Under VB6 it is possible to apply the Len method to a UDT variable, as in this code:

        Type TestUDT
            B As Long
            S As String * 20
        End Type
  
        Dim udt As TestUDT
        Debug.Print Len(udt)     ' displays "24"

This code is translated to VB.NET as follows:

        Structure TestUDT
            Public B As Integer
            Public S As VB6FixedString
		
            Sub InitializeUDT()
                S = New VB6FixedString(20)
            End Sub
        End Type
		
        Dim udt As TestUDT
        Debug.WriteLine(Len6(udt))     ' displays "8"  -- ERROR!!!

The Len6 method internally maps to the Marshal.SizeOf .NET method and returns the number of bytes that the structure would take when marshaled to an unmanaged (non .NET)  DLL. In this case the value is different from the VB6 value because VB6FixedString is a reference type and therefore takes only 4 bytes in the structure. 

The simplest way to solve this problem is using the UseSystemString pragma for the structure, a step that you need to take anyway if you plan to pass the structure to an external DLL.

        Type TestUDT
            '## UseSystemString