Deferred Execution Example

This topic shows how deferred execution and lazy evaluation affect the execution of your LINQ to XML queries.

The following example shows the order of execution when using an extension method that uses deferred execution. The example declares an array of three strings. It then iterates through the collection returned by ConvertCollectionToUpperCase.


The following example uses the yield return construct of C#. Because there is no equivalent feature in Visual Basic 2008, this example is provided only in C#.

public static class LocalExtensions
    public static IEnumerable<string>
      ConvertCollectionToUpperCase(this IEnumerable<string> source)
        foreach (string str in source)
            Console.WriteLine("ToUpper: source {0}", str);
            yield return str.ToUpper();

class Program
    static void Main(string[] args)
        string[] stringArray = { "abc", "def", "ghi" };

        var q = from str in stringArray.ConvertCollectionToUpperCase()
                select str;

        foreach (string str in q)
            Console.WriteLine("Main: str {0}", str);

This example produces the following output:

ToUpper: source abc
Main: str ABC
ToUpper: source def
Main: str DEF
ToUpper: source ghi
Main: str GHI

Notice that when iterating through the collection returned by ConvertCollectionToUpperCase, each item is retrieved from the source string array and converted to uppercase before the next item is retrieved from the source string array.

You can see that the entire array of strings is not converted to uppercase before each item in the returned collection is processed in the foreach loop in Main.

The next topic in this tutorial illustrates chaining queries together: