In this article we will discuss the concept of deferred execution. LINQ queries have two different behaviors of execution
1. Deferred execution
2. Immediate execution
LINQ operators can be broadly classified into 2 categories based on the behaviour of query execution
1. Deferred or Lazy Operators - These query operators use deferred execution.
Examples - select, where, Take, Skip etc
2. Immediate or Greedy Operators - These query operators use immediate execution.
Examples - count, average, min, max, ToList etc
Let us understand these 2 behaviors with examples.
LINQ Deferred Execution Example
Output:
LINQ Immediate Execution Example 1
Output:
LINQ Immediate Execution Example 2
Output:
1. Deferred execution
2. Immediate execution
LINQ operators can be broadly classified into 2 categories based on the behaviour of query execution
1. Deferred or Lazy Operators - These query operators use deferred execution.
Examples - select, where, Take, Skip etc
2. Immediate or Greedy Operators - These query operators use immediate execution.
Examples - count, average, min, max, ToList etc
Let us understand these 2 behaviors with examples.
LINQ Deferred Execution Example
using System;
using System.Collections.Generic;
using System.Linq;
namespace Demo
{
public class Student
{
public int StudentID
{ get; set;
}
public string Name
{ get; set;
}
public int TotalMarks
{ get; set;
}
}
class Program
{
public static void Main()
{
List<Student>
listStudents = new List<Student>
{
new Student { StudentID= 101, Name
= "Tom", TotalMarks
= 800 },
new Student { StudentID= 102, Name
= "Mary", TotalMarks
= 900 },
new Student { StudentID= 103, Name
= "Pam", TotalMarks
= 800 }
};
// LINQ Query is only defined here and
is not executed at this point
// If the query is executed at this
point, the result should not display Tim
IEnumerable<Student>
result = from student
in listStudents
where student.TotalMarks == 800
select student;
// Add a new student object with
TotalMarks = 800 to the source
listStudents.Add(new
Student { StudentID = 104,
Name = "Tim",
TotalMarks = 800 });
// The above query is actually
executed when we iterate thru the sequence
// using the foreach loop. This is
proved as Tim is also included in the result
foreach (Student s in
result)
{
Console.WriteLine(s.StudentID + "\t" + s.Name + "\t" + s.TotalMarks);
}
}
}
}
Output:
LINQ Immediate Execution Example 1
using System;
using System.Collections.Generic;
using System.Linq;
namespace Demo
{
public class Student
{
public int StudentID
{ get; set;
}
public string Name
{ get; set;
}
public int TotalMarks
{ get; set;
}
}
class Program
{
public static void Main()
{
List<Student>
listStudents = new List<Student>
{
new Student { StudentID= 101, Name
= "Tom", TotalMarks
= 800 },
new Student { StudentID= 102, Name
= "Mary", TotalMarks
= 900 },
new Student { StudentID= 103, Name
= "Pam", TotalMarks
= 800 }
};
// Since we are using ToList() which
is a greedy operator
// the LINQ Query is executed
immediately at this point
IEnumerable<Student>
result = (from student
in listStudents
where student.TotalMarks == 800
select student).ToList();
// Adding a new student object with
TotalMarks = 800 to the source
// will have no effect on the result
as the query is already executed
listStudents.Add(new
Student { StudentID = 104,
Name = "Tim",
TotalMarks = 800 });
// The above query is executed at the
point where it is defined.
// This is proved as Tim is not
included in the result
foreach (Student s in
result)
{
Console.WriteLine(s.StudentID + "\t" + s.Name + "\t" + s.TotalMarks);
}
}
}
}
Output:
LINQ Immediate Execution Example 2
using System;
using System.Collections.Generic;
using System.Linq;
namespace Demo
{
public class Student
{
public int StudentID
{ get; set;
}
public string Name
{ get; set;
}
public int TotalMarks
{ get; set;
}
}
class Program
{
public static void Main()
{
List<Student>
listStudents = new List<Student>
{
new Student { StudentID= 101, Name
= "Tom", TotalMarks
= 800 },
new Student { StudentID= 102, Name
= "Mary", TotalMarks
= 900 },
new Student { StudentID= 103, Name
= "Pam", TotalMarks
= 800 }
};
// Since we are using Count()
operator, the LINQ Query is executed at this point
int result = (from
student in listStudents
where student.TotalMarks == 800
select student).Count();
// Adding a new student object with
TotalMarks = 800 to the source
// will have no effect on the result
as the query is already executed
listStudents.Add(new
Student { StudentID = 104,
Name = "Tim",
TotalMarks = 800 });
// The above query is executed at the
point where it is defined.
// This is proved as Tim is not
included in the count
Console.WriteLine("Students with Total Marks = 800 : " +
result);
}
}
}
Output:
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