C#: Understand about IEnumerable vs. IQueryable vs. ICollection vs. IList

In this article we’ll understand about the interfaces (IEnumerable, IQueryable, ICollection and IList) available for holding and querying the data.

IEnumerable: 

·         IEnumerable exists in System.Collections Namespace.

·         IEnumerable is most generic item of all and a core interface which is used to iterate over collection of specified type.

·         IEnumerable provides Enumerator for accessing collection.

·         IEnumerable is forward only collection likes LinkedList. It doesn’t move between items or backward, i.e. one can't get at fifth item without passing first four items.

·         It is read-only collection and it doesn't support add or remove items.

·         IEnumerable is best to query data from in-memory collections like List, Array etc.

·         It mainly implements two methods:

o   MoveNext: This method tells whether there are more records to move on or not.

o   GetCurrent: This method returns the current record from the collection.

·         IEnumerable still might use deferred execution and also supports further filtering.

·         IEnumerable does not run query until it is requested by iteration or enumerator.

·         Using IEnumerable we can find out the no of elements in the collection after iterating the collection.

IQueryable: 

·         IQueryable exists in System.Linq namespace.

·         IQueryable extends IEnumerable interface.

·         IQueryable allows deferred query execution i.e. query generated through IQueryable isn't executed until you iterate, enumerate or add .ToList() method.

·         IQueryable enables a variety of interesting deferred execution scenarios such as paging and composition based queries.

·         IQueryable best suits for remote data source, like a DB or web service.

·         IQueryable is particularly used for LINQ queries.

·         IQueryable doesn’t support custom comparer, like any sting method such as ‘string.IsNullOrWhiteSpace’ etc.

·         IQueryable is not a good place to handle errors, so one must take care while writing complex queries.

ICollection: 

·         ICollection exists in System.Collections Namespace.

·         ICollection implements IEnumerable interface.

·         ICollection is base of Collection<T>, IList<T> and IDictionary objects.

·         It is considered the most basic type for collections and used to manipulate generic collections.

·         As it implements IEnumerable interface so it’s also implements methods MoveNext and GetCurrent, so with this interface you can iterate through collection.

·         ICollection also having its own methods (apart from IEnumerable methods) like:

o   Add: It adds record at the end of collection

o   Remove: It removes specified item from collection

o   Contains: It’s a boolean type method which tells whether collection contains the specified item or not.

·         Some collections that limit access to their elements, i.e. Queue or Stack class, directly implement the ICollection interface.

·         ICollection is normally used where we define EF table relationships and we use this in virtual keyword.

·         ICollection doesn’t support indexing like IList does.

IList: 

·         IList exists in System.Collections Namespace.

·         IList implementations fall into three categories:

o   Read-only: A read-only IList cannot be modified.

o   Fixed-size: A fixed-size IList does not allow the addition or removal of elements, but it allows the modification of existing elements.

o   Variable-size: A variable-size IList allows the addition, removal and modification of elements.

·         IList is used where you need to iterate (read), modify and sort, order a collection

·         With IList random element access is allowed i.e. you can access an element in a specific index in a list. For e.g. you can directly access an element at index 10 instead of first iterating through 0-9 elements.

·         Like IEnumerable, IList is also in memory collection and helps you to query data from in-memory collections like List, Array etc.

·         IList implements two interfaces ICollection and IEnumerable. So it’s also implements the methods of the both the interfaces.

·         IList also having its own methods like:

o   Insert: It insert the given item at specified Index.

o   RemoveAt: It removes the item from specified Index.

o   IndexOf: It retrieves the item from specified Index.

·         IList can give you the no of elements in the collection without iterating the collection.

·         IList supports deferred execution, but it doesn't support further filtering.

·         IList supports custom comparer as comparison is done inside the memory.

Summary

In this post I tried to explain some of the basic difference between IEnumerable, IQueryable, ICollection and IList interfaces. I hope this article will help you to choose specific interface based on your demand. You can share your feedback, question or comments about this article.