Understand about IEnumerable vs. IQueryable vs. ICollection vs. IList

IEnumerable:

• 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:

• 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:

• 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:

• 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.