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In C#, how do I authenticate a user on a network machine?
PrincipalContext pc =
    new PrincipalContext(ContextType.Domain, exampleMachineDomain);
return pc.ValidateCredentials(testUserUsername, testUserPassword);

using System.Security;
using System.DirectoryServices.AccountManagement;
    public struct Credentials
    {
        public string Username;
        public string Password;
    }
    public class Domain_Authentication
    {
        public Credentials Credentials;
        public string Domain;
        public Domain_Authentication(string Username, string Password, string SDomain)
        {
            Credentials.Username = Username;
            Credentials.Password = Password;
            Domain = SDomain;
        }
        public bool IsValid()
        {
            using (PrincipalContext pc = new PrincipalContext(ContextType.Domain, Domain))
            {
                // validate the credentials
                return pc.ValidateCredentials(Credentials.Username, Credentials.Password);
            }
        }
    }

PrincipalContext ctx = new PrincipalContext(ContextType.Domain);

    // define a "query-by-example" principal - here, we search for a UserPrincipal
    UserPrincipal qbeUser = new UserPrincipal(ctx);

    // if you're looking for a particular user - you can limit the search by specifying
    // e.g. a SAMAccountName, a first name - whatever criteria you are looking for
    qbeUser.SamAccountName = "johndoe";

    // create your principal searcher passing in the QBE principal
    PrincipalSearcher srch = new PrincipalSearcher(qbeUser);

    // find all matches
    foreach(var found in srch.FindAll())
    {
        // do whatever here - "found" is of type "Principal" - it could be user, group, computer.....
    }

PrincipalContext pc =
    new PrincipalContext(ContextType.Machine, exampleMachineDomain);
return pc.ValidateCredentials(testUserUsername, testUserPassword);

private static bool AuthenticateUserOnRemote(string server, string userName, string password)
            {
                var connected = PinvokeWindowsNetworking.connectToRemote(server, userName, password);
                var disconnected = PinvokeWindowsNetworking.disconnectRemote(server);
                return connected == null;
            }