IQueryable vs. IEnumerable in .NET

First of all, IQueryable<T> extends the IEnumerable<T> interface, so anything you can do with a “plain” IEnumerable<T>, you can also do with an IQueryable<T>.

What IQueryable<T> has that IEnumerable<T> doesn’t are two properties in particular — one that points to a query provider (e.g., a LINQ to SQL provider) and another one pointing to a query expression representing the IQueryable<T> object as a runtime-traversable abstract syntax tree that can be understood by the given query provider (for the most part, you can’t give a LINQ to SQL expression to a LINQ to Entities provider without an exception being thrown).

• IEnumerable<T> is great for working with sequences that are iterated in-memory, but
• IQueryable<T> allows for out-of-memory things like a remote data source, such as a database or web service.

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 the one big difference which makes the biggest difference. IEnumerable interface is useful when your collection is loaded using LINQ or Entity framework and you want to apply a 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.

EmpEntities ent = new EmpEntities();
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 its scans and gets the record with EmpId is 2.

But now see the below code we have changed IEnumerable to IQueryable. It creates a SQL Query at the server side and only necessary data is sent to the client side.

EmpEntities ent = new EmpEntities();
IQueryable<Employee> emp = ent.Employees;
IQueryable<Employee> temp = emp.Where(x => x.Empid == 2).ToList<Employee>();

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 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: IEnumerable is best suitable for working with in-memory collection (or local queries). IEnumerable doesn’t move between items, it is forward only collection.

IQueryable: IQueryable best suits for remote data source, like a database or web service (or remote queries). IQueryable is a very powerful feature that enables a variety of interesting deferred execution scenarios (like paging and composition based queries).