obtain generic enumerator from an array

问题

In C#, how does one obtain a generic enumerator from a given array?

In the code below, MyArray is an array of MyType objects. I'd like to obtain MyIEnumerator in the fashion shown, but it seems that I obtain an empty enumerator (although I've confirmed that MyArray.Length > 0).

MyType [ ]  MyArray  =  ... ;
IEnumerator<MyType>  MyIEnumerator
  =  ( MyArray.GetEnumerator() as IEnumerator<MyType> ) ;

回答1:

Works on 2.0+:

((IEnumerable<MyType>)myArray).GetEnumerator()

Works on 3.5+ (fancy LINQy, a bit less efficient):

myArray.Cast<MyType>().GetEnumerator()   // returns IEnumerator<MyType>

回答2:

You can decide for yourself whether casting is ugly enough to warrant an extraneous library call:

int[] arr;
IEnumerator<int> Get1()
{
    return ((IEnumerable<int>)arr).GetEnumerator();  // <-- 1 non-local call

    // ldarg.0 
    // ldfld int32[] foo::arr
    // castclass System.Collections.Generic.IEnumerable`1<int32>
    // callvirt instance class System.Collections.Generic.IEnumerator`1<!0> System.Collections.Generic.IEnumerable`1<int32>::GetEnumerator()
}

IEnumerator<int> Get2()
{
    return arr.AsEnumerable().GetEnumerator();   // <-- 2 non-local calls

    // ldarg.0 
    // ldfld int32[] foo::arr
    // call class System.Collections.Generic.IEnumerable`1<!!0> System.Linq.Enumerable::AsEnumerable<int32>(class System.Collections.Generic.IEnumerable`1<!!0>)
    // callvirt instance class System.Collections.Generic.IEnumerator`1<!0> System.Collections.Generic.IEnumerable`1<int32>::GetEnumerator()
}

And for completeness, one should also note that the following is not correct--and will crash at runtime--because T[] chooses the non-generic IEnumerable interface for its default (i.e. non-explicit) implementation of GetEnumerator().

IEnumerator<int> NoGet()                    // error - do not use
{
    return (IEnumerator<int>)arr.GetEnumerator();

    // ldarg.0 
    // ldfld int32[] foo::arr
    // callvirt instance class System.Collections.IEnumerator System.Array::GetEnumerator()
    // castclass System.Collections.Generic.IEnumerator`1<int32>
}

The mystery is, why doesn't SZGenericArrayEnumerator<T> inherit from SZArrayEnumerator--an internal class which is currently marked 'sealed'--since this would allow the (covariant) generic enumerator to be returned by default?

回答3:

Since I don't like casting, a little update:

your_array.AsEnumerable().GetEnumerator();

回答4:

To Make it as clean as possible I like to let the compiler do all of the work. There are no casts (so its actually type-safe). No third party Libraries (System.Linq) are used (No runtime overhead).

    public static IEnumerable<T> GetEnumerable<T>(this T[] arr)
    {
        return arr;
    }

// And to use the code:

    String[] arr = new String[0];
    arr.GetEnumerable().GetEnumerator()

This takes advantage of some compiler magic that keeps everything clean.

The other point to note is that my answer is the only answer that will do compile-time checking.

For any of the other solutions if the type of "arr" changes, then calling code will compile, and fail at runtime, resulting in a runtime bug.

My answer will cause the code to not compile and therefore I have less chance of shipping a bug in my code, as it would signal to me that I am using the wrong type.

回答5:

YourArray.OfType().GetEnumerator();

may perform a little better, since it only has to check the type, and not cast.

回答6:

    MyType[] arr = { new MyType(), new MyType(), new MyType() };

    IEnumerable<MyType> enumerable = arr;

    IEnumerator<MyType> en = enumerable.GetEnumerator();

    foreach (MyType item in enumerable)
    {

    }

回答7:

What you can do, of course, is just implement your own generic enumerator for arrays.

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

namespace SomeNamespace
{
    public class ArrayEnumerator<T> : IEnumerator<T>
    {
        public ArrayEnumerator(T[] arr)
        {
            collection = arr;
            length = arr.Length;
        }
        private readonly T[] collection;
        private int index = -1;
        private readonly int length;

        public T Current { get { return collection[index]; } }

        object IEnumerator.Current { get { return Current; } }

        public bool MoveNext() { index++; return index < length; }

        public void Reset() { index = -1; }

        public void Dispose() {/* Nothing to dispose. */}
    }
}

This is more or less equal to the .NET implemenation of SZGenericArrayEnumerator<T> as mentioned by Glenn Slayden. You should of course only do this, is cases where this is worth the effort. In most cases it is not.

来源：https://stackoverflow.com/questions/1272673/obtain-generic-enumerator-from-an-array

标签

arrays

generics

ienumerator