Question

I am using VB.Net 4 and am successfully using a class with the following signature:

Sub Register(Of TMessage)(recipient As Object, action As
System.Action(Of TMessage))

I want to learn about lambdas in VB so I wanted to see if I can simplify my current code.

CURRENTLY: I have the following in the constructor of a class (simplified for clarity)

Public Sub New()
    Dim myAction As New Action(Of String)(AddressOf HandleMessage)
    Messenger.Default.Register(Of [String])(Me, myAction)            
End Sub

And later in the class I have the following:

Private Function HandleMessage(sMsg As String)
    If sMsg = "String Value" Then
        If Me._Selection.HasChanges Or Me._Selection.HasErrors Then
            Return True
        End If
        Return False
    Else
        Return False
    End If
End Function

QUESTION: Is there a way to simplify this into a lambda like in C# where I don’t have to declare the MyAction variable in the constructor, but just pass the string value to HandleMessage function “inline” with the register sub? (I hope that makes sense)

Answer 1

So your constructor code is equivalent to:

Messenger.[Default].Register(Of String)(Me,
    Function(sMsg)
        If sMsg = "String Value" Then
            If Me._Selection.HasChanges Or Me._Selection.HasErrors Then
                Return True
            End If
            Return False
        Else
            Return False
        End If
    End Function)

It’s worth mentioning though you don’t need to use lambdas just to get rid of your explicit delegate creation. This is perfectly legal too:

Messenger.[Default].Register(Of String)(Me, AddressOf HandleMessage)

There is a bit of strangeness though with your example: you’ve declared your Register method as taking an Action(Of TMessage) which means the function passed needs no return value. Your function however is returning a Boolean. Visual Basic here is doing the fancy “delegate relaxation” and is throwing away the return value. So all your Return True/Return False bits aren’t actually doing anything special.