Visual Basic

<DllImport("winmm.dll")> _
Private Shared Function mciSendString(ByVal strCommand As String, ByVal strReturn As StringBuilder, ByVal iReturnLength As Integer, ByVal hwndCallback As IntPtr) As Long
    End Function 

Private Sub FFTAudio(ByVal RealIn() As Integer, ByVal RealOut() As Single)

<DllImport("winmm.dll")> _
    Private Shared Function mciSendString(ByVal strCommand As String, ByVal strReturn As StringBuilder, ByVal iReturnLength As Integer, ByVal hwndCallback As IntPtr) As Long
    End Function

Private Sub FFTAudio(ByVal RealIn() As Integer, ByVal RealOut() As Single)

Option Explicit

Private Sub btnTest_Click()
    Dim strTest(4) As String
    strTest(1) = "abc"
    strTest(2) = "def"
    strTest(3) = "ghi"
    strTest(4) = "jkl"
    
    On Error GoTo err1
        
    Dim clsTest As ComTest.clsValues
    Set clsTest = New ComTest.clsValues
    
    clsTest.SingleString = "this is my string"
    clsTest.stringArray = strTest   ' <<<ERROR IS HERE
    
    MsgBox "Single string = " & clsTest.SingleString, vbInformation, "Test"
    
    Dim x As Integer
    
    For x = 0 To UBound(clsTest.StringArray)
        MsgBox "Multi string 2 = " & clsTest.StringArray(x), vbInformation, "Test"
    Next x
    
    Exit Sub
    
: err1
    MsgBox "Error: " & Err.Number & vbCrLf & Err.Description, vbCritical, "btnTest_Click"
    On Error Resume Next
    
End Sub

Imports System.Runtime.InteropServices

<ComClass(clsValues.ClassId, clsValues.InterfaceId, clsValues.EventsId)> _
Public Class clsValues

#Region "COM GUIDs"
    ' These  GUIDs provide the COM identity for this class 
    ' and its COM interfaces. If you change them, existing 
    ' clients will no longer be able to access the class.
    Public Const ClassId As String = "095dc64d-141f-46f9-9af2-1c5e633b459b"
    Public Const InterfaceId As String = "86ec1f14-1ef4-446d-9172-b92cf260cd16"
    Public Const EventsId As String = "1605e412-8b11-4ee0-a34c-9ec04464737f"
#End Region

    ' A creatable COM class must have a Public Sub New() 
    ' with no parameters, otherwise, the class will not be 
    ' registered in the COM registry and cannot be created 
    ' via CreateObject.
    Public Sub New()
        MyBase.New()
    End Sub

    Private mSingleString As String
    Public Property SingleString() As String
        Get
            Return mSingleString
        End Get
        Set(ByVal value As String)
            mSingleString = value
        End Set
    End Property

    Private mStringArray() As String
    Public Property StringArray() As <MarshalAs(UnmanagedType.SafeArray, safearraysubtype:=VarEnum.VT_BSTR)> String()
        Get
            Return mStringArray
        End Get
        Set(<MarshalAs(UnmanagedType.SafeArray, safearraysubtype:=VarEnum.VT_BSTR)> ByVal value As String())
            mStringArray = value
        End Set
    End Property

End Class