PropertyDescriptor Class

Definition

Namespace:

System.ComponentModel

Assembly:

System.ComponentModel.TypeConverter.dll

Source:

PropertyDescriptor.cs

Provides an abstraction of a property on a class.

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

public abstract class PropertyDescriptor : System.ComponentModel.MemberDescriptor

type PropertyDescriptor = class
    inherit MemberDescriptor

Public MustInherit Class PropertyDescriptor
Inherits MemberDescriptor

Inheritance

Object

MemberDescriptor

PropertyDescriptor

Derived

System.ComponentModel.DependencyPropertyDescriptor

System.ComponentModel.TypeConverter.SimplePropertyDescriptor

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.
System.ComponentModel.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 & 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.

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

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

        public SerializeReadOnlyPropertyDescriptor(PropertyDescriptor pd)
            : base(pd)
        {
            this._pd = pd;
        }

        public override AttributeCollection Attributes
        {
            get
            {
                return( AppendAttributeCollection(
                    this._pd.Attributes, 
                    ReadOnlyAttribute.Yes) );
            }
        }

        protected override void FillAttributes(IList attributeList)
        {
            attributeList.Add(ReadOnlyAttribute.Yes);
        }

        public override Type ComponentType
        {
            get
            {
                return this._pd.ComponentType;
            }
        }

        // The type converter for this property.
        // A translator can overwrite with its own converter.
        public override TypeConverter Converter
        {
            get
            {
                return this._pd.Converter;
            }
        }

        // Returns the property editor 
        // A translator can overwrite with its own editor.
        public override object GetEditor(Type editorBaseType)
        {
            return this._pd.GetEditor(editorBaseType);
        }

        // Specifies the property is read only.
        public override bool IsReadOnly
        {
            get
            {
                return true;
            }
        }

        public override Type PropertyType
        {
            get
            {
                return this._pd.PropertyType;
            }
        }

        public override bool CanResetValue(object component)
        {
            return this._pd.CanResetValue(component);
        }

        public override object GetValue(object component)
        {
            return this._pd.GetValue(component);
        }

        public override void ResetValue(object component)
        {
            this._pd.ResetValue(component);
        }

        public override void SetValue(object component, object val)
        {
            this._pd.SetValue(component, val);
        }

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

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

            return result;
        }

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

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

            newAttrs ??= new Attribute[0];

            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("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 SerializeReadOnlyPropertyDescriptor(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.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

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.

Constructors

PropertyDescriptor(MemberDescriptor)	Initializes a new instance of the PropertyDescriptor class with the name and attributes in the specified MemberDescriptor.
PropertyDescriptor(MemberDescriptor, Attribute[])	Initializes a new instance of the PropertyDescriptor class with the name in the specified MemberDescriptor and the attributes in both the MemberDescriptor and the Attribute array.
PropertyDescriptor(String, Attribute[])	Initializes a new instance of the PropertyDescriptor class with the specified name and attributes.

Properties

AttributeArray	Gets or sets an array of attributes. (Inherited from MemberDescriptor)
Attributes	Gets the collection of attributes for this member. (Inherited from MemberDescriptor)
Category	Gets the name of the category to which the member belongs, as specified in the CategoryAttribute. (Inherited from MemberDescriptor)
ComponentType	When overridden in a derived class, gets the type of the component this property is bound to.
Converter	Gets the type converter for this property.
Description	Gets the description of the member, as specified in the DescriptionAttribute. (Inherited from MemberDescriptor)
DesignTimeOnly	Gets whether this member should be set only at design time, as specified in the DesignOnlyAttribute. (Inherited from MemberDescriptor)
DisplayName	Gets the name that can be displayed in a window, such as a Properties window. (Inherited from MemberDescriptor)
IsBrowsable	Gets a value indicating whether the member is browsable, as specified in the BrowsableAttribute. (Inherited from MemberDescriptor)
IsLocalizable	Gets a value indicating whether this property should be localized, as specified in the LocalizableAttribute.
IsReadOnly	When overridden in a derived class, gets a value indicating whether this property is read-only.
Name	Gets the name of the member. (Inherited from MemberDescriptor)
NameHashCode	Gets the hash code for the name of the member, as specified in GetHashCode(). (Inherited from MemberDescriptor)
PropertyType	When overridden in a derived class, gets the type of the property.
SerializationVisibility	Gets a value indicating whether this property should be serialized, as specified in the DesignerSerializationVisibilityAttribute.
SupportsChangeEvents	Gets a value indicating whether value change notifications for this property may originate from outside the property descriptor.

Methods

AddValueChanged(Object, EventHandler)	Enables other objects to be notified when this property changes.
CanResetValue(Object)	When overridden in a derived class, returns whether resetting an object changes its value.
CreateAttributeCollection()	Creates a collection of attributes using the array of attributes passed to the constructor. (Inherited from MemberDescriptor)
CreateInstance(Type)	Creates an instance of the specified type.
Equals(Object)	Compares this to another object to see if they are equivalent.
FillAttributes(IList)	Adds the attributes of the PropertyDescriptor to the specified list of attributes in the parent class.
GetChildProperties()	Returns the default PropertyDescriptorCollection.
GetChildProperties(Attribute[])	Returns a PropertyDescriptorCollection using a specified array of attributes as a filter.
GetChildProperties(Object)	Returns a PropertyDescriptorCollection for a given object.
GetChildProperties(Object, Attribute[])	Returns a PropertyDescriptorCollection for a given object using a specified array of attributes as a filter.
GetEditor(Type)	Gets an editor of the specified type.
GetHashCode()	Returns the hash code for this object.
GetInvocationTarget(Type, Object)	This method returns the object that should be used during invocation of members.
GetType()	Gets the Type of the current instance. (Inherited from Object)
GetTypeFromName(String)	Returns a type using its name.
GetValue(Object)	When overridden in a derived class, gets the current value of the property on a component.
GetValueChangedHandler(Object)	Retrieves the current set of ValueChanged event handlers for a specific component.
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
OnValueChanged(Object, EventArgs)	Raises the ValueChanged event that you implemented.
RemoveValueChanged(Object, EventHandler)	Enables other objects to be notified when this property changes.
ResetValue(Object)	When overridden in a derived class, resets the value for this property of the component to the default value.
SetValue(Object, Object)	When overridden in a derived class, sets the value of the component to a different value.
ShouldSerializeValue(Object)	When overridden in a derived class, determines a value indicating whether the value of this property needs to be persisted.
ToString()	Returns a string that represents the current object. (Inherited from Object)

See also

• MemberDescriptor
• Attribute
• DefaultValueAttribute