PropertyDescriptor Class

Definition

Provides an abstraction of a property on a class.

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

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 = myProperty.DisplayName + '\n';
textBox1.Text += myProperty.Description + '\n';
textBox1.Text += myProperty.Category + '\n';

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;
        }
    }
}

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);
        }
    }
}
using System.ComponentModel;
using System.Windows.Forms;

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

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:

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, 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(MemberDescriptor)

Initializes a new instance of the PropertyDescriptor class with the name and attributes in the specified MemberDescriptor.

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.

ConverterFromRegisteredType

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.

FillAttributes(IList)

When overridden in a derived class, adds the attributes of the inheriting class to the specified list of attributes in the parent class.

(Inherited from MemberDescriptor)
GetChildProperties()

Returns the default PropertyDescriptorCollection.

GetChildProperties(Attribute[])

Returns a PropertyDescriptorCollection using a specified array of attributes as a filter.

GetChildProperties(Object, Attribute[])

Returns a PropertyDescriptorCollection for a given object using a specified array of attributes as a filter.

GetChildProperties(Object)

Returns a PropertyDescriptorCollection for a given object.

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.

GetInvocationTarget(Type, Object)

Retrieves the object that should be used during invocation of members.

(Inherited from MemberDescriptor)
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)

Applies to

Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7 (package-provided), 4.7, 4.7.1 (package-provided), 4.7.1, 4.7.2 (package-provided), 4.7.2, 4.8 (package-provided), 4.8, 4.8.1
.NET Standard 2.0, 2.1

See also