101 LINQ Samples: Query Execution

Deferred Execution

The following sample shows how query execution is deferred until the query is enumerated at a foreach statement.

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 Execution

The following sample shows how queries can be executed immediately with operators such as ToList().

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

The following sample shows how, because of deferred execution, queries can be used again after data changes and will then operate on the new data.

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