public class AD
{
    // Fields
    private static string ADPassword = ConfigurationManager.AppSettings["ADPassword"].ToString();
    private static string ADPath = ConfigurationManager.AppSettings["ADConnection"].ToString();
    private static string ADServerName = ConfigurationManager.AppSettings["ADServerName"].ToString();
    private static string ADUserName = ConfigurationManager.AppSettings["ADUserName"].ToString();

    // Methods
    public static string GetLogin(string sUserName, string sPassword)
    {
        try
        {
            DirectoryEntry entry = new DirectoryEntry(ADPath, ADServerName + sUserName, sPassword);
            object nativeObject = entry.NativeObject;
            return string.Empty;
        }
        catch
        {
            return "Invalid Username or Password";
        }
    }

    public static string Update(string sUserName, string sOldPassword, string sNewPassword)
    {
        string message;
        try
        {
            DirectoryEntry searchRoot = new DirectoryEntry();
            searchRoot.Path = ADPath;
            searchRoot.Username = ADServerName + ADUserName;
            searchRoot.Password = ADPassword;
            DirectorySearcher searcher = new DirectorySearcher(searchRoot);
            searcher.Filter = "(SAMAccountName=" + sUserName + ")";
            DirectoryEntry directoryEntry = searcher.FindOne().GetDirectoryEntry();
            directoryEntry.Invoke("ChangePassword", new object[] { sOldPassword, sNewPassword });
            directoryEntry.CommitChanges();
            directoryEntry.Close();
            message = string.Empty;
        }
        catch (Exception exception)
        {
            try
            {
                message = exception.InnerException.Message;
            }
            catch
            {
                message = exception.Message;
            }
        }
        return message;
    }
}

public bool UserExists(string userName)
{
  DirectoryEntry searchRoot = new DirectoryEntry("LDAP://dc=yourcompany,dc=com", userName, password);
  DirectorySearcher searchForUser = new DirectorySearcher(searchRoot);

  searchForUser.SearchScope = SearchScope.SubTree;
  searchForUser.Filter = string.Format("(&(objectCategory=Person)(anr={0}))", userName);

  if(searchForUser.FindOne() != null)
  {
     return true;
  }
  else
  {
     return false;
  }
}

LDAP://dc=yourcompany,dc=com

LDAP://cn=Users,dc=yourcompany,dc=com

PrincipalContext ctx = new PrincipalContext(ContextType.Domain, "YOURDOMAIN");

UserPrincipal foundUser = UserPrincipal.FindByIdentity(ctx, "your user name");