Content update - Safe constructor patterns for DependencyObjects (user story 1878471) (#1254)

* Add article, code projects, and redirects * Further edits * Update dotnet-desktop-guide/net/wpf/properties/safe-constructor-patterns-for-dependencyobjects.md Co-authored-by: Andy (Steve) De George <[email protected]>
2026-09-06 23:43:07 +02:00 · 2022-01-04 14:35:40 -08:00
parent 71675bec48
commit 96056b1cdf
18 changed files with 768 additions and 0 deletions
@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Configuration;
+using System.Data;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+    }
+}
@@ -0,0 +1,10 @@
+using System.Windows;
+
+[assembly: ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+                                     //(used if a resource is not found in the page,
+                                     // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+                                              //(used if a resource is not found in the page,
+                                              // app, or any theme specific resource dictionaries)
+)]
@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net6.0-windows</TargetFramework>
+    <UseWPF>true</UseWPF>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Update="Properties\Resources.Designer.cs">
+      <DesignTime>True</DesignTime>
+      <AutoGen>True</AutoGen>
+      <DependentUpon>Resources.resx</DependentUpon>
+    </Compile>
+  </ItemGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Update="Properties\Resources.resx">
+      <Generator>ResXFileCodeGenerator</Generator>
+      <LastGenOutput>Resources.Designer.cs</LastGenOutput>
+    </EmbeddedResource>
+  </ItemGroup>
+
+</Project>
@@ -0,0 +1,9 @@
+<Window x:Class="CodeSampleCsharp.MainWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Safe constructor patterns for DependencyObjects" Height="100" Width="400">
+
+    <StackPanel>
+
+    </StackPanel>
+</Window>
@@ -0,0 +1,181 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Windows;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for MainWindow.xaml.
+    /// </summary>
+    public partial class MainWindow : Window
+    {
+        public MainWindow()
+        {
+            InitializeComponent();
+            TestUnsafeConstructorPattern();
+        }
+
+        //<TestUnsafeConstructorPattern>
+        private static void TestUnsafeConstructorPattern()
+        {
+            //Aquarium aquarium = new();
+            //Debug.WriteLine($"Aquarium temperature (C): {aquarium.TempCelcius}");
+
+            // Instantiate and set tropical aquarium temperature.
+            TropicalAquarium tropicalAquarium = new(tempCelcius: 25);
+            Debug.WriteLine($"Tropical aquarium temperature (C): " +
+                $"{tropicalAquarium.TempCelcius}");
+
+            /* Test output:
+            Derived class static constructor running.
+            Base class ValidateValueCallback running.
+            Base class ValidateValueCallback running.
+            Base class ValidateValueCallback running.
+            Base class parameterless constructor running.
+            Base class ValidateValueCallback running.
+            Derived class CoerceValueCallback running.
+            Derived class CoerceValueCallback: null reference exception.
+            Derived class OnPropertyChanged event running.
+            Derived class OnPropertyChanged event: null reference exception.
+            Derived class PropertyChangedCallback running.
+            Derived class PropertyChangedCallback: null reference exception.
+            Aquarium temperature (C): 20
+            Derived class parameterless constructor running.
+            Derived class parameter constructor running.
+            Base class ValidateValueCallback running.
+            Derived class CoerceValueCallback running.
+            Derived class OnPropertyChanged event running.
+            Derived class PropertyChangedCallback running.
+            Tropical aquarium temperature (C): 25
+            */
+        }
+    }
+
+    public class Aquarium : DependencyObject
+    {
+        // Register a dependency property with the specified property name,
+        // property type, owner type, property metadata with default value,
+        // and validate-value callback.
+        public static readonly DependencyProperty TempCelciusProperty =
+            DependencyProperty.Register(
+                name: "TempCelcius",
+                propertyType: typeof(int),
+                ownerType: typeof(Aquarium),
+                typeMetadata: new PropertyMetadata(defaultValue: 0),
+                validateValueCallback: 
+                    new ValidateValueCallback(ValidateValueCallback));
+
+        // Parameterless constructor.
+        public Aquarium()
+        {
+            Debug.WriteLine("Base class parameterless constructor running.");
+
+            // Set typical aquarium temperature.
+            TempCelcius = 20;
+
+            Debug.WriteLine($"Aquarium temperature (C): {TempCelcius}");
+        }
+
+        // Declare public read-write accessors.
+        public int TempCelcius
+        {
+            get => (int)GetValue(TempCelciusProperty);
+            set => SetValue(TempCelciusProperty, value);
+        }
+
+        // Validate-value callback.
+        public static bool ValidateValueCallback(object value)
+        {
+            Debug.WriteLine("Base class ValidateValueCallback running.");
+            double val = (int)value;
+            return val >= 0;
+        }
+    }
+
+    public class TropicalAquarium : Aquarium
+    {
+        // Class field.
+        private static List<int> s_temperatureLog;
+
+        // Static constructor.
+        static TropicalAquarium()
+        {
+            Debug.WriteLine("Derived class static constructor running.");
+
+            // Create a new metadata instance with callbacks specified.
+            PropertyMetadata newPropertyMetadata = new(
+                defaultValue: 0,
+                propertyChangedCallback: new PropertyChangedCallback(PropertyChangedCallback),
+                coerceValueCallback: new CoerceValueCallback(CoerceValueCallback));
+
+            // Call OverrideMetadata on the dependency property identifier.
+            TempCelciusProperty.OverrideMetadata(
+                forType: typeof(TropicalAquarium),
+                typeMetadata: newPropertyMetadata);
+        }
+
+        // Parameterless constructor.
+        public TropicalAquarium()
+        {
+            Debug.WriteLine("Derived class parameterless constructor running.");
+            s_temperatureLog = new List<int>();
+        }
+
+        // Parameter constructor.
+        public TropicalAquarium(int tempCelcius) : this()
+        {
+            Debug.WriteLine("Derived class parameter constructor running.");
+            TempCelcius = tempCelcius;
+            s_temperatureLog.Add(tempCelcius);
+        }
+
+        // Property-changed callback.
+        private static void PropertyChangedCallback(DependencyObject depObj, 
+            DependencyPropertyChangedEventArgs e)
+        {
+            Debug.WriteLine("Derived class PropertyChangedCallback running.");
+            try
+            {
+                s_temperatureLog.Add((int)e.NewValue);
+            }
+            catch (NullReferenceException)
+            {
+                Debug.WriteLine("Derived class PropertyChangedCallback: null reference exception.");
+            }
+        }
+
+        // Coerce-value callback.
+        private static object CoerceValueCallback(DependencyObject depObj, object value)
+        {
+            Debug.WriteLine("Derived class CoerceValueCallback running.");
+            try
+            {
+                s_temperatureLog.Add((int)value);
+            }
+            catch (NullReferenceException)
+            {
+                Debug.WriteLine("Derived class CoerceValueCallback: null reference exception.");
+            }
+            return value;
+        }
+
+        // OnPropertyChanged event.
+        protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e)
+        {
+            Debug.WriteLine("Derived class OnPropertyChanged event running.");
+            try
+            {
+                s_temperatureLog.Add((int)e.NewValue);
+            }
+            catch (NullReferenceException)
+            {
+                Debug.WriteLine("Derived class OnPropertyChanged event: null reference exception.");
+            }
+
+            // Mandatory call to base implementation.
+            base.OnPropertyChanged(e);
+        }
+    }
+    //</TestUnsafeConstructorPattern>
+}
@@ -0,0 +1,63 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//     Runtime Version:4.0.30319.42000
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace CodeSampleCsharp.Properties {
+    using System;
+    
+    
+    /// <summary>
+    ///   A strongly-typed resource class, for looking up localized strings, etc.
