C# Definitions:

// Use this when the unmanaged API expects the structure passed by-value, or
// or if you want to pass it by-reference as a pointer to a structure
struct OSVERSIONINFO {
   public int OSVersionInfoSize;
   [ MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )]    
   public String versionString;
}

// Use this when you want to pass it by-value even though the unmanaged API
// expects a pointer to a structure.  Being a class adds an extra level of indirection.
[StructLayout(LayoutKind.Sequential)]  
class OSVERSIONINFO
{
   public int OSVersionInfoSize;
   [ MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )]
   public String versionString;
}

VB Definitions:

' Use this when the unmanaged API expects the structure passed by-value, or
' or if you want to pass it by-reference as a pointer to a structure
Structure OSVERSIONINFO
   Public OSVersionInfoSize As Integer
   < MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )> _
   Public versionString As String
End Structure

' Use this when you want to pass it by-value even though the unmanaged API
' expects a pointer to a structure.  Being a class adds an extra level of indirection.
<StructLayout(LayoutKind.Sequential)> _
Class OSVERSIONINFO
   Public OSVersionInfoSize As Integer
   < MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )> _
   Public versionString As String
End Class

Notes:

None.