Populating Local Computer "Users" and "Groups" using C# DotNet 4.0

 You need to have include reference to following two dlls:

using System.DirectoryServices;
using System.DirectoryServices.AccountManagement;

Adding reference for DLLs

1:     static void Main(string[] args) 
2:      { 
3:        List<string> usersList = new List<string>(); 
4:        List<string> groupsList = new List<string>(); 
5:   
6:        // Populating Local Computer "Users" and "Groups"  
7:        Console.WriteLine("================================================="); 
8:         
9:        //Create your domain context to Machine level 
10:        PrincipalContext pContext = new PrincipalContext(ContextType.Machine); 
11:   
12:        //Query as UserPrincipal  
13:        UserPrincipal uPrincipal = new UserPrincipal(pContext); 
14:   
15:        //Create principal searcher passing in the User principal   
16:        PrincipalSearcher pSearcher = new PrincipalSearcher(uPrincipal); 
17:   
18:        // find all matches 
19:        foreach (Principal principal in pSearcher.FindAll()) 
20:        { 
21:          if(!usersList.Contains(principal.Name)) 
22:            usersList.Add(principal.Name); 
23:           
24:          foreach (Principal grpPrincipal in principal.GetGroups()) 
25:          { 
26:            if (!groupsList.Contains(grpPrincipal.Name)) 
27:              groupsList.Add(grpPrincipal.Name); 
28:             
29:          } 
30:        } 
31:   
32:        //Groups 
33:        foreach(string group in groupsList ) 
34:          Console.WriteLine(" Group Name :: " + group); 
35:        Console.WriteLine("***************"); 
36:   
37:        //Users 
38:        foreach (string user in usersList) 
39:          Console.WriteLine(" User Name :: " + user); 
40:   
41:        Console.WriteLine("================================================="); 
42:        foreach (Principal principal in pSearcher.FindAll()) 
43:        { 
44:   
45:          foreach (Principal grpPrincipal in principal.GetGroups()) 
46:          { 
47:            using (DirectoryEntry dGroupEntry = new DirectoryEntry(string.Format("WinNT://./{0},group", grpPrincipal.Name))) 
48:            {             
49:              foreach (Object member in (IEnumerable)dGroupEntry.Invoke("Members")) 
50:              { 
51:                using (DirectoryEntry dMemberEntry = new DirectoryEntry(member)) 
52:                { 
53:                  Console.WriteLine(string.Format(" User {0} is Member of Group {1}.", dMemberEntry.Name, grpPrincipal.Name)); 
54:                } 
55:              } 
56:            } 
57:   
58:          } 
59:        } 
60:   
61:        Console.WriteLine("================================================="); 
62:        //clear lists 
63:        usersList.Clear(); 
64:        groupsList.Clear(); 
65:   
66:        using (DirectoryEntry dEntry = new 
67:               DirectoryEntry("WinNT://" + Environment.MachineName + ",computer")) 
68:        { 
69:          //Filter only "user" and "Groups" objects            
70:          dEntry.Children.SchemaFilter.Add("user"); 
71:          dEntry.Children.SchemaFilter.Add("group"); 
72:          foreach (DirectoryEntry dChildEntry in dEntry.Children) 
73:          { 
74:            if (dChildEntry.SchemaClassName.Equals("User", StringComparison.OrdinalIgnoreCase)) 
75:            { 
76:              usersList.Add(dChildEntry.Name); 
77:            } 
78:            else 
79:            { 
80:              groupsList.Add(dChildEntry.Name); 
81:            } 
82:          } 
83:   
84:        } 
85:   
86:        //Groups 
87:        foreach (string group in groupsList) 
88:          Console.WriteLine(" Group Name :: " + group); 
89:        Console.WriteLine("***************"); 
90:   
91:        //Users 
92:        foreach (string user in usersList) 
93:          Console.WriteLine(" User Name :: " + user); 
94:        Console.WriteLine("================================================="); 
95:        Console.ReadKey(); 
96:      } 

OUTPUT:

