C# 0 The Complete Reference

PART I

C h a p t e r   1 9 :  

L I N Q  

549

PART IPART I

class SimpQuery {

  static void Main() {

    int[] nums =  { 1, -2, 3, 0, -4, 5 };

    // Create a query that obtains only positive numbers.

    var posNums = from n in nums

                  where n > 0

                  select n;

    Console.Write("The positive values in nums: ");

    // Execute the query and display the results.

    foreach(int i in posNums) Console.Write(i + " ");

    Console.WriteLine();

    // Change nums.

    Console.WriteLine("\nSetting nums[1] to 99.");

    nums[1] = 99;

    Console.Write("The positive values in nums after change: ");

    // Execute the query a second time.

    foreach(int i in posNums) Console.Write(i + " ");

    Console.WriteLine();

  }

}

The following output is produced:

The positive values in nums: 1 3 5

Setting nums[1] to 99.

The positive values in nums after change: 1 99 3 5

As the output confirms, after the value in 

nums[1]

was changed from –2 to 99, the result 

of rerunning the query reflects the change. This is a key point that must be emphasized. 

Each execution of a query produces its own results, which are obtained by enumerating the 

current contents of the data source. Therefore, if the data source changes, so, too, might the 

results of executing a query. The benefits of this approach are quite significant. For example, 

if you are obtaining a list of pending orders for an online store, then you want each execution 

of your query to produce all orders, including those just entered.