7.4. ListBox, CheckedListBox, and ComboBox Classes

List boxes may also contain objects. Because an object may have many members , this raises the question of what is displayed in the TextBox list. Because by default a ListBox displays the results of an item's ToString method, it is necessary to override this System.Object method to return the string you want displayed. The following class is used to create ListBox items:

// Instances of this class will be placed in a ListBox

public class Artist

{

   public string BDate, DDate, Country;

   private string firstname;

   private string lastname;

   public Artist(string birth, string death, string fname,

                 string lname, string ctry)

   {

      BDate = birth;

      DDate = death;

      Country = ctry;

      firstname = fname;

      lastname = lname;

   }

   public override string ToString() {

      return (lastname+" , "+firstname);

   }

   public string GetLName    {

      get{ return lastname;} 

   }

   public string GetFName    {

      get{ return firstname;} 

   }

}

ToString has been overridden to return the artist's last and first names , which are displayed in the ListBox . The ListBox (Figure 7-9) is populated using these statements:

lstArtists.Items.Add

   (new Artist("1832", "1883", "Edouard", "Manet","Fr" ));

lstArtists.Items.Add

   (new Artist("1840", "1926", "Claude", "Monet","Fr"));

lstArtists.Items.Add

   (new Artist("1606", "1669", "Von Rijn", "Rembrandt","Ne"));

lstArtists.Items.Add

   (new Artist("1445", "1510", "Sandre", "Botticelli","It"));

Selecting and Searching for Items in a ListBox

The SelectionMode property determines the number of items a ListBox allows to be selected at one time. It takes four values from the SelectionMode enumeration: None , Single , MultiSingle , and MultiExtended . MultiSingle allows selection by clicking an item or pressing the space bar; MultiExtended permits the use of the Shift and Ctrl keys.

The SelectedIndexChanged event provides an easy way to detect when an item in a ListBox is selected. It is fired when the user clicks on an item or uses the arrow keys to traverse a list. A common use is to display further information about the selection in other controls on the form. Here is code that displays an artist's dates of birth and death when the artist's name is selected from the ListBox in Figure 7-9:

// Set up event handler in constructor

lstArtists.SelectedIndexChanged += new EventHandler(ShowArtist);

//

private void ShowArtist(object sender, EventArgs e)

{

   // Cast to artist object in order to access properties

   Artist myArtist = lstArtists.

SelectedItem

as Artist;

   if (myArtist != null) {

      txtBirth.Text = myArtist.Dob; // Place dates in text boxes

      txtDeath.Text = myArtist.Dod;

   }

}

The SelectedItem property returns the item selected in the ListBox . This object is assigned to myArtist using the as operator, which ensures the object is an Artist type. The SelectedIndex property can also be used to reference the selected item:

myArtist = lstArtists.Items[lstArtists.SelectedIndex] as Artist;

Working with a multi-selection ListBox requires a different approach. You typically do not want to respond to a selection event until all items have been selected. One approach is to have the user click a button to signal that all choices have been made and the next action is required. All selections are exposed as part of the SelectedItems collection, so it is an easy matter to enumerate the items:

foreach (Artist a in lstArtists.SelectedItems)

   MessageBox.Show(a.GetLName);

The SetSelected method provides a way to programatically select an item or items in a ListBox . It highlights the item(s) and fires the SelectedIndexChanged event. In this example, SetSelected is used to highlight all artists who were born in France:

lstArtists.

ClearSelected

();   // Clear selected items

for (int ndx =0; ndx < lstArtists.Items.Count-1; ndx ++)

{

   Artist a = lstArtists.Items[ndx] as Artist;

   if (a.country == "Fr") lstArtists.

SetSelected

(ndx,true);

}

Customizing the Appearance of a ListBox

The ListBox , along with the ComboBox , MenuItem , and TabControl controls, is an owner-drawn control. This means that by setting a control property, you can have it fire an event when the control's contents need to be drawn. A custom event handler takes care of the actual drawing.

To enable owner drawing of the ListBox , the DrawMode property must be set to one of two DrawMode enumeration values: OwnerDrawFixed or OwnerDrawVariable . The former draws each item a fixed size ; the latter permits variable- sized items. Both of these cause the DrawItem event to be fired and rely on its event handler to perform the drawing.

Using the ListBox from the previous example, we can use the constructor to set DrawMode and register an event handler for the DrawItem event:

lstArtists.DrawMode = DrawMode.OwnerDrawFixed;

lstArtists.ItemHeight = 16;  // Height (pixels) of item

lstArtists.DrawItem += new DrawItemEventHandler(DrawList);

The DrawItemEventHandler delegate has two parameters: the familiar sender object and the DrawItemEventArgs object. The latter is of more interest. It contains properties related to the control's appearance and state as well as a couple of useful drawing methods. Table 7-2 summarizes these.

Table 7-2. DrawItemEventArgs Properties

private void DrawList(object sender, DrawItemEventArgs e)

{

   // Draw ListBox Items

   string ctry;

   Rectangle rect = e.Bounds;

   Artist a = lstArtists.Items[e.Index] as Artist;

   string artistName = a.ToString();

   if ( (e.State & DrawItemState.Selected) == 

                   DrawItemState.Selected )

   {

      // Draw Black border around the selected item

      e.Graphics.DrawRectangle(Pens.Black,rect); 

   } else {

      ctry = a.Country;

      Brush b;   // Object used to define backcolor

      // Each country will have a different backcolor

      b = Brushes.LightYellow;      // Netherlands

      if (ctry == "Fr")  b = Brushes.LightGreen;

      if (ctry == "It")  b = Brushes.Yellow;

      e.Graphics.FillRectangle(b,rect);}

      e.Graphics.DrawString(artistName,e.Font,

                            Brushes.Black,rect);

   }

}

ComboBox cbArtists = new ComboBox();

cbArtists.Size = new System.Drawing.Size(120, 21);

cbArtists.MaxDropDownItems= 4;  // Max number of items to display

cbArtists.DropDownWidth = 140;  // Width of drop-down portion

cbArtists.Items.Add(new Artist("1832", "1883",

                               "Edouard", "Manet","Fr" ));

// Add other items here...

// List all items with checked box.

foreach (Artist a in clBox.

CheckedItems

)

   MessageBox.Show(a.ToString()); //   -> Monet, Claude

For (int i=0; I< clBox.Items.Count; i++)

{

   if(clBox.GetItemCheckState(i) == CheckState.Checked) 

   { Do something }  else {do something if not checked }

}