excel vba: Special Types - Functions as Arguments of Functions

Question

There is no special type for functions in VBA. It is hard for me to see how to add functions as arguments to functions in Excel VBA.

What I am trying to accomplish i

Answer 1

It is possible to pass a function as an argument, and get exactly the behaviour you're after, but it's a bit hacky. This is achieved by exploiting the default property of a class,

Create a class myFunction containing your function

Public Function sayHi(ByVal s As String) As String
    sayHi = "Hi " & s
End Function

Export it, and open the .cls file in a text editor

You should see somewhere in the file, a line that defines your function

Public Function sayHi(ByVal s As String) As String

Beneath it, add the line Attribute Value.VB_UserMemId = 0, so that now it looks like this:

Public Function sayHi(ByVal s As String) As String
Attribute Value.VB_UserMemId = 0
    sayHi = "Hi " & s
End Function

What you've done here is to mark sayHi as the default property of the class. Now you can call the function by the class object alone class(args), rather than class.function(args)

Save the amended file and import it into your VBA project

Test module

Sub test()
    Dim parameterFunction As Object         'this is what we'll be passing to our routine
    Set parameterFunction = New myFunction  'create instance of the function object
    MsgBox f(parameterFunction, "Greedo")
End Sub

Function f(ByVal g As Object, ByVal x As String) As String
        f = g(x)
End Function

*NB, this way you can pass any function; but you may instead specify ByVal g As IStringFunction to get only the subset with the correct interface as per Gary McGill's answer