+    /// </summary>
+    // This class was auto-generated by the StronglyTypedResourceBuilder
+    // class via a tool like ResGen or Visual Studio.
+    // To add or remove a member, edit your .ResX file then rerun ResGen
+    // with the /str option, or rebuild your VS project.
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "16.0.0.0")]
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+    internal class Resources {
+        
+        private static global::System.Resources.ResourceManager resourceMan;
+        
+        private static global::System.Globalization.CultureInfo resourceCulture;
+        
+        [global::System.Diagnostics.CodeAnalysis.SuppressMessageAttribute("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
+        internal Resources() {
+        }
+        
+        /// <summary>
+        ///   Returns the cached ResourceManager instance used by this class.
+        /// </summary>
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Advanced)]
+        internal static global::System.Resources.ResourceManager ResourceManager {
+            get {
+                if (object.ReferenceEquals(resourceMan, null)) {
+                    global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("CodeSampleCsharp.Properties.Resources", typeof(Resources).Assembly);
+                    resourceMan = temp;
+                }
+                return resourceMan;
+            }
+        }
+        
+        /// <summary>
+        ///   Overrides the current thread's CurrentUICulture property for all
+        ///   resource lookups using this strongly typed resource class.
+        /// </summary>
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Advanced)]
+        internal static global::System.Globalization.CultureInfo Culture {
+            get {
+                return resourceCulture;
+            }
+            set {
+                resourceCulture = value;
+            }
+        }
+    }
+}
@@ -0,0 +1,120 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <!-- 
+    Microsoft ResX Schema 
+    
+    Version 2.0
+    
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
+    associated with the data types.
+    
+    Example:
+    
+    ... ado.net/XML headers & schema ...
+    <resheader name="resmimetype">text/microsoft-resx</resheader>
+    <resheader name="version">2.0</resheader>
+    <resheader name="reader">System.Resources.ResXResourceReader, System.Windows.Forms, ...</resheader>
+    <resheader name="writer">System.Resources.ResXResourceWriter, System.Windows.Forms, ...</resheader>
+    <data name="Name1"><value>this is my long string</value><comment>this is a comment</comment></data>
+    <data name="Color1" type="System.Drawing.Color, System.Drawing">Blue</data>
+    <data name="Bitmap1" mimetype="application/x-microsoft.net.object.binary.base64">
+        <value>[base64 mime encoded serialized .NET Framework object]</value>
+    </data>
+    <data name="Icon1" type="System.Drawing.Icon, System.Drawing" mimetype="application/x-microsoft.net.object.bytearray.base64">
+        <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
+        <comment>This is a comment</comment>
+    </data>
+                
+    There are any number of "resheader" rows that contain simple 
+    name/value pairs.
+    
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
+    mimetype set.
+    
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
+    extensible. For a given mimetype the value must be set accordingly:
+    
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
+    read any of the formats listed below.
+    
+    mimetype: application/x-microsoft.net.object.binary.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
+            : and then encoded with base64 encoding.
+    
+    mimetype: application/x-microsoft.net.object.soap.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.bytearray.base64
+    value   : The object must be serialized into a byte array 
+            : using a System.ComponentModel.TypeConverter
+            : and then encoded with base64 encoding.
+    -->
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+</root>
@@ -0,0 +1,9 @@
+<Application x:Class="CodeSampleCsharp.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:CodeSampleCsharp"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+         
+    </Application.Resources>
+</Application>
@@ -0,0 +1,9 @@
+<Application x:Class="Application"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:CodeSampleVb"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+         
+    </Application.Resources>
+</Application>
@@ -0,0 +1,6 @@
+Class Application
+
+    ' Application-level events, such as Startup, Exit, and DispatcherUnhandledException
+    ' can be handled in this file.
+
+End Class
@@ -0,0 +1,11 @@
+Imports System.Windows
+
+'The ThemeInfo attribute describes where any theme specific and generic resource dictionaries can be found.