=================================================
 Group Name :: Administrators
 Group Name :: Remote Desktop Users
 Group Name :: Users
 Group Name :: Guests
 Group Name :: HelpServicesGroup
***************
 User Name :: Administrator
 User Name :: ASPNET
 User Name :: Guest
 User Name :: HelpAssistant
 User Name :: IUSR_VIRTUAL01
 User Name :: IWAM_VIRTUAL01
 User Name :: SUPPORT_388945a0
 User Name :: TestAdmin
=================================================
 User Administrator is Member of Group Administrators.
 User TestAdmin is Member of Group Administrators.
 User Administrator is Member of Group Remote Desktop Users.
 User INTERACTIVE is Member of Group Users.
 User Authenticated Users is Member of Group Users.
 User ASPNET is Member of Group Users.
 User TestAdmin is Member of Group Users.
 User Guest is Member of Group Guests.
 User IUSR_VIRTUAL01 is Member of Group Guests.
 User Guest is Member of Group Guests.
 User IUSR_VIRTUAL01 is Member of Group Guests.
 User SUPPORT_388945a0 is Member of Group HelpServicesGroup.
 User Administrator is Member of Group Administrators.
 User TestAdmin is Member of Group Administrators.
 User INTERACTIVE is Member of Group Users.
 User Authenticated Users is Member of Group Users.
 User ASPNET is Member of Group Users.
 User TestAdmin is Member of Group Users.
=================================================
 Group Name :: Administrators
 Group Name :: Backup Operators
 Group Name :: Guests
 Group Name :: Network Configuration Operators
 Group Name :: Power Users
 Group Name :: Remote Desktop Users
 Group Name :: Replicator
 Group Name :: Users
 Group Name :: DB2ADMNS
 Group Name :: DB2USERS
 Group Name :: HelpLibraryUpdaters
 Group Name :: HelpServicesGroup
 Group Name :: SQLServer2005SQLBrowserUser$VIRTUAL01
 Group Name :: SQLServerDTSUser$VIRTUAL01
 Group Name :: SQLServerFDHostUser$virtual01$MSSQLSERVER
 Group Name :: SQLServerMSASUser$VIRTUAL01$MSSQLSERVER
 Group Name :: SQLServerMSSQLServerADHelperUser$VIRTUAL01
 Group Name :: SQLServerMSSQLUser$virtual01$MSSQLSERVER
 Group Name :: SQLServerMSSQLUser$virtual01$SQLEXPRESS
 Group Name :: SQLServerReportServerUser$VIRTUAL01$MSRS10_50.MSSQLSERVER2008
 Group Name :: SQLServerSQLAgentUser$VIRTUAL01$MSSQLSERVER
 Group Name :: SQLServerSQLAgentUser$VIRTUAL01$SQLEXPRESS
***************
 User Name :: Administrator
 User Name :: ASPNET
 User Name :: Guest
 User Name :: HelpAssistant
 User Name :: IUSR_VIRTUAL01
 User Name :: IWAM_VIRTUAL01
 User Name :: SUPPORT_388945a0
 User Name :: TestAdmin
=================================================

Authenticate a Local User using UserName/Password pair:

1:   
2:      static void Main(string[] args) 
3:      { 
4:        string userName = "TestAdmin"; 
5:        string password = "********"; 
6:   
7:        //Authenticate a Local User using UserName/Password pair 
8:        using (PrincipalContext context = new PrincipalContext(ContextType.Machine)) 
9:        { 
10:          bool isAuthenticated = context.ValidateCredentials(userName, password); 
11:          if (isAuthenticated) 
12:          { 
13:            Console.WriteLine(string.Format("User Name ({0}) is a authenticated user!", userName)); 
14:          } 
15:          else 
16:          { 
17:            Console.WriteLine(string.Format("User Name ({0}) is a not authenticated, bad user name or password!", userName));  
18:          } 
19:        } 
20:   
21:        Console.ReadKey(); 
22:      } 

OUTPUT:

User Name (TestAdmin) is a authenticated user!
 
or


User Name (TestAdmin) is a not authenticated, bad user name or password!