Deferred Execution

Query

public void Linq99()
{
    // Sequence operators form first-class queries that
    // are not executed until you enumerate over them.
  
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
  
    int i = 0;
    var q =
        from n in numbers
        select ++i;
  
    // Note, the local variable 'i' is not incremented
    // until each element is evaluated (as a side-effect):
    foreach (var v in q)
    {
        Console.WriteLine("v = {0}, i = {1}", v, i);
    }
}

Lambda Expression

public void Linq99()
{
 
    // Queries are not executed until you enumerate over them.
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
 
    int i = 0;
    var simpleQuery = numbers.Select(num => ++i);
 
    // The local variable 'i' is not incremented
    // until the query is executed in the foreach loop.
    Console.WriteLine("The current value of i is {0}", i); //i is still zero
 
    foreach (var item in simpleQuery)
    {
        Console.WriteLine("v = {0}, i = {1}", item, i); // now i is incremented         
    }
}

Output

v = 1, i = 1
v = 2, i = 2
v = 3, i = 3
v = 4, i = 4
v = 5, i = 5
v = 6, i = 6
v = 7, i = 7
v = 8, i = 8
v = 9, i = 9
v = 10, i = 10