If you have an interface class object and you implement several types of the object, what can you do when your object has no handler? That’s where the Null Object pattern comes into play.

Here’s an example of how you could implement the Null Object Design Pattern in VBA to detect whether a file type processor based on a processing interface was found:

Step 1: Define the Processing Interface (Put in a class module called IProcessor)

' IProcessor Interface
' Public Interface IProcessor 
    Sub ProcessFile(ByVal filePath As String) 
' End Interface 

Step 2: Implement Concrete Processors (Put in two classes, TextFileProcessor and ImageProcessor respectively)

' TextFileProcessor Clas
'Public Class TextFileProcessor 
    Implements IProcessor 

    Public Sub ProcessFile(ByVal filePath As String) Implements IProcessor.ProcessFile 
        ' Implementation for processing text files
        MsgBox  "Processing text file: " & filePath 
    End Sub 
' End Class 

' ImageFileProcessor Class
' Public Class ImageFileProcessor  
    Implements IProcessor 

    Public Sub ProcessFile(ByVal filePath As String) Implements IProcessor.ProcessFile 
        ' Implementation for processing image files
        MsgBox  "Processing image file: " & filePath 
    End Sub 
' End Class 

Step 3: Implement the Null Object (Put in a class module called NullProcessor)

' NullProcessor Class
'Public Class NullProcessor 
    Implements IProcessor 

    Public Sub ProcessFile(ByVal filePath As String) Implements IProcessor.ProcessFile 
        ' Do nothing
    End Sub 
'End Class 

Step 4: Factory Method to Get the Appropriate Processor (Put in a class module called ProcessorFactory)

' ProcessorFactory Class
'Public Class ProcessorFactory
    Public Function GetProcessor(ByVal fileType As String) As IProcessor 
        Select Case fileType 
            Case  "txt"
                Set GetProcessor = New TextFileProcessor 
            Case  "jpg"
                Set GetProcessor = New ImageFileProcessor 
            Case Else 
                Set GetProcessor = New NullProcessor 
        End Select 
    End Function 
'End Class

Step 5: Usage Example (Put in a normal module)

Sub TestProcessor() 
    Dim factory As ProcessorFactory 
    Set factory = New ProcessorFactory 
    
    Dim processor As IProcessor 
    Set processor = factory.GetProcessor( "txt") 
    processor.ProcessFile  "example.txt"
    
    Set processor = factory.GetProcessor( "unknown") 
    processor.ProcessFile  "example.unknown"
    
    ' Check if processor is NullProcessor
    If TypeName(processor) =  "NullProcessor" Then 
        MsgBox  "Processor is NullProcessor"
    Else 
        MsgBox  "Processor is not NullProcessor"
    End If 
End Sub 

In this example, the ProcessorFactory class returns the appropriate processor based on the file type. If the file type is not recognized, it returns a NullProcessor that does nothing. This way, you avoid null checks and simplify the code. Note that you can use TypeName to get the actual type name to check to see if it is the null type if you need to. Also you can use TypeIs NullProcessor as another alternative.