understanding IEnumerable<T> in C#

问题

I am trying to understand the IEnumerable interface in C#. As it is stated in MSDN
IEnumerable exposes an enumerator, which supports a simple iteration over a non-generic collection. This is quite simple. When we have a collection that implements this interface then, we get an enumerator, which implements the IEnumerator interface, (http://msdn.microsoft.com/en-us/library/system.collections.ienumerable.getenumerator(v=vs.110).aspx), and the using this enumerator we can iterate through all the elements of the collection in a way like the below:

foreach(var item in items)
{
    // code goes here
}

In the above snippet of code, items is a collection that implements the IEnumerable interface. This is possible because the enumerator has a property called Current, which has the current element of the collection and a method MoveNext, which is used by the enumerator, to move to the next element. Last but not least, it has a method called Reset, which sets the enumerator to its initial position, which is before the first element in the collection, as it is stated in MSDN.

Let we have the following snippet of code:

int[] array = new int[10] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
IEnumerable<int> numbers = array;
foreach (var number in numbers)
{
    Console.WriteLine(number);
}

In the above snippet of code we initialize an array of integers and then we define a type of IEnumerable that points to the array. Since numbers implements the IEnumerable, we can iterate through it's elements using the above foreach.

I think that the compiler behind the scenes emits some code, when we define the numbers and then try to iterate through it's elements. Actually I found that Array implements IEnumerable<T>.

My question is every time we define an array, since an array is an object that implements IEnumerable and IEnumerator, the corresponding methods are implemented by the compiler?

Thanks in advance for any help !

回答1:

Your question,

My question is every time we define an array, since an array is an object that implements IEnumerable and IEnumerator, the corresponding methods are implemented by the compiler?

As per the MSDN documentation under Remarks section it is clearly mentioned,

Starting with the .NET Framework 2.0, the Array class implements the System.Collections.Generic.IList<T>, System.Collections.Generic.ICollection<T>, and System.Collections.Generic.IEnumerable<T> generic interfaces. The implementations are provided to arrays at run time, and therefore are not visible to the documentation build tools. As a result, the generic interfaces do not appear in the declaration syntax for the Array class, and there are no reference topics for interface members that are accessible only by casting an array to the generic interface type (explicit interface implementations). The key thing to be aware of when you cast an array to one of these interfaces is that members which add, insert, or remove elements throw NotSupportedException.

Yes, the implementation are provided to arrays at run time by the compiler.

EDIT Your question in comment

That happens with all the non-generic collections?

After reading the posts,

• Why do C# Multidimensional arrays not implement IEnumerable<T>?
• Why doesn't Array class expose its indexer directly? especially the answer by dasblinkenlight

and after looking at all the Collections, I found that ArrayList, BitArray etc all are collections but do not implement any of generic interfaces.

Correct me if I am wrong at this I don't think so that implementation of all the Collection will be provided at run time.

来源：https://stackoverflow.com/questions/20543151/understanding-ienumerablet-in-c-sharp