerator precedence involved - it goes like if you have a public GetEnumerator method, then that is the default choice of foreach irrespective of who is implementing it. For example:

class Test : IEnumerable<int>
{
    public SomethingEnumerator GetEnumerator()
    {
        //this one is called
    }

    IEnumerator<int> IEnumerable<int>.GetEnumerator()
    {

    }
}

(If you don't have a public implementation (ie only explicit implementation), then precedence goes like IEnumerator<T> > IEnumerator.)

Answer 6

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