using System;

using System.Collections.Generic;

using System.Linq;

​

namespace demo 

{

    public class Program 

    {

        public void Main()

        {

            //create a reference to our user manager

            var userManager = new UserManager();

            //run some demos with different search criteria

            SearchAndDisplay(userManager, null, "Anne", "Droid"); //should return true

            SearchAndDisplay(userManager, null, "Anne", "Borgue"); //should return false

            SearchAndDisplay(userManager, null, "Simon", "Borgue"); //should return true

            SearchAndDisplay(userManager, "Two", null, null); //sshould return true

            SearchAndDisplay(userManager, "Four", null, null); //should return false

            SearchAndDisplay(userManager, null, null, null); //should return true (as we've not given any filter criteria, and users do exist)

            SearchAndDisplay(new UserManager(true), null, null, null); //should return true (as we've not given any criteria, but no users exist so there is no match)

        }

        void SearchAndDisplay(UserManager userManager, string searchByUsername, string searchByGivenName, string searchBySurname)

        {

            //build up our query

            Predicate<User> searchCriteria = PredicateBuilder.And<User>(); //`And` with no arguments is simply `true`

            if (!string.IsNullOrEmpty(searchByUsername)) searchCriteria = PredicateBuilder.And<User>(searchCriteria, user => searchByUsername.Equals(user.Username, StringComparison.InvariantCultureIgnoreCase));

            if (!string.IsNullOrEmpty(searchByGivenName)) searchCriteria = PredicateBuilder.And<User>(searchCriteria, user => searchByGivenName.Equals(user.GivenName, StringComparison.InvariantCultureIgnoreCase));

            if (!string.IsNullOrEmpty(searchBySurname)) searchCriteria = PredicateBuilder.And<User>(searchCriteria, user => searchBySurname.Equals(user.Surname, StringComparison.InvariantCultureIgnoreCase));

            //search the users using the criteria defined above; NB: The user manager does not need to know anything about our query logic, so our client has lots of flexibility in what queries it can use / is not restricted by the UserManager's implementation

            var result = userManager.UserMatchingCriteriaExists(searchCriteria);

            //display some output to show the user what we've done

            Console.WriteLine("Search Criteria:\n\tUsername: {0}\n\tGivenName: {1}\n\tSurname: {2}\nFound: {3}", searchByUsername ?? "not searched", searchByGivenName ?? "not searched", searchBySurname ?? "not searched", result);

        }

    }

​

    ///Holds basic user information

    class User 

    {

        public User(){}

        public string Username{get; set;}

        public string GivenName{get; set;}

        public string Surname{get; set;}

    }

    ///Holds the collection of users & allows operations against them

    class UserManager 

    {

        readonly IEnumerable<User> users;

        public UserManager(): this(false) {}

        public UserManager(bool noUsers)

        {

            if (noUsers) 

            {

                users = new User[]{};

            }

            else

            {

                users = new User[]

                {

                    new User(){Username = "one", GivenName = "Anne", Surname = "Droid"}

                    ,new User(){Username = "two", GivenName = "Simon", Surname = "Borgue"}

                    ,new User(){Username = "three", GivenName = "Alex", Surname = "Arnold"}

                };

            }

        }

        public bool UserMatchingCriteriaExists(Predicate<User> criteria)

        {

            return users.Any(user => criteria(user));

        }

    }

    static class PredicateBuilder

    {

        ///Allows us to combine predicates

        public static Predicate<T> And<T>(params Predicate<T>[] predicates)

        {

            return delegate (T item)

            {

                foreach (var predicate in predicates)

                {

                    if (!predicate(item))

                    {

                        return false;

                    }

                }

                return true;

            };

        }

    }

}