Unit testing INotifyPropertyChanged [tip]

Introduction

One common mistake I make in programming classes that implement INotifyPropertyChanged is to forget to raise the change event in the property setter.  Therefore I added this to the list of things I commonly unit test.

For example, given the following class that implements INotifyPropertyChanged:

Public NotInheritable Class AggregateModel
    Implements System.ComponentModel.INotifyPropertyChanged

    <DataMember(Name:=NameOf(Name))>
    Private ReadOnly m_name As String
    ''' <summary>
    ''' The unique name by which the event sourcing model is known
    ''' </summary>
    ''' <remarks>
    ''' This is used to refer to this model in code and also in the default filename
    ''' </remarks>
    <XmlAttribute(AttributeName:="AggregateName")>
    Public ReadOnly Property Name As String
        Get
            Return m_name
        End Get
    End Property


    <DataMember(Name:=NameOf(KeyName))>
    Private m_keyName As String = "Key"
    ''' <summary>
    ''' The name by which the property which provides the unique key for this aggregate is known
    ''' </summary>
    ''' <remarks>
    ''' This defaults to "Key" if there is no meaningful business key name
    ''' </remarks>
    <XmlAttribute(AttributeName:="KeyName")>
    Public Property KeyName As String
        Get
            Return m_keyName
        End Get
        Set(value As String)
            If Not (m_keyName.Equals(value)) Then
                m_keyName = value
                OnPropertyChanged(NameOf(KeyName))
            End If
        End Set
    End Property    
    
End Class    

A unit test would look like

<TestMethod> 
Public Sub Keyname_ChangeNotification_TestMethod() 
     
     Dim expected As String = "KeyName"
     Dim actual As String = "Not Set"
     
     Dim testObj As New AggregateModel("TestObject")
     
     AddHandler testObj.PropertyChanged, New PropertyChangedEventHandler( 
         Sub(ByVal sender As Object, ByVal e As PropertyChangedEventArgs) 
            actual = e.PropertyName 
        End Sub ) 
        
    testObj.KeyName = "My key"
    testObj = Nothing 
    
    Assert.AreEqual(expected, actual) 
    
End Sub