Part 14 - LINQ query deferred execution

Suggested Videos
Part 11 - Ordering Operators in LINQ - II
Part 12 - Partitioning Operators
Part 13 - Implement paging using skip and take operators



In this video 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

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: