How to treat group propertys of float as float?

Question

There is a group names "groupname" with a extra property
(Zusatzeigenschaft) named "X" of type float64. It's value is 
"1,5"
Try this script:
msgbox GroupPropGet(GroupIndexGet("groupname") ,"X")
msgbox CStr(GroupPropGet(GroupIndexGet("groupname") ,"X"))
msgbox CDbl(CStr(GroupPropGet(GroupIndexGet("groupname") ,"X")))
msgbox CDbl(GroupPropGet(GroupIndexGet("groupname") ,"X"))
The last two outputs are different and wrong!
I need a (SUD-)dialog to edit a propperty of type float. For this I
have to convert it to string and after editing to convert back  to
float.

Answer 1

I guess the problem is the decimalsign (dot / comma)
Try this little function
Option Explicit
msgbox GroupPropGet(GroupIndexGet("groupname") ,"X")
msgbox CStr(GroupPropGet(GroupIndexGet("groupname") ,"X"))
msgbox StringToFloat(CStr(GroupPropGet(GroupIndexGet("groupname") ,"X")))
'''Function to convert a string to a float value
Function StringToFloat(sInput)
  Dim sDecimalCharacter
  Dim sTempValue
  If IsNumeric(sInput) Then
    'Determine the decimalsign (which depends on the regional settings)
    sDecimalCharacter = Mid(1.1, 2, 1)
    If Instr(sInput, ".") > 0 Then
      sTempValue = Replace(sInput, ".", sDecimalCharacter)
    Else
      sTempValue = Replace(sInput, ",", sDecimalCharacter)
    End if
    StringToFloat = CDbl(sTempValue)
  Else
    StringToFloat = Null
  End If
End Function