PropertyDescriptor Class

Definition

public ref class PropertyDescriptor abstract : System::ComponentModel::MemberDescriptor

public abstract class PropertyDescriptor : System.ComponentModel.MemberDescriptor

[System.Runtime.InteropServices.ComVisible(true)]
public abstract class PropertyDescriptor : System.ComponentModel.MemberDescriptor

type PropertyDescriptor = class
 inherit MemberDescriptor

[<System.Runtime.InteropServices.ComVisible(true)>]
type PropertyDescriptor = class
 inherit MemberDescriptor

Public MustInherit Class PropertyDescriptor
Inherits MemberDescriptor

Inheritance

Object

MemberDescriptor

PropertyDescriptor

Derived

System.ComponentModel.DependencyPropertyDescriptor

System.ComponentModel.TypeConverter.SimplePropertyDescriptor

Attributes

ComVisibleAttribute

Examples

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.
PropertyDescriptorCollection^ properties = TypeDescriptor::GetProperties( button1 );

// Sets an PropertyDescriptor to the specific property.
System::ComponentModel::PropertyDescriptor^ myProperty = properties->Find( "Text", false );

// Prints the property and the property description.
textBox1->Text = String::Concat( myProperty->DisplayName, "\n" );
textBox1->Text = String::Concat( textBox1->Text, myProperty->Description, "\n" );
textBox1->Text = String::Concat( textBox1->Text, myProperty->Category, "\n" );

// Creates a new collection and assign it the properties for button1.
PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(button1);

// Sets an PropertyDescriptor to the specific property.
PropertyDescriptor myProperty = properties.Find("Text", false);

// Prints the property and the property description.
textBox1.Text = myProperty.DisplayName + '\n';
textBox1.Text += myProperty.Description + '\n';
textBox1.Text += myProperty.Category + '\n';

' 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 & ControlChars.Cr
TextBox1.Text += myProperty.Description & ControlChars.Cr
TextBox1.Text += myProperty.Category & 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.

using System;
using System.Collections;
using System.ComponentModel;

namespace ReadOnlyPropertyDescriptorTest;

// The SerializeReadOnlyPropertyDescriptor shows how to implement a 
// custom property descriptor. It provides a read-only wrapper 
// around the specified PropertyDescriptor. 
sealed class SerializeReadOnlyPropertyDescriptor : PropertyDescriptor
{
 readonly PropertyDescriptor _pd;

 public SerializeReadOnlyPropertyDescriptor(PropertyDescriptor pd)
 : base(pd) => _pd = pd;

 public override AttributeCollection Attributes => AppendAttributeCollection(
 _pd.Attributes,
 ReadOnlyAttribute.Yes);

 protected override void FillAttributes(IList attributeList) => attributeList.Add(ReadOnlyAttribute.Yes);

 public override Type ComponentType => _pd.ComponentType;

 // The type converter for this property.
 // A translator can overwrite with its own converter.
 public override TypeConverter Converter => _pd.Converter;

 // Returns the property editor 
 // A translator can overwrite with its own editor.
 public override object GetEditor(Type editorBaseType) => _pd.GetEditor(editorBaseType);

 // Specifies the property is read only.
 public override bool IsReadOnly => true;

 public override Type PropertyType => _pd.PropertyType;

 public override bool CanResetValue(object component) => _pd.CanResetValue(component);

 public override object GetValue(object component) => _pd.GetValue(component);

 public override void ResetValue(object component) => _pd.ResetValue(component);

 public override void SetValue(object component, object val) => _pd.SetValue(component, val);

 // Determines whether a value should be serialized.
 public override bool ShouldSerializeValue(object component)
 {
 bool result = _pd.ShouldSerializeValue(component);

 if (!result)
 {
 DefaultValueAttribute dva = (DefaultValueAttribute)_pd.Attributes[typeof(DefaultValueAttribute)];
 result = dva == null || !Equals(_pd.GetValue(component), dva.Value);
 }

 return result;
 }

 // The following Utility methods create a new AttributeCollection
 // by appending the specified attributes to an existing collection.
 public static AttributeCollection AppendAttributeCollection(
 AttributeCollection existing,
 params Attribute[] newAttrs) => new(AppendAttributes(existing, newAttrs));

 public static Attribute[] AppendAttributes(
 AttributeCollection existing,
 params Attribute[] newAttrs)
 {
 if (existing == null)
 {
 throw new ArgumentNullException(nameof(existing));
 }

 newAttrs ??= [];

 Attribute[] attributes;

 Attribute[] newArray = new Attribute[existing.Count + newAttrs.Length];
 int actualCount = existing.Count;
 existing.CopyTo(newArray, 0);

 for (int idx = 0; idx < newAttrs.Length; idx++)
 {
 if (newAttrs[idx] == null)
 {
 throw new ArgumentNullException(nameof(newAttrs));
 }

 // Check if this attribute is already in the existing
 // array. If it is, replace it.
 bool match = false;
 for (int existingIdx = 0; existingIdx < existing.Count; existingIdx++)
 {
 if (newArray[existingIdx].TypeId.Equals(newAttrs[idx].TypeId))
 {
 match = true;
 newArray[existingIdx] = newAttrs[idx];
 break;
 }
 }

 if (!match)
 {
 newArray[actualCount++] = newAttrs[idx];
 }
 }

 // If some attributes were collapsed, create a new array.
 if (actualCount < newArray.Length)
 {
 attributes = new Attribute[actualCount];
 Array.Copy(newArray, 0, attributes, 0, actualCount);
 }
 else
 {
 attributes = newArray;
 }

 return attributes;
 }
}

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 - 1
 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 SerializeReadOnlyPropertyDescriptor in a custom designer.

using System.Collections;
using System.ComponentModel;
using System.Windows.Forms.Design;

namespace ReadOnlyPropertyDescriptorTest;

class DemoControlDesigner : ControlDesigner
{
 // The PostFilterProperties method replaces the control's 
 // Size property with a read-only Size property by using 
 // the SerializeReadOnlyPropertyDescriptor class.
 protected override void PostFilterProperties(IDictionary properties)
 {
 if (properties.Contains("Size"))
 {
 PropertyDescriptor original = properties["Size"] as PropertyDescriptor;
 SerializeReadOnlyPropertyDescriptor readOnlyDescriptor =
 new(original);

 properties["Size"] = readOnlyDescriptor;
 }

 base.PostFilterProperties(properties);
 }
}

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

using System.ComponentModel;
using System.Windows.Forms;

namespace ReadOnlyPropertyDescriptorTest;

[Designer(typeof(DemoControlDesigner))]
public class DemoControl : Control
{
 public DemoControl()
 {
 }
}

Imports System.ComponentModel
Imports System.Windows.Forms

<Designer(GetType(DemoControlDesigner))>
Public Class DemoControl
 Inherits Control

 Public Sub New()

 End Sub
End Class

Remarks

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.

See also

• MemberDescriptor
• Attribute
• DefaultValueAttribute