Difference Between IQueryable and IEnumerable

In LINQ to query data from database and collections, we use IEnumerable and IQueryable for data manipulation. IEnumerable is inherited by IQueryable, hence it has all the features of it and except this, it has its own features. Both have their own importance to query data and data manipulation. Let’s see both the features and take the advantage of both the features to boost your LINQ Query performance.

IEnumerable
IEnumerable exists in System.Collections Namespace.
IEnumerable can move forward only over a collection, it can’t move backward and between the items.
IEnumerable is best to query data from in-memory collections like List, Array etc.
While query data from database, IEnumerable execute select query on server side, load data in-memory on client side and then filter data.
IEnumerable is suitable for LINQ to Object and LINQ to XML queries.
IEnumerable supports deferred execution.
IEnumerable doesn’t supports custom query.
IEnumerable doesn’t support lazy loading. Hence not suitable for paging like scenarios.
Extension methods supports by IEnumerable takes functional objects.

Example
MyDataContext dc = new MyDataContext ();
IEnumerable<Employee> list = dc.Employees.Where(p => p.Name.StartsWith("S"));
list = list.Take<Employee>(10);


IQueryable
IQueryable exists in System.Linq Namespace.
IQueryable can move forward only over a collection, it can’t move backward and between the items.
IQueryable is best to query data from out-memory (like remote database, service) collections.
While query data from database, IQueryable execute select query on server side with all filters.
IQueryable is suitable for LINQ to SQL queries.
IQueryable supports deferred execution.
IQueryable supports custom query using CreateQuery and Execute methods.
IQueryable support lazy loading. Hence it is suitable for paging like scenarios.
Extension methods supports by IEnumerable takes expression objects means expression tree

Example
MyDataContext dc = new MyDataContext ();
IQueryable<Employee> list = dc.Employees.Where(p => p.Name.StartsWith("S"));
list = list.Take<Employee>(10);

*Notice that in this query "top 10" is existing since IQueryable executes query in SQL server with all filters.

1. IEnumerable<T> represents a forward-only cursor of T, .NET 3.5 added extension methods that included the LINQ standard query operators like Where and First, with any operators that require predicates or anonymous functions taking Func<T>.
2. IQueryable<T> implements the same LINQ standard query operators, but accepts Expression<Func<T>> for predicates and anonymous functions. Expression<T> is a compiled expression tree, a broken-up version of the method ("half-compiled" if you will) that can be parsed by the queryable's provider and used accordingly.