C# IEnumerable and IQueryable

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. IEnumerable 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 Wherefilter 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 interface can be found under System.Collections Name space.

                       In C#, there is a non-generic IEnumerable interface and generic type safe IEnumerable<T> interface as below.




interface IEnumerable

{

    IEnumerator GetEnumerator();

}




GetEnumerator function always return an instance of an object of a class which implements the IEnumerator interface.




IEnumerator is the base interface for all non-generic enumerators.

For the generic version of this interface IEnumerable<T>.

Manipulating with Enumerable

            The foreach statement of the C# language hides the complexity of the enumerators. Therefore, using foreach is recommended instead of directly manipulating the enumerator.