101 LINQ Samples: Query Execution

Deferred

This sample shows how query execution is deferred until the query is enumerated at a foreach statement. Sequence operators form first-class queries that are not executed until you enumerate them.

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);        
    }
}

Result

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

Immediate

This sample shows how queries can be executed immediately with operators such as ToList(). Methods like ToList() cause the query to be executed immediately, caching the results.

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);
    }
}

Result

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

Query Reuse

This sample shows how, because of deferred execution, queries can be reused. Deferred execution lets you define a query once and reuse it, even after data changes.

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);
    }
}

Result

First run numbers <= 3:
1
3
2
0

Second run numbers <= 3:
-5
-4
-1
-3
-9
-8
-6
-7
-2
0