Query Execution

linq99: Deferred Execution

//c#
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); 
    } 
}

Whilst the results is the same the behavior is different, each filter in the pipeline is immediately executed in batches where the result of one filter is passed as the first argument in the next filter, i.e. it's not lazily executed in a chain like LINQ.

linq100: Immediate Execution

//c#
public void Linq100() 
{ 
    // Methods like ToList() cause the query to be 
    // executed immediately, caching the results. 
  
    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) 
        .ToList(); 
  
    // The local variable i has already been fully 
    // incremented before we iterate the results: 
    foreach (var v in q) 
    { 
        Console.WriteLine("v = {0}, i = {1}", v, i); 
    } 
} 

linq101: Query Reuse

//c#
public void Linq101() 
{ 
    // Deferred execution lets us define a query once 
    // and then reuse it later after data changes. 
  
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 
    var lowNumbers = 
        from n in numbers 
        where n <= 3 
        select n; 
  
    Console.WriteLine("First run numbers <= 3:"); 
    foreach (int n in lowNumbers) 
    { 
        Console.WriteLine(n); 
    } 
  
    for (int i = 0; i < 10; i++) 
    { 
        numbers[i] = -numbers[i]; 
    } 
  
    // During this second run, the same query object, 
    // lowNumbers, will be iterating over the new state 
    // of numbers[], producing different results: 
    Console.WriteLine("Second run numbers <= 3:"); 
    foreach (int n in lowNumbers) 
    { 
        Console.WriteLine(n); 
    } 
}