pinvoke.net: Edit: netapi32.NetWkstaUserEnum

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Summary: The NetWkstaUserEnum function lists information about all users currently logged on to the workstation. This list includes interactive, service and batch logons.

!!!!C# Signature:
        [DllImport("netapi32.dll", CharSet = CharSet.Unicode, SetLastError=true)]
        static extern int NetWkstaUserEnum(
           string servername,
           int level,
           out IntPtr bufptr,
           int prefmaxlen,
           out int entriesread,
           out int totalentries,
           ref int resume_handle);
!!!!VB Signature:
<DllImport("netapi32.dll", CharSet := CharSet.Unicode, SetLastError:=True)> _
Shared Function NetWkstaUserEnum(ByVal servername As String, ByVal level As Integer, <System.Runtime.InteropServices.Out()> ByRef bufptr As IntPtr, ByVal prefmaxlen As Integer, <System.Runtime.InteropServices.Out()> ByRef entriesread As Integer, <System.Runtime.InteropServices.Out()> ByRef totalentries As Integer, ByRef resume_handle As Integer) As Integer
End Function
!!!!User-Defined Types:
See sample code for types and constants.

!!!!Alternative Managed API:
WMI can do this sort of thing (the System.Management namespace in .Net 2.0)

!!!!Notes:
If you want to find the user logged in to the workstation, consider instead a WMI query ("select UserName from Win32_ComputerSystem"), which  has certain advantages (runs faster, less ambigious results, doesn't require Interop if using .Net 2.0 System.Management namespace, etc.)

!!!!Tips & Tricks:
Please add some!

* On Windows 8+ when users log in with a MicrosoftAccount the username is returned like "MicrosoftAccount\user.email@thedomain.com" rather than the ID. This means for example that it won't match their directory name in C:\Users.

!!!!Sample Code:
  class NetWorkstationUserEnum
  {
        [DllImport("netapi32.dll", CharSet = CharSet.Unicode, SetLastError=true)]
        static extern int NetWkstaUserEnum(
           string servername,
           int level,
           out IntPtr bufptr,
           int prefmaxlen,
           out int entriesread,
           out int totalentries,
           ref int resume_handle);

[DllImport("netapi32.dll")]
        static extern int NetApiBufferFree(
           IntPtr Buffer);

const int NERR_SUCCESS = 0;
        const int ERROR_MORE_DATA = 234;

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public struct WKSTA_USER_INFO_0
        {
            public string wkui0_username;
        }

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public struct WKSTA_USER_INFO_1
        {
            public string wkui1_username;
            public string wkui1_logon_domain;
            public string wkui1_oth_domains;
            public string wkui1_logon_server;
        }
        void ScanHost(string hostName)
        {

IntPtr bufptr = IntPtr.Zero;
            int dwEntriesread;
            int dwTotalentries = 0;
            int dwResumehandle = 0;
            int nStatus;
            Type tWui1 = typeof(WKSTA_USER_INFO_1);
            int nStructSize = Marshal.SizeOf(tWui1);
            WKSTA_USER_INFO_1 wui1;

//this.listView1.Items.Clear();

do
            {
                nStatus = NetWkstaUserEnum(
                  hostName,
                  1,
                  out bufptr,
                  32768,
                  out dwEntriesread,
                  out dwTotalentries,
                  ref dwResumehandle);

//
                // If the call succeeds,
                //
                if ((nStatus == NERR_SUCCESS) | (nStatus == ERROR_MORE_DATA))
                {
                    if (dwEntriesread > 0)
                    {
                        IntPtr pstruct = bufptr;

//
                        // Loop through the entries.
                        //
                        for (int i = 0; i < dwEntriesread; i++)
                        {
                            wui1 = (WKSTA_USER_INFO_1)Marshal.PtrToStructure(pstruct, tWui1);
                            Console.WriteLine(wui1.wkui1_logon_domain + "\\" + wui1.wkui1_username);
                            pstruct = (IntPtr)((int)pstruct + nStructSize);
                        }
                    }
                    else
                    {
                        Console.WriteLine("A system error has occurred : " + nStatus);
                    }
                }

if (bufptr != IntPtr.Zero)
                    NetApiBufferFree(bufptr);

} while (nStatus == ERROR_MORE_DATA);
        }
        static public void Main(string[] args)
        {
            string host = Environment.MachineName;
            if (args.Length > 0)
            {
                host = args[0];
            }
            NetWorkstationUserEnum nws = new NetWorkstationUserEnum();
            nws.ScanHost(host);
        }
  }

Documentation: NetWkstaUserEnum@msdn on MSDN
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/netmgmt/netmgmt/netwkstauserenum.asp