IEnumerable vs IQueryable

Many  developers gets confused between IEnumerable and IQueryable. When it comes to writing code, both looks very similar. However there are many difference between them which needs to be taken care of while writing code. Both have some intended usability scenarios for which they are made.

The first important point to remember is “IQueryable” interface inherits from “IEnumerable”, so whatever “IEnumerable” can do, “IQueryable” can also do.

There are many differences but let us discuss about the one big difference which makes the biggest difference. “IQueryable” interface is useful when your collection is loaded using LINQ or Entity framework and you want to apply filter on the collection.

Consider the below simple code which uses “IEnumerable” with entity framework. It’s using a “where” filter to get records whose “EmpId” is “2”.

IEnumerable<Employee> emp = ent.Employees;

IEnumerable<Employee> temp = emp.Where(x => x.Empid == 2).ToList<Employee>();

This where filter is executed on the client side where the “IEnumerable” code is. In other words, all the data is fetched from the database and then at the client it scans and gets the record with “EmpId” is “2”.

But now see the below code we have changed “IEnumerable” to “IQueryable”.

IQueryable<Employee> emp = ent.Employees;

IEnumerable<Employee> temp = emp.Where(x => x.Empid == 2).ToList<Employee>();

In this case, the filter is applied on the database using the “SQL” query. So the client sends a request and on the server side, a select query is fired on the database and only necessary data is returned.

So the difference between “IQueryable” and “IEnumerable” is about where the filter logic is executed. One executes on the client side and the other executes on the database.

So if you are working with only in-memory data collection “IEnumerable” is a good choice but if you want to query data collection which is connected with database, “IQueryable” is a better choice as it reduces network traffic and uses the power of SQL language.

IEnumerable vs IQueryable vs IEnumerator

Below lists the differences between them based on their properties :

	IEnumerable	IQueryable
Namespace	System.Collections Namespace	System.Linq Namespace
Derives from	No base interface	Derives from IEnumerable
Deferred Execution	Supported	Supported
Lazy Loading	Not Supported	Supported
How does it work	While querying data from database, IEnumerable execute select query on server side, load data in-memory on client side and then filter data. Hence does more work and becomes slow.	While querying data from database, IQueryable execute select query on server side with all filters. Hence does less work and becomes fast.
Suitable for	LINQ to Object and LINQ to XML queries.	LINQ to SQL queries.
Custom Query	Doesn’t supports.	Supports using CreateQuery and Execute methods.
Extension mehtod parameter	Extension methods supported in IEnumerable takes functional objects.	Extension methods supported in IEnumerable takes expression objects i.e. expression tree.
When to use	when querying data from in-memory collections like List, Array etc.	when querying data from out-memory (like remote database, service) collections.
Best Uses	In-memory traversal	Paging