What is the best way to convert an IEnumerator to

问题:

I am writing a custom ConfigurationElementCollection for a custom ConfigurationHandler in C#.NET 3.5 and I am wanting to expose the IEnumerator as a generic IEnumerator.

What would be the best way to achieve this?

I am currently using the code:

public new IEnumerator<GenericObject> GetEnumerator()
{
  var list = new List();
  var baseEnum = base.GetEnumerator();
  while(baseEnum.MoveNext())
  {
    var obj = baseEnum.Current as GenericObject;
    if (obj != null)
      list.Add(obj);
  }
  return list.GetEnumerator();
}

Cheers

回答1:

I don't believe there's anything in the framework, but you could easily write one:

IEnumerator<T> Cast<T>(IEnumerator iterator)
{
    while (iterator.MoveNext())
    {
        yield return (T) iterator.Current;
    }
}

It's tempting to just call Enumerable.Cast<T> from LINQ and then call GetEnumerator() on the result - but if your class already implements IEnumerable<T> and T is a value type, that acts as a no-op, so the GetEnumerator() call recurses and throws a StackOverflowException. It's safe to use return foo.Cast<T>.GetEnumerator(); when foo is definitely a different object (which doesn't delegate back to this one) but otherwise, you're probably best off using the code above.

回答2:

IEnumerable<T> already derives from IEnumerable so there's no need to do any conversion. You can simply cast to it...well actually it's implicit no cast necessary.

IEnumerable<T> enumerable = GetGenericFromSomewhere();
IEnumerable sadOldEnumerable = enumerable;
return sadOldEnumerable.GetEnumerator();

Going the other way round isn't much more difficult with LINQ:

var fancyEnumerable = list.OfType<GenericObject>();
return fancyEnumerable.GetEnumerator();

回答3:

You can use OfType<T> and Cast<T>.

public static IEnumerable Digits()
{
    return new[]{1, 15, 68, 1235, 12390, 1239};
}

var enumerable = Digits().OfType<int>();
foreach (var item in enumerable)
    // var is here an int. Without the OfType<int(), it would be an object
    Console.WriteLine(i);

To get an IEnumerator<T> instead of an IEnumerable<T> you can just make a call to GetEnumerator()

var enumerator = Digits().OfType<int>().GetEnumerator();

回答4:

This works for me.

IEnumerator<DataColumn> columnEnumerator = dt.Columns.Cast<DataColumn>().GetEnumerator();

回答5:

I was running into the same Stack Overflow problem mentioned is some of the comments. In my case it was due to the fact that the GetEnumerator call needed to be to the base.GetEnumerator otherwise you loop within your own GetEnumerator redefinition.

This is the code that was Throwing the Stack Overflow. The use of the foreach statement call the same GetEnumerator function I'm trying to overload:

public new IEnumerator<T> GetEnumerator()
{
    foreach (T type in this)
    {
        yield return type;
    }
}

I've ended up with a simplified version of the original post as you don't need to use a List holder.

public class ElementCollection<T> : ConfigurationElementCollection, IList<T>
    ...
    public new IEnumerator<T> GetEnumerator()
    {
        var baseEnum = base.GetEnumerator();
        while (baseEnum.MoveNext())
        {
            yield return baseEnum.Current as T;
        }
    }
    ...
}