Deferred Execution and Immediate Execution in LINQ

Here the result variable does not contain the actual result. To get the result, we may iterate through the result (query) variable in loop as many time as we want. We do deferred execution mainly when multiple values is being returned from the query.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication3
{
 class Program
 {
 static void Main(string[] args)
 {
 DataClasses1DataContext context = new DataClasses1DataContext();
 var result = from r in context.Persons select r;
 foreach (var a in result)
 {
 Console.WriteLine(a.FirstName + "" + a.LastName);
 }
 Console.ReadKey(true);

 }
 }
}

  

Immediate Execution happens when LINQ query returns a single value.

Above query is returning a single value so it can be executed immediately.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication3
{
 class Program
 {
 static void Main(string[] args)
 {
 DataClasses1DataContext context = new DataClasses1DataContext();
 var numberOfRecords = (from r in context.Persons select r).Count();
 Console.WriteLine(numberOfRecords);
 Console.ReadKey(true);

}
 }
}

If we want to make a query returning multiple values or sequence of values as immediate then we need to explicitly convert the result in ToList().

So above query is returning multiple values and would be executed immediately.

Example

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;namespace ConsoleApplication3
{
 class Program
 {
 static void Main(string[] args)
 {
 DataClasses1DataContext context = new DataClasses1DataContext();
 var result1 = (from r in context.Persons select r).ToList();
 foreach (var a in result1)
 {
 Console.WriteLine(a.FirstName + "" + a.LastName);
 }
 Console.ReadKey(true);
 }
 }
}