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When the user has selected an item in the list box, you can determine which item was selected by using the ListBox s SelectedItem property. This property returns an object reference to the first selected item, or null, if no item is selected. I'll cover multiple selections in the "Multiple Selections" section. The markup and code-behind listed produce the window shown in Figure 6-21. In this example, the Button has an event handler, called Button_Click, which determines which item in the ListBox is selected, and displays that item.

namespace ColorCollectionIterator { class ColorCollection { string[] Colors={"Red", "Orange", "Yellow", "Green", "Blue", "Purple"}; public IEnumerable<string> Forward() { for (int i = 0; i < Colors.Length; i++) yield return Colors[i]; } // Enumerable iterator

.NET 4.0 introduces ISet<T>, a new interface utilized by SortedSet and HashSet and surprisingly enough for implementing set classes.

public IEnumerable<string> Reverse() { // Enumerable iterator for (int i = Colors.Length - 1; i >= 0; i--) yield return Colors[i]; } }

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Figure 6-21. A ListBox with a button that determines which item is selected <StackPanel> <ListBox Name="lstbxCats" HorizontalAlignment="Left" Width="100"> <ListBoxItem>Sweetie</ListBoxItem> <ListBoxItem>Darwin</ListBoxItem> <ListBoxItem>Florence</ListBoxItem> </ListBox> <Button Click="Button_Click" HorizontalAlignment="Left" Width="100" Padding="10,3" Margin="0,5">Enter</Button> </StackPanel> Notice in the following event handler that the reference returned is of type object, so it must be cast to the correct type before you can use it. Notice how the code does this by casting the reference to type ListBoxItem and then using the Content property of the result. The event handler must have the signature and return type shown in the code. Also, make sure to handle the case where a null value is returned. private void Button_Click( object sender, RoutedEventArgs e ) { Get the selected item.

A tuple is a typed collection of fixed size. Tuples were introduced for interoperability with F# and IronPython, but can also make your code more concise. Tuples are very easy to create: Tuple<int, int, int, int, int> MultiplesOfTwo = Tuple.Create(2, 4, 6, 8, 10); Individual items in the tuple can then be queried with the Item property: Console.WriteLine(MultiplesOfTwo.Item2);

class Program { static void Main() { ColorCollection cc = new ColorCollection(); Return enumerable to the foreach statement foreach (string color in cc.Forward()) Console.Write("{0} ", color); Console.WriteLine(); Return enumerable to the foreach statement foreach (string color in cc.Reverse()) Console.Write("{0} ", color); Console.WriteLine(); // Skip the foreach and manually use the enumerable and enumerator. IEnumerable<string> ieable = cc.Reverse(); IEnumerator<string> ieator = ieable.GetEnumerator(); while (ieator.MoveNext()) Console.Write("{0} ", ieator.Current); Console.WriteLine(); } } } This code produces the following output: Red Orange Yellow Green Blue Purple Purple Blue Green Yellow Orange Red Purple Blue Green Yellow Orange Red

Tuples might contain up to seven elements; if you want to add more items, you have to pass in another tuple to the Rest parameter: var multiples = new Tuple<int, int, int, int, int, int, int,Tuple<int,int,int>>(2, 4, 6, 8, 10, 12, 14, new Tuple<int,int,int>(3,6,9)); Items in the second tuple can be accessed by querying the Rest property: Console.WriteLine(multiples.Rest.Item1);

object obj = lstbxCats.SelectedItem; string selected = ( obj == null ) "No item selected." : (string) ( (ListBoxItem) obj ).Content;

The previous example used iterators to produce a class with two enumerables. This example shows two things. First, it uses iterators to produce a class with two enumerators. Second, it shows how iterators can be implemented as properties rather than methods. The code declares two properties that define two different enumerators. The GetEnumerator method returns one or the other of the two enumerators, depending on the value of the Boolean variable ColorFlag. If ColorFlag is true, the Colors enumerator is returned. Otherwise, the BlackAndWhite enumerator is returned. class MyClass: IEnumerable<string> { bool ColorFlag = true; public MyClass(bool flag) { ColorFlag = flag; } IEnumerator<string> BlackAndWhite { get { yield return "black"; yield return "gray"; yield return "white"; } } // Constructor

Mathematicians will be glad of the addition of the new Complex type: a structure for representing and manipulating complex numbers, meaning that they will no longer have to utilize open source libraries or projects. Complex represents both a real and imaginary number, and contains support for both rectangular and polar coordinates: Complex c1 = new Complex(8, 2); Complex c2 = new Complex(8, 2); Complex c3 = c1 + c2; And I am afraid my math skills aren t up to saying much more about this type, so let s move on.

IEnumerator<string> Colors // Property--enumerator iterator { get { string[] theColors = { "blue", "red", "yellow" }; for (int i = 0; i < theColors.Length; i++) yield return theColors[i]; } }

MessageBox.Show( selected, "Selected Item" ); }

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