You can call PropertyChange of Greetings from setter of  FirstName and LastName



    Public Property FirstName() As String
        Get
            Return _firstName
        End Get
        Set(ByVal value As String)
            _firstName = value
            OnPropertyChanged("FirstName")
            OnPropertyChanged("Greeting")
        End Set
    End Property

OR 
=

You can subscribe to `PropertyChanged` of your ViewModel in itself 



    AddHandler this.PropertyChanged, AddressOf PropertyChanged

and in `PropertyChanged` you can check which property is changed depending on that you can RaisePropertyChanged for Greeting

Let's say I have a property called `Greeting` that is principally composed of two bound properties: `LastName` and `FirstName`.  Can I subscribe to updates on first and last name so I can force a refresh with `OnPropertyChanged()` for my Greeting property.  Here's a simple example:

**View**



    <TextBox Text="{Binding FirstName}" />
    <TextBox Text="{Binding LastName}" />
    <TextBlock Text="{Binding Greeting}" />

**ViewModel**



    Public Property FirstName() As String
        Get
            Return _firstName
        End Get
    	Set(ByVal value As String)
            _firstName = value
            OnPropertyChanged("FirstName")
        End Set
    End Property

    '... Omitting LastName for brevity ...

    Public ReadOnly Property Greeting() As String
        Get
            Return String.Format("Hello {0} {1}", Firstname, LastName)
        End Get
    End Property

The way this is currently set up, nothing will ever update the Greeting binding.  I could put `OnPropertyChanged("Greeting")` in the setter for `FirstName` and `LastName`, but this feels wrong.  In a more complex example, I'd rather each object just take care of refreshing itself when something changes.  

**Q:)** *Can I force an update for a `ReadOnly` Property when one of the Properties it's composed of changes?*