Question

Can anyone explain IEnumerable and IEnumerator to me?

For example, when to use it over foreach? what’s the difference between IEnumerable and IEnumerator? Why do we need to use it?

Answer 1

for example, when to use it over foreach?

You don’t use IEnumerable "over" foreach. Implementing IEnumerable makes using foreach possible.

When you write code like:

foreach (Foo bar in baz) {    ... } 

it’s functionally equivalent to writing:

IEnumerator bat = baz.GetEnumerator(); while (bat.MoveNext()) {    Foo bar = (Foo)bat.Current;    ... } 

By "functionally equivalent," I mean that’s actually what the compiler turns the code into. You can’t use foreach on baz in this example unless baz implements IEnumerable.

IEnumerable means that baz implements the method

IEnumerator GetEnumerator() 

The IEnumerator object that this method returns must implement the methods

bool MoveNext() 

and

Object Current() 

The first method advances to the next object in the IEnumerable object that created the enumerator, returning false if it’s done, and the second returns the current object.

Anything in .NET that you can iterate over implements IEnumerable. If you’re building your own class, and it doesn’t already inherit from a class that implements IEnumerable, you can make your class usable in foreach statements by implementing IEnumerable (and by creating an enumerator class that its new GetEnumerator method will return).