Someone can tell me what's the difference between List, Collection and Enumerable?

Question

While I do a program, sometimes I've got this doubt. I have been using List<T> but I haven't used the others.

I'd like to know when each one is better to use, and under what circumstances.

Answer 1

I'm sure you can read the documentation by yourself. I'll give a short summary here:

1. IEnumerable is an interface that exposes an enumerator over a collection. Implement this interface if want to be able to support iteration, for example inside a foreach loop.
2. Collection is an implementation of IEnumerable (therefore you can iterate over it) that is generally further extended by user-defined classes that want to have collection-like behavior (ie use Add , Remove , Contains , etc.). It can also be used "as-is".
3. List is also an implementation of IEnumerable (therefore you can iterate over it) that is generally used "as-is" as a container for objects of a certain type. Uses a dynamically adjusting array in the background and is the generic equivalent of ArrayList .

Note that while IEnumerable can be both generic and non-generic, Collection and List can only be used as generic classes.

Answer 2

MSDN可以：

• List<T>
• IList<T>
• ICollection<T>
• IEnumerable<T>

Answer 3

List is instantiable type that holds items in a linear fashion. (Edit: I'm told it's implemented with a dynamic array, rather than with a Linked List, which was just my guess).

ICollection / CollectionBase and IEnumerable aren't directly instantiable types.

CollectionBase / ICollection is a base class/interface that is inherited/implemented by any class that considers itself a collection and holds multiple items. CollectionBase / ICollection also allows you to pass around collections without knowing their implementation.

IEnumerable is also a interface that provides methods for iterating over the collection. IEnumerable basically lets you use foreach loops on the collection. You can use the interface methods directly to get some C++ style iterators going, but the foreach loop is less error prone.

Answer 4

If you look at the definitions of the three (see below) you will notice that List implements Enumerable and ICollection and ICollection implements IEnumerable . More clearly:

List is CLASS which can store a variable number of items of the same type plus the functionality specified by the other two interfaces.

ICollection is an interface which specifies a generic way to manipulate collections. NOTE: this is an interface and thus is NOT capable of being instantiated.

IEnumerable is an interface which specifies a means of iterating over a collection. NOTE: this is an interface and thus is NOT capable of being instantiated.

List

Represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort, and manipulate lists.

public class List<T> : IList<T>, ICollection<T>, 
IEnumerable<T>, IList, ICollection, IEnumerable

ICollection

Defines methods to manipulate generic collections.

public interface ICollection<T> : IEnumerable<T>, 
IEnumerable

IEnumerable

Exposes the enumerator, which supports a simple iteration over a collection of a specified type.

public interface IEnumerable<out T> : IEnumerable