Extension Method:
=================

public static class MyExtension
{
    public static IEnumerable<string> ToUpper(this IEnumerable<string> source)
    {
        foreach (string s in source)
        {
            Console.WriteLine("Yield returning: {0}", s);
            yield return s.ToUpper();
        }
    }
}

Iteration Code:
===============
string[] sa = new[] {
    "aaa",
    "Bbb",
    "CCc"
};

Console.WriteLine("Before using ToUpper()");
var sb = sa.ToUpper();

Console.WriteLine("After using ToUpper()");
Console.WriteLine("Before iterating the collection in sb");

foreach (string s in sb)
    Console.WriteLine("Within iteration, s: {0}", s);

Chained Queries:
================
string[] sa = new[] {
    "#33",
    "#22",
    "% this is text",
    "#18"
};

var justNumbersAsStrings =
    from s in sa
    where s.StartsWith("#")
    select s.Substring(1);

var numbersAsInts =
    from j in justNumbersAsStrings
    select Int32.Parse(j);

var sum =
    numbersAsInts.Sum();

Console.WriteLine(sum);