C# Signature:

[DllImport("advapi32.dll", SetLastError=true)]
static extern bool GetTokenInformation(
    IntPtr TokenHandle,
    TOKEN_INFORMATION_CLASS TokenInformationClass,
    IntPtr TokenInformation,
    uint TokenInformationLength,
    out uint ReturnLength);

User-Defined Types:

TOKEN_INFORMATION_CLASS

Notes:

Call once with zero for the third and fourth parameters to obtain the required size, then allocate the buffer and call again supplying these parameters.

Tips & Tricks:

Please add some!

Sample Code:

// Prints out sid of current user

using System;

using System.Runtime.InteropServices;

using System.Security.Principal;

using System.Text;

namespace test

{

    class clsLookupAccountName
    {

        enum TOKEN_INFORMATION_CLASS
        {
            TokenUser = 1,
            TokenGroups,
            TokenPrivileges,
            TokenOwner,
            TokenPrimaryGroup,
            TokenDefaultDacl,
            TokenSource,
            TokenType,
            TokenImpersonationLevel,
            TokenStatistics,
            TokenRestrictedSids,
            TokenSessionId,
            TokenGroupsAndPrivileges,
            TokenSessionReference,
            TokenSandBoxInert,
            TokenAuditPolicy,
            TokenOrigin
        }

        public struct TOKEN_USER
        {
            public SID_AND_ATTRIBUTES User ;
        }

        public struct SID_AND_ATTRIBUTES
        {

            public IntPtr Sid ;
            public int Attributes ;
        }

        // Using IntPtr for pSID insted of Byte[]
        [DllImport("advapi32", CharSet=CharSet.Auto, SetLastError=true)]
        static extern bool ConvertSidToStringSid(
            IntPtr pSID,
            out IntPtr ptrSid);

        [DllImport("kernel32.dll")]
        static extern IntPtr LocalFree(IntPtr hMem);

        [DllImport("advapi32.dll", SetLastError=true)]
        static extern bool GetTokenInformation(
            IntPtr TokenHandle,
            TOKEN_INFORMATION_CLASS TokenInformationClass,
            IntPtr TokenInformation,
            int TokenInformationLength,
            out int ReturnLength);

        [STAThread]
        static void Main(string[] args)
        {
            int TokenInfLength = 0 ;
            bool Result ;

            // first call gets lenght of TokenInformation
            Result = GetTokenInformation( WindowsIdentity.GetCurrent().Token , TOKEN_INFORMATION_CLASS.TokenUser , IntPtr.Zero , TokenInfLength , out TokenInfLength );

            IntPtr TokenInformation = Marshal.AllocHGlobal( TokenInfLength ) ;

            Result = GetTokenInformation( WindowsIdentity.GetCurrent().Token  , TOKEN_INFORMATION_CLASS.TokenUser , TokenInformation , TokenInfLength , out TokenInfLength ) ;

            if( Result )
            {
                TOKEN_USER TokenUser = ( TOKEN_USER )Marshal.PtrToStructure( TokenInformation , typeof( TOKEN_USER ) ) ;

                IntPtr pstr = IntPtr.Zero;
                Boolean ok = ConvertSidToStringSid( TokenUser.User.Sid  , out pstr );
                string sidstr = Marshal.PtrToStringAuto( pstr );
                LocalFree(pstr);
                Console.WriteLine(@"Found sid {0}",sidstr);
            }

            Marshal.FreeHGlobal( TokenInformation );

            Console.ReadLine();
        }
    }

}

Alternative Managed API:

Do you know one? Please contribute it!