Declare a 0-Length String Array in VBA - Impossible?

Question

Is it really not possible to declare a 0-length array in VBA? If I try this:

Dim lStringArr(-1) As String

I get a compile error saying range has no values. If I try to trick the compiler and redim at runtime like this:

ReDim lStringArr(-1)

I get a subscript out of range error.

I've varied the above around a bit but with no luck e.g.

Dim lStringArr(0 To -1) As String

Use Case

I want to convert a variant array to a string array. The variant array may be empty as it comes from the Keys property of a dictionary. The keys property gives back an array of variants. I want an array of strings to use in my code, as I have some functions for processing string arrays I'd like to use. Here's the conversion function I'm using. This throws a subscript out of range error due to lMaxIndex being = -1:

Public Function mVariantArrayToStringArray(pVariants() As Variant) As String()
    Dim lStringArr() As String
    Dim lMaxIndex As Long, lMinIndex As Long
    lMaxIndex = UBound(pVariants)
    lMinIndex = LBound(pVariants)
    ReDim lStringArr(lMaxIndex)
    Dim lVal As Variant
    Dim lIndex As Long
    For lIndex = lMinIndex To lMaxIndex
        lStringArr(lIndex) = pVariants(lIndex)
    Next
    mVariantArrayToStringArray = lStringArr
End Function

Hack

Return a singleton array containing an empty string. Note- this isn't what we want. We want an empty array- such that looping over it is like doing nothing. But a singleton array containing an empty string will often work e.g. if we later want to join all the strings together in the string array.

Public Function mVariantArrayToStringArray(pVariants() As Variant) As String()
    Dim lStringArr() As String
    Dim lMaxIndex As Long, lMinIndex As Long
    lMaxIndex = UBound(pVariants)
    lMinIndex = LBound(pVariants)
    If lMaxIndex < 0 Then
        ReDim lStringArr(1)
        lStringArr(1) = ""
    Else
        ReDim lStringArr(lMaxIndex)
    End If
    Dim lVal As Variant
    Dim lIndex As Long
    For lIndex = lMinIndex To lMaxIndex
        lStringArr(lIndex) = pVariants(lIndex)
    Next
    mVariantArrayToStringArray = lStringArr
End Function

Update since answer

Here is the function I'm using for converting a variant array to a string array. Comintern's solution seems more advanced and general, and I may switch to that one day if I'm still stuck coding in VBA:

Public Function mVariantArrayToStringArray(pVariants() As Variant) As String()
    Dim lStringArr() As String
    Dim lMaxIndex As Long, lMinIndex As Long
    lMaxIndex = UBound(pVariants)
    lMinIndex = LBound(pVariants)
    If lMaxIndex < 0 Then
        mVariantArrayToStringArray = Split(vbNullString)
    Else
        ReDim lStringArr(lMaxIndex)
    End If
    Dim lVal As Variant
    Dim lIndex As Long
    For lIndex = lMinIndex To lMaxIndex
        lStringArr(lIndex) = pVariants(lIndex)
    Next
    mVariantArrayToStringArray = lStringArr
End Function

Notes

• I use Option Explicit. This can't change as it safeguards the rest of the code in the module.

Answer 1

As noted in the comments, you can do this "natively" by calling Split on a vbNullString, as documented here:

expression - Required. String expression containing substrings and delimiters. If expression is a zero-length string(""), Split returns an empty array, that is, an array with no elements and no data.

If you need a more general solution (i.e., other data types, you can call the SafeArrayRedim function in oleaut32.dll directly and request that it re-dimensions the passed array to 0 elements. You do have to jump through a couple of hoops to get the base address of the array (this is due to a quirk of the VarPtr function).

In the module declarations section:

'Headers
Private Type SafeBound
    cElements As Long
    lLbound As Long
End Type

Private Const VT_BY_REF = &H4000&
Private Const PVDATA_OFFSET = 8

Private Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias _
    "RtlMoveMemory" (ByRef Destination As Any, ByRef Source As Any, _
    ByVal length As Long)

Private Declare Sub SafeArrayRedim Lib "oleaut32" (ByVal psa As LongPtr, _
    ByRef rgsabound As SafeBound)

---

The procedure - pass it an initialized array (any type) and it will remove all elements from it:

Private Sub EmptyArray(ByRef vbArray As Variant)
    Dim vtype As Integer
    CopyMemory vtype, vbArray, LenB(vtype)
    Dim lp As LongPtr
    CopyMemory lp, ByVal VarPtr(vbArray) + PVDATA_OFFSET, LenB(lp)
    If Not (vtype And VT_BY_REF) Then
        CopyMemory lp, ByVal lp, LenB(lp)
        Dim bound As SafeBound
        SafeArrayRedim lp, bound
    End If
End Sub

---

Sample usage:

Private Sub Testing()
    Dim test() As Long
    ReDim test(0)
    EmptyArray test
    Debug.Print LBound(test)    '0
    Debug.Print UBound(test)    '-1
End Sub