static extern bool CopySid(uint nDestinationSidLength, IntPtr pDestinationSid,

static extern bool CopySid(uint nDestinationSidLength, byte[] pDestinationSid,

Declare Function CopySid Lib "advapi32.dll" (ByVal nDestinationSidLength As Integer, _

        UnsafeNativeMethods.CopySid(length, bytes, pSid);

Consider also the use of CopySid instead of Marshal.Copy below. MSDN isn't clear if that is necessary or not, but better safe than sorry.