VB Forum / General 2 / October 2003

Ok, so I figured this about about 7 seconds after I posted the
message.  The cause is pretty obvious, it turns out, but I'll post it
just so anyone else who tries this doesn't have to go through what I
did.

Yes, the problem is that referencing the property's value with
CallByName in the GetPropertyValue causes the app to run the
GetPropertyValue function, which calls the CallByName function, which
runs the GetPropertyValue function...  Endless loop.  In this case, an
infinitely deep stack, resulting in the stack space error.

You can avoid the problem by passing the local property value to the
GetPropertyValue function, so it knows what the existing value is
without using the Property Get.

Don't do drugs.

That error usually indicates you've entered an endless loop. Without
seeing more of your program, I can't tell how or if that is happening.
But here's something else to think about: Do your properties always
return non-object values, or do they sometimes return objects? If they
ever WILL return objects, you'll have to do some fancy footwork to
allow for that, because manipulating objects in VB6 requires the use
of the 'Set' keyword.

Here's a little example I created to illustrate the problem.

Option Explicit

Private Function GetProperty(ByVal PropertyName As String, ByVal
Person As Object) As Variant
   ' You've got to know whether the property returns an object or
not!
   ' If so, you have to use 'set GetProperty = '.
   ' If not, you can use 'GetProperty ='
   ' You end up getting the property twice this way, once to see if
   ' it's an object or not, and another to actually get it.
   ' You could rewrite it as in the 'LookupProperty' subroutine
below.
   If IsObject(CallByName(Person, PropertyName, VbGet)) Then
       Set GetProperty = CallByName(Person, PropertyName, VbGet)
   Else
       GetProperty = CallByName(Person, PropertyName, VbGet)
   End If
End Function

Private Sub LookupProperty(ByVal PropertyName As String, ByVal Person
As Object, ByRef ReturnData As Variant)
   ' This method doesn't check the object to see whether or not
   ' it's an object, it checks the ReturnData instead.
   If IsObject(ReturnData) Then
       Set ReturnData = CallByName(Person, PropertyName, VbGet)
   Else
       ReturnData = CallByName(Person, PropertyName, VbGet)
   End If
End Sub

Private Sub Command1_Click()
   Dim person1 As clsPerson
   Dim person2 As clsPerson
   Dim o1 As Object
   Dim o2 As Object
   Dim n1 As String
   Dim n2 As String
   
   Set person1 = New clsPerson
   person1.Name = "Fred"
   
   Set person2 = New clsPerson
   person2.Name = "Wilma"
   
   Set person1.Spouse = person2
   Set person2.Spouse = person1
   
   ' Use a function to return the value
   Set o1 = person1
   n1 = GetProperty("name", o1)
   Set o2 = GetProperty("spouse", o1)
   n2 = GetProperty("name", o2)
   MsgBox _
       "Object 1: " & Hex(ObjPtr(o1)) & " Name: " & n1 & vbCrLf & _
       "Object 2: " & Hex(ObjPtr(o2)) & " Name: " & n2
   
   Set o1 = person2
   n1 = GetProperty("name", o1)
   Set o2 = GetProperty("spouse", o1)
   n2 = GetProperty("name", o2)
   MsgBox _
       "Object 1: " & Hex(ObjPtr(o1)) & " Name: " & n1 & vbCrLf & _
       "Object 2: " & Hex(ObjPtr(o2)) & " Name: " & n2
       
       
   ' Use a subroutine to return the value
   Set o1 = person1
   LookupProperty "name", o1, n1
   LookupProperty "spouse", o1, o2
   MsgBox _
       "Object 1: " & Hex(ObjPtr(o1)) & " Name: " & n1 & vbCrLf & _
       "Object 2: " & Hex(ObjPtr(o2)) & " Name: " & n2
 
   
   Set o1 = person2
   LookupProperty "name", o1, n1
   LookupProperty "spouse", o1, o2
   MsgBox _
       "Object 1: " & Hex(ObjPtr(o1)) & " Name: " & n1 & vbCrLf & _
       "Object 2: " & Hex(ObjPtr(o2)) & " Name: " & n2
 
   
End Sub

And here's the code in clsPerson (a contrived example, you'd probably
never want to set the spouse as an object instead of an ID number):

Option Explicit

Private mName As String
Private mSpouse As clsPerson

Public Property Let Name(ByVal value As String)
   mName = value
End Property

Public Property Get Name() As String
   Name = mName
End Property

Public Property Set Spouse(ByVal value As clsPerson)
   Set mSpouse = value
End Property

Public Property Get Spouse() As clsPerson
   Set Spouse = mSpouse
End Property

I misspoke on my first reply. It's not the sign of an endless loop,
but rather, too much recursion, a subtle difference! An example would
be a function that calls itself repeatedly, or calls another function
that ends up calling the first function. A different kind of 'endless
loop.'

And there was a slight bug in the code I posted. Add the following
lines to the last two blocks of statements, just before the 'MsgBox'
statement.

  LookupProperty "name", o2, n2

Should have reviewed it more carefully before I hit the 'Post Message'
button, sorry about that.