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End Sub To have code outside of the MDI parent form class notified when an MDI child form becomes active,
write a handler for the MDI parent forms MdiChildActivate event. This event is defined in the Form class as:
Public Event MdiChildActivate _ ByVal sender As Object, _
ByVal e As EventArgs _
The sender
parameter is the MDI parent form, not the MDI child form that has been activated. The e
parameter does not contain any additional information about the event. As when overriding the OnMdiChildActivate method, read the MDI parent forms ActiveMdiChild property to discover which
MDI child form has been activated.
4.5 Component Attributes
As explained in Chapt er 2
, attributes can be added to code elements to provide additional information about those elements. The System.ComponentModel namespace defines several
attributes for use in component, control, and form declarations. These attributes dont affect component behavior. Rather, they provide information that is used or displayed by the Visual
Studio .NET IDE. The following is a description of each attribute:
AmbientValueAttribute For ambient properties, specifies the property value that will mean get this propertys actual
value from wherever ambient values come from for this property. Ambient properties are properties able to get their values from another source. For example, the BackColor property
of a Label control can be set either to a specific Color value or to the special value Color.Empty, which causes the Labels background color to be the same as the background
color of the form on which it is placed.
Putting this attribute on a property definition isnt what causes the property to behave as an ambient property: the control itself must be written such that when the special value is written
to the property, the control gets the actual value from the appropriate location. This attribute simply provides the Windows Forms Designer with a way to discover what the special value is.
When specifying this attribute, pass the special value to the attributes constructor. For example,
AmbientValue0 .
BindableAttribute Indicates whether a property is typically usable for data binding. Specify
BindableTrue to indicate that the property can be used for data binding or
BindableFalse to indicate
that the property typically is not used for data binding. This attribute affects how a property is displayed in the IDE, but does not affect whether a property can be bound at runtime. By
default, properties are considered not bindable.
BrowsableAttribute Indicates whether a property should be viewable in the IDEs Properties window. Specify
BrowsableTrue to indicate that the property should appear in the Properties window or
BrowsableFalse to indicate that it should not. By default, properties are considered
browsable.
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CategoryAttribute Indicates the category to which a property or event belongs Appearance, Behavior, etc..
The IDE uses this attribute to sort the properties and events in the Properties window. Specify the category name as a string argument to the attribute. For example,
CategoryAppearance . The argument can be any string. If it is not one of the
standard strings, the Properties window will add a new group for it. The standard strings are: Action
Used for events that indicate a user action, such as the Click event. Appearance
Used for properties that affect the appearance of a component, such as the BackColor property.
Behavior Used for properties that affect the behavior of a component, such as the AllowDrop property.
Data Used for properties that relate to data, such as the DecimalPlaces property of the
NumericUpDown control. Design
Used for properties that relate to the design-time appearance or behavior of a component. DragDrop
Used for properties and events that relate to drag and drop. No Windows Forms components have any properties or events marked with this category.
Focus Used for properties and events that relate to input focus, such as the CanFocus property and
the GotFocus event. Format
Used for properties and events that relate to formats. No Windows Forms components have any properties or events marked with this category.
Key Used for events that relate to keyboard input, such as the KeyPress event.
Layout Used for properties and events that relate to the visual layout of a component, such as the
Height property and the Resize event. Mouse
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Used for events that relate to mouse input, such as the MouseMove event. WindowStyle
Used for properties and events that relate to the window style of top-level forms. No Windows Forms components have any properties or events marked with this category.
If no CategoryAttribute
is specified, the property is considered to have a category of Misc
. DefaultEventAttribute
Indicates the name of the event that is to be considered the default event of a component. For example,
DefaultEventClick . When a component is double-clicked in the
Windows Forms Designer, the designer switches to code view and displays the event handler for the default event. This attribute can be used only on class declarations.
DefaultPropertyAttribute Indicates the name of the property that is to be considered the default property of a
component. For example, DefaultPropertyText
. This attribute can be used only on class declarations. The default property is the property that the Windows Forms Designer
highlights in the Properties window when a component is clicked in design view.
Dont confuse this usage of the term default property with the usage associated with the Default modifier of a property
declaration. The two concepts are unrelated. Refer to Chapt er 2
for details on the Default modifier.
DefaultValueAttribute Indicates the default value of a property. For example,
DefaultValue0 . If the IDE is
used to set a property value to something other than the default value, the code generator will generate the appropriate assignment statement in code. However, if the IDE is used to set a
property to the default value, no assignment statement is generated.
DescriptionAttribute Provides a description for the code element. For example,
DescriptionThe text
contained in the control. . The IDE uses this description in tool tips and
IntelliSense. DesignerAttribute
Identifies the class that acts as the designer for a component. For example, DesignerMyNamespace.MyClass
. The designer class must implement the IDesigner
interface. This attribute can be used only on class declarations and is needed only if the built-in designer isnt sufficient. Creating custom designers is not discussed in this
book. DesignerCategoryAttribute
Used with custom designers to specify the category to which the class designer belongs. DesignerSerializationVisibilityAttribute
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Used with custom designers to specify how a property on a component is saved by the designer.