+'1st parameter: where theme specific resource dictionaries are located
+'(used if a resource is not found in the page,
+' or application resource dictionaries)
+
+'2nd parameter: where the generic resource dictionary is located
+'(used if a resource is not found in the page,
+'app, and any theme specific resource dictionaries)
+<Assembly: ThemeInfo(ResourceDictionaryLocation.None, ResourceDictionaryLocation.SourceAssembly)>
@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net6.0-windows</TargetFramework>
+    <RootNamespace>CodeSampleVb</RootNamespace>
+    <UseWPF>true</UseWPF>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Import Include="System.Windows" />
+    <Import Include="System.Windows.Controls" />
+    <Import Include="System.Windows.Data" />
+    <Import Include="System.Windows.Documents" />
+    <Import Include="System.Windows.Input" />
+    <Import Include="System.Windows.Media" />
+    <Import Include="System.Windows.Media.Imaging" />
+    <Import Include="System.Windows.Navigation" />
+    <Import Include="System.Windows.Shapes" />
+  </ItemGroup>
+
+</Project>
@@ -0,0 +1,9 @@
+<Window x:Class="CodeSampleVb.MainWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Safe constructor patterns for DependencyObjects" Height="100" Width="400">
+
+    <StackPanel>
+
+    </StackPanel>
+</Window>
@@ -0,0 +1,173 @@
+Namespace CodeSampleVb
+
+    ' <summary>
+    ' Interaction logic for MainWindow.xaml.
+    ' </summary>
+    Partial Public Class MainWindow
+        Inherits Window
+
+        Public Sub New()
+            InitializeComponent()
+            TestUnsafeConstructorPattern()
+        End Sub
+
+        '<TestUnsafeConstructorPattern>
+        Private Shared Sub TestUnsafeConstructorPattern()
+            'Aquarium aquarium = new Aquarium();
+            'Debug.WriteLine($"Aquarium temperature (C): {aquarium.TempCelcius}");
+
+            ' Instantiate And set tropical aquarium temperature.
+            Dim tropicalAquarium As New TropicalAquarium(tempCelc:=25)
+            Debug.WriteLine($"Tropical aquarium temperature (C): 
+                {tropicalAquarium.TempCelcius}")
+
+            ' Test output:
+            ' Derived class static constructor running.
+            ' Base class ValidateValueCallback running.
+            ' Base class ValidateValueCallback running.
+            ' Base class ValidateValueCallback running.
+            ' Base class parameterless constructor running.
+            ' Base class ValidateValueCallback running.
+            ' Derived class CoerceValueCallback running.
+            ' Derived class CoerceValueCallback: null reference exception.
+            ' Derived class OnPropertyChanged event running.
+            ' Derived class OnPropertyChanged event: null reference exception.
+            ' Derived class PropertyChangedCallback running.
+            ' Derived class PropertyChangedCallback: null reference exception.
+            ' Aquarium temperature(C):  20
+            ' Derived class parameterless constructor running.
+            ' Derived class parameter constructor running.
+            ' Base class ValidateValueCallback running.
+            ' Derived class CoerceValueCallback running.
+            ' Derived class OnPropertyChanged event running.
+            ' Derived class PropertyChangedCallback running.
+            ' Tropical Aquarium temperature (C): 25
+
+        End Sub
+    End Class
+
+    Public Class Aquarium
+        Inherits DependencyObject
+
+        'Register a dependency property with the specified property name,
+        ' property type, owner type, property metadata with default value,
+        ' and validate-value callback.
+        Public Shared ReadOnly TempCelciusProperty As DependencyProperty =
+            DependencyProperty.Register(
+            name:="TempCelcius",
+            propertyType:=GetType(Integer),
+            ownerType:=GetType(Aquarium),
+            typeMetadata:=New PropertyMetadata(defaultValue:=0),
+            validateValueCallback:=
+                New ValidateValueCallback(AddressOf ValidateValueCallback))
+
+        ' Parameterless constructor.
