Malditos Atributos

El compilador no me reconoce ningún atributo, pongo el siguiente ejemplo:

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto)> _
Public Class FindData
Public fileAttributes As Integer = 0
' creationTime was a by-value FILETIME structure.
Public creationTime_lowDateTime As Integer = 0
Public creationTime_highDateTime As Integer = 0
' lastAccessTime was a by-value FILETIME structure.
Public lastAccessTime_lowDateTime As Integer = 0
Public lastAccessTime_highDateTime As Integer = 0
' lastWriteTime was a by-value FILETIME structure.
Public lastWriteTime_lowDateTime As Integer = 0
Public lastWriteTime_highDateTime As Integer = 0
Public nFileSizeHigh As Integer = 0
Public nFileSizeLow As Integer = 0
Public dwReserved0 As Integer = 0
Public dwReserved1 As Integer = 0
<MarshalAs(UnmanagedType.ByValTStr, SizeConst:%6)> _
Public fileName As String = Nothing
<MarshalAs(UnmanagedType.ByValTStr, SizeConst:)> _
Public alternateFileName As String = Nothing
End Class 'FindData

El compilador me dice que "MarshalAs" y "StructLayout" no están definidos,
he visto otros atributos que serían geniales para poner las descripciones a
los metodos de las clases, pero no hay maneras de que funcionen, alguien me
sabe decir que me estoy dejando ???

Muchas gracias.