C# Signature:

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

VB.NET Definition:

    Public Declare Function GetTokenInformation Lib "advapi32.dll" ( _
    ByVal TokenHandle As IntPtr, ByVal TokenInformationClass As TOKEN_INFORMATION_CLASS, _
    ByVal TokenInformation As IntPtr, ByVal TokenInformationLength As System.UInt32, _
    ByRef ReturnLength As System.UInt32) As Boolean

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 ;
      }

          [StructLayout(LayoutKind.Sequential)]
      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();
      }
      }
  }

There's a sample based on this at http://stackoverflow.com/questions/2146153/how-to-get-the-logon-sid-in-c/2146418#2146418 that gets the LogonSID instead.

Alternative Managed API:

Do you know one? Please contribute it!