DesignOnlyAttribute Indicates when a property can be set. Specify
DesignOnlyTrue to indicate that the
property can be set at design time only or DesignOnlyFalse
to indicate that the property can be set at both design time and runtime the default.
EditorAttribute Identifies the editor to use in the IDE to allow the user to set the values of properties that
have the type on which the EditorAttribute
attribute appears. In this way, a component can declare new types and can declare properties having those types, yet still allow the user
to set the values of the properties at design time. Creating custom type editors is not discussed in this book.
EditorBrowsableAttribute Indicates whether a property is viewable in an editor. The argument to the attributes
constructor must be one of the values defined by the EditorBrowsableState enumeration defined in the System.ComponentModel namespace. The values of this enumeration are:
Advanced Only advanced users should see the property. It is up to the editor to determine when its
appropriate to display advanced properties. Always
The property should always be visible within the editor. Never
The property should never be shown within the editor. ImmutableObjectAttribute
Indicates whether a type declaration defines a state that can change after an object of that type is constructed. Specify
ImmutableObjectTrue to indicate that an object of the
given type is immutable. Specify ImmutableObjectFalse
to indicate that an object of the given type is not immutable. The IDE uses this information to determine whether to render
a property as read-only. InheritanceAttribute
Used to document an inheritance hierarchy that can be read using the IInheritanceService
interface. This facility is not discussed in this book. InstallerTypeAttribute
Used on type declarations to specify the installer for the type. Installers are not discussed in this book.
LicenseProviderAttribute
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Indicates the license provider for a component. ListBindableAttribute
Indicates whether a property can be used as a data source. A data source is any object that exposes the
IList interface. Specify
ListBindableTrue to indicate that the property
can be used as a data source. Specify ListBindableFalse
to indicate that the property cant be used as a data source.
LocalizableAttribute Indicates whether a propertys value should be localized when the application is localized.
Specify LocalizableTrue
to indicate that the propertys value should be localized. Specify
LocalizableFalse or omit the
LocalizableAttribute attribute to indicate
that the propertys value should not be localized. The values of properties declared with LocalizableTrue
are stored in a resource file, which can be localized. MergablePropertyAttribute
Indicates where property attributes can be merged. By default, when two or more components are selected in the Windows Forms Designer, the Properties window typically shows the
properties that are common to all of the selected components. If the user changes a value in the Properties window, the value is changed for that property in all of the selected components.
Placing
MergablePropertyFalse on a property declaration changes this behavior.
Any property declared in this way is omitted from the Properties window when two or more components are selected. Specifying
MergablePropertyTrue is the same as omitting
the attribute altogether. NotifyParentPropertyAttribute
Indicates whether the display of a propertys parent property should be refreshed when the given property changes its value.
ParenthesizePropertyNameAttribute Indicates whether the property name should be parenthesized in the Properties window.
Specify ParenthesizePropertyNameTrue
to indicate that the property name should appear within parentheses. Specify
ParenthesizePropertyNameFalse to indicate
that the property name should not appear within parentheses. Omitting the attribute is the same as specifying
ParenthesizePropertyNameFalse .
The only benefit to parenthesizing property names in the property window is that they are sorted to the top of the list. Microsoft has no specific recommendations for when to
parenthesize a property name.
PropertyTabAttribute Specifies a type that implements a custom property tab or tabs for a component. This facility
is not discussed in this book. ProvidePropertyAttribute
Used with extender providers—i.e., classes that provide properties for other objects. Extender providers are not discussed in this book.
ReadOnlyAttribute
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Indicates whether a property is read-only at design time. Specify ReadOnlyTrue
to indicate that the property is read-only at design time. Specify
ReadOnlyFalse to
indicate that the propertys ability to be modified at design time is determined by whether a Set method is defined for the property. Omitting the attribute is the same as specifying
ReadOnlyFalse .
RecommendedAsConfigurableAttribute Indicates whether a property is configurable. Configurable properties arent discussed in this
book. RefreshPropertiesAttribute
Determines how the Properties window is refreshed when the value of the given property changes.
RunInstallerAttribute Indicates whether an installer should be invoked during installation of the assembly that
contains the associated class. This attribute can be used only on class declarations, and the associated class must inherit from the Installer class defined in the
System.Configuration.Install namespace. Installers are not discussed in this book.
ToolboxItemAttribute Specifies a type that implements a toolbox item related to the declaration on which the
attribute is placed. This facility is not discussed in this book. ToolboxItemFilterAttribute
Specifies a filter string for a toolbox-item filter. This facility is not discussed in this book. TypeConverterAttribute
Indicates the type converter to be used with the associated item. Type converters are not discussed in this book.
4.6 2-D Graphics Programming with GDI+
