Are nested functions possible in VBA?

问题

I'm trying to clean up code by stripping parameters from a function within a private scope, like this:

Function complicatedFunction(x as Double, param1 as Double, param2 as Double)
    ...
End Function

Function mainActionHappensHere(L as Double, U as Double ...)
    Function cleaner(x)
        cleaner = complicatedFunction(x, L, U)
    End Function
    ...
    cleaner(x)                       'Many calls to this function
    ...
End Function

Is this possible? Compiler complains, "Expected End Function", since I'm beginning a function before ending the outer one. And Google is no help :( PS I can't define cleaner() outside of mainActionHappensHere(), since then the correct L and U won't get passed into it.

回答1:

VB.Net can do this, but I don't believe VBA can.

Two features that might help you simplify this code in other ways are overloaded functions or optional parameters. Here's an example using optional parameters:

Function complicatedFunction(x as Double, Optional param1 as Double = L, Optional param2 as Double = U) As Object
...
End Function

complicatedFunction(x)

However, L and U must be constants for this to work.

FWIW, and in case it turns out that you're really working with a VB.Net dialect, the VB.Net syntax looks like this:

Sub complicatedFunction(x as Double, param1 as Double, param2 as Double) 
    ...
End Sub

Function mainActionHappensHere(L as Double, U as Double ...)
    Dim cleaner As Func(Of Double, Object) = 
        Function(x) 
            Return complicatedFunction(x, L, U)
        End Function

    Dim y = cleaner(x)                       'Many calls to this function
    ...
End Function

回答2:

There are no nested functions in VB, either VBA or VB6 or VB.NET.

Limiting the scope to VBA, your options would be:

• Use GoSub, one of the oldest VB command, that is deprecated, frowned upon and has no upgrade equivalent in VB.NET:

  Function mainActionHappensHere(L as Double, U as Double ...)
      Dim ResultOfCleaner As Variant
      ...
      x = 5 : GoSub cleaner                       'Many calls to this function
      'Use ResultOfCleaner here
      ...
      x = 42 : GoSub cleaner                      'Many calls to this function
      'Use ResultOfCleaner here
      ...
      Exit Function
  
  cleaner:
      ResultOfCleaner = complicatedFunction(x, L, U)
      Return
  End Function
  

• Manually create a closure.

  Define a class that exposes L and U as fields or properties. Instantiate the class, set L and U once, then call function Cleaner, also defined in that class, that calls complicatedFunction with the stored L and U.

  Obviously this creates some overhead.

回答3:

You can use this syntax

Dim fSomeFunction As Func(Of String, Boolean) = Function(ByVal something As String) As Boolean
                                                                     Return True
                                                                 End Function

来源：https://stackoverflow.com/questions/23615581/are-nested-functions-possible-in-vba