HTML clipboardConsistency and adaptability are two often-undeclared but implicit software
design goals. In an object-oriented world, a frequently employed mechanism to
achieve these objectives is inheritance.
In the System.Windows.Forms namespace, all controls derive from the Control
class. The raw Control class is not normally instantiated but rather forms the
basis for further refining the user interface hierarchy.
The Control class implements the basic functionality of controls and, where
appropriate, provides for members to be overridden. This approach promotes not
only reusability but also standardization. This can be seen in two Control class
properties, Name and Text. The Name property, the equivalent to a control ID in
Win32, applies to all controls regardless of type and therefore is not declared
"virtual," whereas a Text property implementation differs depending on the type
and can be overridden. For example, Form.Text refers to the caption in the title
bar, while TextBox.Text returns the user's keyboard input.
In addition to many controls directly deriving from the Control class, a number
of classes act as intermediaries to collect controls into what loosely could be
termed a behavioral unit. Figure 9.16 shows a number of these intermediate
classes.
Figure 9.16: Intermediate Classes
The ButtonBase class, for instance, is the root of the Button, CheckBox, and
RadioButton classes, which exhibit a similar behavior. The ButtonBase class
handles common chores such as raising mouse and focus events. Other classes that
use this method of control intercession will be pointed out as they arise.
Another gauge of consistency in the Windows.Forms namespace is its treatment of
collections. While objects range from the general to the very specific, the
means of accessing an object in a collection remains consistent throughout the
namespace. Each collection in the namespace has an indexer, as well as identical
methods to add and remove objects. The most commonly referenced collection is
the nested Control.ControlCollection, accessible through the read-only property
called Controls. This is a container for Control objects.
Some of the properties of the Control classes are defined in Table 9.2.
Table 9.2: Control Class Properties
Some Control class methods are itemized in Table 9.3.
Table 9.3: Control Class Methods
The Control class implements basic mouse and keyboard events, some of which are
defined in Table 9.4.
Table 9.4: Control Class Events
Besides the methods defined in Table 9.4, there are overridable methods for
raising events programmatically, such as OnClick, OnEnter, and OnKeyUp. When
overriding any of these eventtriggering methods, the base class's method must be
called, so that any registered delegate receives the event.
Control Classes
Table 9.5 lists some of the common control classes.
Table 9.5: Windows Forms Common Control Classes
As has been seen earlier, you can create these controls programmatically as well
as from the VS.NET Form Designer. The Form Designer allows you to drag and drop
controls from the toolbox onto a form. Here we will show you how to create a
control manually and set its properties.
Creating a Control
Use a constructor to create an instance of a control. Most controls are
constructed with a default constructor, that is, a constructor with no
parameters. Here is an example of creating a Button control:
Dim
btn1 As Button
= New System.Windows.Forms.Button()
Setting Properties
After creating the object, set the control's properties. The code in Listing 9.5
sets the button properties.
Listing 9.5: Setting the properties of a Button Control
btn1.ImageAlign =
System.Drawing.ContentAlignment.TopLeft
btn1.Dock = System.Windows.Forms.DockStyle.Left
btn1.ForeColor = System.Drawing.Color.Red
btn1.BackColor = System.Drawing.SystemColors.Desktop
btn1.DialogResult = System.Windows.Forms.DialogResult.OK
btn1.AllowDrop = True
btn1.FlatStyle = System.Windows.Forms.FlatStyle.Flat
btn1.Size = New System.Drawing.Size(336, 568)
btn1.TabIndex = 0
btn1.Font = New System.Drawing.Font("Verdana",
10, System.Drawing.FontStyle.Bold)
btn1.Text = "Click Me"
Once you have set the properties, you can write an event handler for the
control.