IEnumerable VS IQueryable

IEnumerable

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

IEnumerable Example

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

Generated SQL statements of above query will be :

 SELECT [t0].[EmpID], [t0].[EmpName], [t0].[Salary] FROM [Employee] AS [t0]
WHERE [t0].[EmpName] LIKE @p0

Notice that in this query "top 10" is missing since IEnumerable filters records on client side

IQueryable

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

IQueryable Example

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

Generated SQL statements of above query will be :

 SELECT TOP 10 [t0].[EmpID], [t0].[EmpName], [t0].[Salary] FROM [Employee] AS [t0]
WHERE [t0].[EmpName] LIKE @p0

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