Option Explicit
 
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" ( _
  Destination As Any, _
Source As Any, ByVal Length As Long)
 
Private Type LngArray
    Byte0 As Byte
    Byte256 As Byte
    Byte64K As Byte
    Byte16M As Byte
End Type
 
Public Function ConvertULToString(ByVal UL As Long) As String
 
    Dim lng As LngArray
    Dim Buffer As Byte
 
    Call CopyMemory(lng, UL, 4)
    Buffer = lng.Byte0
    lng.Byte0 = lng.Byte16M
    lng.Byte16M = Buffer
    Buffer = lng.Byte256
    lng.Byte256 = lng.Byte64K
    lng.Byte64K = Buffer
    ConvertULToString = Space(4)
    Call CopyMemory(ByVal ConvertULToString, lng, 4)
 
End Function
 
Public Function ConvertStringToUL(ByVal Str As String) As Long
 
    Dim lng As LngArray
 
    Call ReverseLongString(Str)
    Call CopyMemory(lng, ByVal Str, 4)
    Call CopyMemory(ConvertStringToUL, lng, 4)
 
End Function
 
Public Function ConvertDoubleToUL(ByVal Dbl As Double) As Long
 
    Dim Max As Double
 
    Max = (CDbl(&H7FFFFFFF) + 1) * 2
    If Dbl > Max Then
        ' No Conversion possible
    Else
        If Dbl < Max / 2 Then
            ConvertDoubleToUL = CLng(Dbl)
        Else
            ConvertDoubleToUL = Dbl - Max
        End If
    End If
 
End Function
 
Public Function ConvertULToDouble(ByVal UL As Long) As Double
 
    If UL < 0 Then
        ConvertULToDouble = (CDbl(&H7FFFFFFF) + 1) * 2 + UL
    Else
        ConvertULToDouble = UL
    End If
 
End Function
 
Private Sub ReverseLongString(ByRef Str As String)
 
    Dim lng As LngArray
 
    Call CopyMemory(lng, ByVal Str, 4)
    Str = Chr(lng.Byte16M) & Chr(lng.Byte64K) & Chr(lng.Byte256) & Chr( _
      lng.Byte0)
 
End Sub

Dim Str As String
Dim LongVal As Long
Dim Dbl As Double
 
Str = "ÁÎ>Õ"
LongVal = ConvertStringToUL(Str)
Dbl = ConvertULToDouble(LongVal)
 
Dbl = 3251519189# ' Dbl hat bereits diesen Wert (nur zu Verdeutlichung)
LongVal = ConvertDoubleToUL(Dbl)
Str = ConvertULToString(LongVal)