PropertyDescriptor Class
Provides an abstraction of a property on a class.
Assembly: System (in System.dll)
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The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: SharedState. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes. |
A description of a property consists of a name, its attributes, the component class that the property is associated with, and the type of the property.
PropertyDescriptor provides the following properties and methods:
Converter contains the TypeConverter for this property.
IsLocalizable indicates whether this property should be localized.
GetEditor returns an editor of the specified type.
PropertyDescriptor also provides the following abstract properties and methods:
ComponentType contains the type of component this property is bound to.
IsReadOnly indicates whether this property is read-only.
PropertyType gets the type of the property.
CanResetValue indicates whether resetting the component changes the value of the component.
GetValue returns the current value of the property on a component.
ResetValue resets the value for this property of the component.
SetValue sets the value of the component to a different value.
ShouldSerializeValue indicates whether the value of this property needs to be persisted.
Typically, the abstract members are implemented through reflection. For more information about reflection, see the topics in Reflection.
The following code example is built upon the example in the PropertyDescriptorCollection class. It prints the information (category, description, display name) of the text of a button in a text box. It assumes that button1 and textbox1 have been instantiated on a form.
' Creates a new collection and assign it the properties for button1. Dim properties As PropertyDescriptorCollection = TypeDescriptor.GetProperties(Button1) ' Sets an PropertyDescriptor to the specific property. Dim myProperty As PropertyDescriptor = properties.Find("Text", False) ' Prints the property and the property description. TextBox1.Text += myProperty.DisplayName & Microsoft.VisualBasic.ControlChars.Cr TextBox1.Text += myProperty.Description & Microsoft.VisualBasic.ControlChars.Cr TextBox1.Text += myProperty.Category & Microsoft.VisualBasic.ControlChars.Cr
The following code example shows how to implement a custom property descriptor that provides a read-only wrapper around a property. The SerializeReadOnlyPropertyDescriptor is used in a custom designer to provide a read-only property descriptor for the control's Size property.
Imports System Imports System.Collections Imports System.ComponentModel Imports System.Text ' The SerializeReadOnlyPropertyDescriptor shows how to implement a ' custom property descriptor. It provides a read-only wrapper ' around the specified PropertyDescriptor. Friend NotInheritable Class SerializeReadOnlyPropertyDescriptor Inherits PropertyDescriptor Private _pd As PropertyDescriptor = Nothing Public Sub New(ByVal pd As PropertyDescriptor) MyBase.New(pd) Me._pd = pd End Sub Public Overrides ReadOnly Property Attributes() As AttributeCollection Get Return AppendAttributeCollection(Me._pd.Attributes, ReadOnlyAttribute.Yes) End Get End Property Protected Overrides Sub FillAttributes(ByVal attributeList As IList) attributeList.Add(ReadOnlyAttribute.Yes) End Sub Public Overrides ReadOnly Property ComponentType() As Type Get Return Me._pd.ComponentType End Get End Property ' The type converter for this property. ' A translator can overwrite with its own converter. Public Overrides ReadOnly Property Converter() As TypeConverter Get Return Me._pd.Converter End Get End Property ' Returns the property editor ' A translator can overwrite with its own editor. Public Overrides Function GetEditor(ByVal editorBaseType As Type) As Object Return Me._pd.GetEditor(editorBaseType) End Function ' Specifies the property is read only. Public Overrides ReadOnly Property IsReadOnly() As Boolean Get Return True End Get End Property Public Overrides ReadOnly Property PropertyType() As Type Get Return Me._pd.PropertyType End Get End Property Public Overrides Function CanResetValue(ByVal component As Object) As Boolean Return Me._pd.CanResetValue(component) End Function Public Overrides Function GetValue(ByVal component As Object) As Object Return Me._pd.GetValue(component) End Function Public Overrides Sub ResetValue(ByVal component As Object) Me._pd.ResetValue(component) End Sub Public Overrides Sub SetValue(ByVal component As Object, ByVal val As Object) Me._pd.SetValue(component, val) End Sub ' Determines whether a value should be serialized. Public Overrides Function ShouldSerializeValue(ByVal component As Object) As Boolean Dim result As Boolean = Me._pd.ShouldSerializeValue(component) If Not result Then Dim dva As DefaultValueAttribute = _ CType(_pd.Attributes(GetType(DefaultValueAttribute)), DefaultValueAttribute) If Not (dva Is Nothing) Then result = Not [Object].Equals(Me._pd.GetValue(component), dva.Value) Else result = True End If End If Return result End Function ' The following Utility methods create a new AttributeCollection ' by appending the specified attributes to an existing collection. Public Shared Function AppendAttributeCollection( _ ByVal existing As AttributeCollection, _ ByVal ParamArray newAttrs() As Attribute) As AttributeCollection Return New AttributeCollection(AppendAttributes(existing, newAttrs)) End Function Public Shared Function AppendAttributes( _ ByVal existing As AttributeCollection, _ ByVal ParamArray newAttrs() As Attribute) As Attribute() If existing Is Nothing Then Throw New ArgumentNullException("existing") End If If newAttrs Is Nothing Then newAttrs = New Attribute(-1) {} End If Dim attributes() As Attribute Dim newArray(existing.Count + newAttrs.Length) As Attribute Dim actualCount As Integer = existing.Count existing.CopyTo(newArray, 0) Dim idx As Integer For idx = 0 To newAttrs.Length If newAttrs(idx) Is Nothing Then Throw New ArgumentNullException("newAttrs") End If ' Check if this attribute is already in the existing ' array. If it is, replace it. Dim match As Boolean = False Dim existingIdx As Integer For existingIdx = 0 To existing.Count If newArray(existingIdx).TypeId.Equals(newAttrs(idx).TypeId) Then match = True newArray(existingIdx) = newAttrs(idx) Exit For End If Next existingIdx If Not match Then actualCount += 1 newArray(actualCount) = newAttrs(idx) End If Next idx ' If some attributes were collapsed, create a new array. If actualCount < newArray.Length Then attributes = New Attribute(actualCount) {} Array.Copy(newArray, 0, attributes, 0, actualCount) Else attributes = newArray End If Return attributes End Function End Class
The following code examples show how to use the The SerializeReadOnlyPropertyDescriptor in a custom designer.
Imports System Imports System.Collections Imports System.ComponentModel Imports System.Text Imports System.Windows.Forms.Design Class DemoControlDesigner Inherits ControlDesigner ' The PostFilterProperties method replaces the control's ' Size property with a read-only Size property by using ' the SerializeReadOnlyPropertyDescriptor class. Protected Overrides Sub PostFilterProperties(ByVal properties As IDictionary) If properties.Contains("Size") Then Dim original As PropertyDescriptor = properties("Size") Dim readOnlyDescriptor As New SerializeReadOnlyPropertyDescriptor(original) properties("Size") = readOnlyDescriptor End If MyBase.PostFilterProperties(properties) End Sub End Class ... Imports System Imports System.ComponentModel Imports System.ComponentModel.Design Imports System.Text Imports System.Windows.Forms Imports System.Windows.Forms.Design <Designer(GetType(DemoControlDesigner))> _ Public Class DemoControl Inherits Control Public Sub New() End Sub End Class
System.ComponentModel.MemberDescriptor
System.ComponentModel.PropertyDescriptor
System.ComponentModel.DependencyPropertyDescriptor
System.ComponentModel.TypeConverter.SimplePropertyDescriptor
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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