Understanding Var and IEnumerable with LINQ

In this article I would like to share my opinion on Var and IEnumerable with LINQ. IEnumerable is an interface that can move forward only over a collection, it can’t move backward and between the items. Var is used to declare implicitly typed local variable means it tells the compiler to figure out the type of the variable at compilation time. A var variable must be initialized at the time of declaration. Both have its own importance to query data and data manipulation.

Var Type with LINQ

Since Var is anonymous types, hence use it whenever you don't know the type of output or it is anonymous. In LINQ, suppose you are joining two tables and retrieving data from both the tables then the result will be an Anonymous type.

 var q =(from e in tblEmployee 
 join d in tblDept on e.DeptID equals d.DeptID 
 select new 
 { 
 e.EmpID,
 e.FirstName,
 d.DeptName,
 }); 

In above query, result is coming from both the tables so use Var type.

 var q =(from e in tblEmployee where e.City=="Delhi" select new { 
 e.EmpID,
 FullName=e.FirstName+" "+e.LastName, 
 e.Salary 
}); 

In above query, result is coming only from single table but we are combining the employee's FirstName and LastName to new type FullName that is annonymous type so use Var type. Hence use Var type when you want to make a "custom" type on the fly.
More over Var acts like as IQueryable since it execute select query on server side with all filters. Refer below examples for explanation.

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

Var Example

 MyDataContext dc = new MyDataContext ();
var 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 var is a IQueryable type that executes query in SQL server with all filters.

IEnumerable Type with LINQ

IEnumerable is a forward only collection and is useful when we already know the type of query result. In below query the result will be a list of employee that can be mapped (type cast) to employee table.

 IEnumerable<tblEmployee> lst =(from e in tblEmployee 
where e.City=="Delhi" 
select e); 

Note

1. In LINQ query, use Var type when you want to make a "custom" type on the fly.
2. In LINQ query, use IEnumerable when you already know the type of query result.
3. In LINQ query, Var is also good for remote collection since it behaves like IQuerable.
4. IEnumerable is good for in-memory collection.