Linq- Query 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 behavior 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

Deferred Execution Example

// 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 list

            listStudents.Add(new Student { StudentID = 104, Name = "Tim", TotalMarks = 800 });

       // The above query is actually executed when we iterate through 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);

            }

Immediate Execution Example

// 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);

            }

Immediate Execution Example 2

// 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);

        }

Posted in: LINQ