C# Definition:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
struct OSVERSIONINFOEX {
   public int dwOSVersionInfoSize;
   public int dwMajorVersion;
   public int dwMinorVersion;
   public int dwBuildNumber;
   public int dwPlatformId;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
   public string szCSDVersion;
   public UInt16 wServicePackMajor;
   public UInt16 wServicePackMinor;
   public UInt16 wSuiteMask;
   public byte wProductType;
   public byte wReserved;

}

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 uint dwOSVersionInfoSize;
    public uint dwMajorVersion;
    public uint dwMinorVersion;
    public uint dwBuildNumber;
    public uint dwPlatformId;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    public string szCSDVersion;
    public Int16 wServicePackMajor;
    public Int16 wServicePackMinor;
    public Int16 wSuiteMask;
    public Byte wProductType;
    public Byte wReserved;
}

C# Definition (with Enums):

  [StructLayout(LayoutKind.Sequential)]
  struct OSVERSIONINFOEX
  {
    public uint dwOSVersionInfoSize;
    public uint dwMajorVersion;
    public uint dwMinorVersion;
    public uint dwBuildNumber;
    public uint dwPlatformId;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)] public string szCSDVersion;
    public ushort wServicePackMajor;
    public ushort wServicePackMinor;
    public SuiteMask wSuiteMask;
    public ProductType wProductType;
    public byte wReserved;
  }
// 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 uint dwOSVersionInfoSize;
    public uint dwMajorVersion;
    public uint dwMinorVersion;
    public uint dwBuildNumber;
    public uint dwPlatformId;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    public string szCSDVersion;
    public Int16 wServicePackMajor;
    public Int16 wServicePackMinor;
    public Int16 wSuiteMask;
    public Byte wProductType;
    public Byte wReserved;
}

  enum ProductType : byte
  {
    VER_NT_DOMAIN_CONTROLLER = 0x0000002,
    VER_NT_SERVER = 0x0000003,
    VER_NT_WORKSTATION = 0x0000001,
  }

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
' If you plan to call "GetVersionExW" Use CharSet.Unicode
' If you are calling "GetVersionExA", Use CharSet.Ansi
  <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _
  Structure OSVERSIONINFO
    Public dwOSVersionInfoSize As Int32
    Public dwMajorVersion As Int32
    Public dwMinorVersion As Int32
    Public dwBuildNumber As Int32
    Public dwPlatformId As Int32
    <VBFixedString(128), MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> Public szCSDVersion As String
    Public wServicePackMajor As Int16
    Public wServicePackMinor As Int16
    Public wSuiteMask As Int16
    Public wProductType As Byte
    Public wReserved As Byte
End Structure

  [Flags]
  enum SuiteMask : ushort
  {
    VER_SUITE_BACKOFFICE = 0x00000004,
    VER_SUITE_BLADE = 0x00000400,
    VER_SUITE_COMPUTE_SERVER = 0x00004000,
    VER_SUITE_DATACENTER = 0x00000080,
    VER_SUITE_ENTERPRISE = 0x00000002,
    VER_SUITE_EMBEDDEDNT = 0x00000040,
    VER_SUITE_PERSONAL = 0x00000200,
    VER_SUITE_SINGLEUSERTS = 0x00000100,
    VER_SUITE_SMALLBUSINESS = 0x00000001,
    VER_SUITE_SMALLBUSINESS_RESTRICTED = 0x00000020,
    VER_SUITE_STORAGE_SERVER = 0x00002000,
    VER_SUITE_TERMINAL = 0x00000010,
    VER_SUITE_WH_SERVER = 0x00008000,
  }
' 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 dwOSVersionInfoSize As Int32
    Public dwMajorVersion As Int32
    Public dwMinorVersion As Int32
    Public dwBuildNumber As Int32
    Public dwPlatformId As Int32
    <VBFixedString(128), MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> Public szCSDVersion As String
    Public wServicePackMajor As Int16
    Public wServicePackMinor As Int16
    Public wSuiteMask As Int16
    Public wProductType As Byte
    Public wReserved As Byte
End Class

VB Definition:

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> _

Structure OSVERSIONINFOEX

   Public dwOSVersionInfoSize As Integer
   Public dwMajorVersion As Integer
   Public dwMinorVersion As Integer
   Public dwBuildNumber As Integer
   Public dwPlatformId As Integer
   <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _
   Public szCSDVersion As String
   Public wServicePackMajor As UInt16
   Public wServicePackMinor As UInt16
   Public wSuiteMask As UInt16
   Public wProductType As Byte
   Public wReserved As Byte

End Structure 'OSVERSIONINFOEX