Deferred Vs Immediate Query Execution in LINQ

In Deferred Execution, the query is not executed when declared. It is executed when the query object is iterated over a loop.

In Immediate Execution, the query is executed when it is declared.

Deferred Execution:

Now we will see both the executions using an example. For this I created a Visual Studio Solution. Here in the solution I added a new Class Employee as in the following screenshot:


Figure 1: Class Employee

Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace DeferredVsImmediate_Query
{
    public class Employee
    {
        public int Emp_ID { get; set; }
        public string Name { get; set; }
        public string Email { get; set; }
        public string Country { get; set; }
    }
}

Add a New Web Page Default.aspx and add the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace DeferredVsImmediate_Query
{
    public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            var Emp_List = new List<Employee>(
                new Employee[]
                {
                    new Employee{Emp_ID=1, Name="Shambhu Sharma", Email="[email protected]", Country="India"},
                    new Employee{Emp_ID=2, Name="Manu Khanna", Email="[email protected]", Country="India"},
                    new Employee{Emp_ID=3, Name="Abhishek Nigam", Email="[email protected]", Country="USA"},
                    new Employee{Emp_ID=4, Name="Yogesh Gupta", Email="[email protected]", Country="USA"},
                    new Employee{Emp_ID=5, Name="Shweta Kashyap", Email="[email protected]", Country="India"},
                    new Employee{Emp_ID=6, Name="Shraddha Gaur", Email="[email protected]", Country="India"},
                    new Employee{Emp_ID=7, Name="Akhilesh Atwal", Email="[email protected]", Country="India"},
                    new Employee{Emp_ID=6, Name="Mayank Dhulekar", Email="[email protected]", Country="USA"},
                    new Employee{Emp_ID=7, Name="Saurabh Mehrotra", Email="[email protected]", Country="USA"},
                    new Employee{Emp_ID=7, Name="Mehak Jain", Email="[email protected]", Country="India"},
                });

            var Result = from a in Emp_List
                         where a.Country.Equals("India")
                         select new { a.Name };

            foreach (var EMP in Result)
                Response.Write(EMP.Name + "</br>");
        }
    }
}

Now run your application:


Figure 2: Output

Now you can see the following when your query executes:


Figure 3: Query Execution

Now add a new record after your query to see the deferred execution:


Figure 4: Deferred Execution 

Now run your app:


Figure 5: Output

Immediate Execution: We can force our query to execute immediately as in the following:


Figure 6: Immediate Execution

Now run your application:


Figure 7: Output

Now let us see both the executions:


Figure 8: Both the executions