+        Public Sub New()
+            Debug.WriteLine("Base class parameterless constructor running.")
+
+            ' Set typical aquarium temperature.
+            TempCelcius = 20
+
+            Debug.WriteLine($"Aquarium temperature (C): {TempCelcius}")
+        End Sub
+
+        ' Declare public read-write accessors.
+        Public Property TempCelcius As Integer
+            Get
+                Return GetValue(TempCelciusProperty)
+            End Get
+            Set(value As Integer)
+                SetValue(TempCelciusProperty, value)
+            End Set
+        End Property
+
+        ' Validate-value callback.
+        Public Shared Function ValidateValueCallback(value As Object) As Boolean
+            Debug.WriteLine("Base class ValidateValueCallback running.")
+            Dim val As Double = CInt(value)
+            Return val >= 0
+        End Function
+
+    End Class
+
+    Public Class TropicalAquarium
+        Inherits Aquarium
+
+        ' Class field.
+        Private Shared s_temperatureLog As List(Of Integer)
+
+        ' Static constructor.
+        Shared Sub New()
+            Debug.WriteLine("Derived class static constructor running.")
+
+            ' Create a new metadata instance with callbacks specified.
+            Dim newPropertyMetadata As New PropertyMetadata(
+                    defaultValue:=0,
+                    propertyChangedCallback:=
+                        New PropertyChangedCallback(AddressOf PropertyChangedCallback),
+                    coerceValueCallback:=
+                        New CoerceValueCallback(AddressOf CoerceValueCallback))
+
+            ' Call OverrideMetadata on the dependency property identifier.
+            TempCelciusProperty.OverrideMetadata(
+                    forType:=GetType(TropicalAquarium),
+                    typeMetadata:=newPropertyMetadata)
+        End Sub
+
+        ' Parameterless constructor.
+        Public Sub New()
+            Debug.WriteLine("Derived class parameterless constructor running.")
+            s_temperatureLog = New List(Of Integer)()
+        End Sub
+
+        ' Parameter constructor.
+        Public Sub New(tempCelc As Integer)
+            Me.New()
+            Debug.WriteLine("Derived class parameter constructor running.")
+            TempCelcius = tempCelc
+            s_temperatureLog.Add(TempCelcius)
+        End Sub
+
+        ' Property-changed callback.
+        Private Shared Sub PropertyChangedCallback(depObj As DependencyObject,
+            e As DependencyPropertyChangedEventArgs)
+            Debug.WriteLine("Derived class PropertyChangedCallback running.")
+
+            Try
+                s_temperatureLog.Add(e.NewValue)
+            Catch ex As NullReferenceException
+                Debug.WriteLine("Derived class PropertyChangedCallback: null reference exception.")
+            End Try
+        End Sub
+
+        ' Coerce-value callback.
+        Private Shared Function CoerceValueCallback(depObj As DependencyObject, value As Object) As Object
+            Debug.WriteLine("Derived class CoerceValueCallback running.")
+
+            Try
+                s_temperatureLog.Add(value)
+            Catch ex As NullReferenceException
+                Debug.WriteLine("Derived class CoerceValueCallback: null reference exception.")
+            End Try
+
+            Return value
+        End Function
+
+        ' OnPropertyChanged event.
+        Protected Overrides Sub OnPropertyChanged(e As DependencyPropertyChangedEventArgs)
+            Debug.WriteLine("Derived class OnPropertyChanged event running.")
+
+            Try
+                s_temperatureLog.Add(e.NewValue)
+            Catch ex As NullReferenceException
+                Debug.WriteLine("Derived class OnPropertyChanged event: null reference exception.")
+            End Try
+
+            ' Mandatory call to base implementation.
+            MyBase.OnPropertyChanged(e)
+        End Sub
+
+    End Class
+    '</TestUnsafeConstructorPattern>
+
+End Namespace