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Sandeep Kumar
Sandeep Kumar

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

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


  • IEnumerable exists in System.Collections Namespace.
  • IEnumerable is the most generic item of all and a core interface that is used to iterate over a collection of the specified type.
  • IEnumerable provides an Enumerator for accessing the collection.
  • IEnumerable is forward only collection like LinkedList. It doesn’t move between items or backward, i.e. one can't get at the fifth item without passing the first four items.
  • It is a read-only collection and it doesn't support adding or removing items.
  • IEnumerable is best to query data from in-memory collections like List, Array, etc.
  • It mainly implements two methods:
    • MoveNext: This method tells whether there are more records to move on or not.
    • 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 exists in System.Linq namespace.
  • IQueryable extends the IEnumerable interface.
  • IQueryable allows deferred query execution i.e. query generated through IQueryable isn't executed until you iterate, enumerate, or add any method such as ToList(), First(), Single(), etc.
  • IQueryable enables a variety of interesting deferred execution scenarios such as paging and composition-based queries.
  • IQueryable best suits remote data sources, 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 exists in System.Collections Namespace.
  • ICollection implements the IEnumerable interface.
  • ICollection is the base of Collection<T>, IList<T>, and IDictionary objects.
  • It is considered the most basic type for collections and is used to manipulate generic collections.
  • As it implements the IEnumerable interface so it also implements methods MoveNext and GetCurrent, so with this interface you can iterate through the collection.
  • ICollection also has its own methods (apart from IEnumerable methods) like:
    • Add: It adds a record at the end of the collection
    • Remove: It removes the specified item from the collection
    • Contains: It’s a boolean type method that tells whether the 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 the virtual keyword.
  • ICollection doesn’t support indexing as IList does.


  • IList exists in System.Collections Namespace.
  • IList implementations fall into three categories:
    • Read-only: A read-only IList cannot be modified.
    • Fixed-size: A fixed-size IList does not allow the addition or removal of elements, but it allows the modification of existing elements.
    • 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 also implements the methods of both the interfaces.
  • IList also has its own methods like:
    • Insert: It inserts the given item at the specified Index.
    • RemoveAt: It removes the item from the specified Index.
    • IndexOf: It retrieves the item from the 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 the comparison is done inside the memory.

In this post, we saw some of the basic differences between IEnumerable, IQueryable, ICollection, and IList interfaces.

I hope this article will help you to choose a specific interface based on your demand. You can share your feedback, question, or comments about this article.

Originally published at on
March 14, 2014.

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