Why do arrays in .net only implement IEnumerable and not IEnumerable<T>?

Question

I was implementing my own ArrayList class and was left surprised when I realised that

public System.Collections.Generic.IEnumerator<T> GetEnumerator() {
    return _array.GetEnumerator();
}

didn't work. What is the reason arrays don't implement IEnumerator in .NET?

Is there any work-around?

Thanks

Why would you implement your own ArrayList class when there are already over a dozen existing collection classes? — Aaronaught
– Aaronaught, Commented May 5, 2010 at 13:49
We don't ask these kind of questions to annoy you, but to find out what it is you are trying to do. This helps us supplying you with a better answer. — Steven
– Steven, Commented May 5, 2010 at 14:08
Yes, I am implementing an ArrayList because although all languages have some sort of ArrayList provided in its framework, I assumed .NET didn't have one. Thanks guys for clearing it up. — devoured elysium
– devoured elysium, Commented May 5, 2010 at 14:11
These kind of questions are annoying though, because they seek to divert the conversation and the answers away from what is being asked. I always find it's better to answer what was asked no matter how silly it "seems". — Owl
– Owl, Commented Sep 19, 2017 at 16:43

Richard · Accepted Answer · 2018-07-25 11:14:13Z

71

Arrays do implement IEnumerable<T>, but it is done as part of the special knowledge the CLI has for arrays. This works as if it were an explicit implementation (but isn't: it is done at runtime). Many tools will not show this implementation, this is described in the Remarks section of the Array class overview.

You could add a cast:

return ((IEnumerable<T>)_array).GetEnumerator();

Note, older MSDN (pre learn.microsoft.com) coverage of this changed a few times with different .NET versions, check for the remarks section.

edited Jul 25, 2018 at 11:14

answered May 5, 2010 at 13:51

Richard

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3 Comments

Jeppe Stig Nielsen Over a year ago

It works just as if the "class" T[] implemented IEnumerable<T> by an explicit interface implementation. If you simply say _array.GetEnumerator() you get instead the public method which has the wrong return type.

Richard Over a year ago

@RayL See the .NET 2.0 version of the documentation :-) updating.

Wouter Over a year ago

Just a heads up. From the referenced msdn.microsoft.com/en-us/library/system.array.aspx#Remarks: it implements IList<T>, ICollection<T> and IEnumerable<T> generic interfaces.

alcohol is evil · Accepted Answer · 2016-05-07 08:05:57Z

-2

You can use generic method IEnumerable<T> OfType<T>() from System.Linq namespace, which extends IEnumerable interface. It will filter out all elements which type is different than T and return IEnumerable<T> collection. If you use (IEnumerable<T>)_array conversion operator, it might not be safe, because System.Array (and other nongeneric types) stores items of type System.Object.

answered May 7, 2016 at 8:05

alcohol is evil

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1 Comment

jmoreno Over a year ago

Only arrays of Objects (i.e. Object[] arr) stores the item as an object. Other than those, array's are type safe. That makes the OfType and Cast methods redundant.

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