using System;

using System.Collections.Generic;

using System.Linq;

using System.Linq.Expressions;

public class Program

{

    public static void Main()

    {

        var persons = new List<Person>

        {

            new Person { Id = 1, FirstName = "Fox", LastName = "Mulder", Age = 40 },

            new Person { Id = 2, FirstName = "Dana", LastName = "Scully", Age = 35 }

        };

        var person = new Person(){FirstName = "Dana"};

        var results = Search(persons,person);

        foreach(var result in results)

        {

            Console.WriteLine($"{result.FirstName} {result.LastName} aged {result.Age}");

        }

        ;       

    }

    private static List<Person> Search(List<Person> persons, Person person)

    {

        IEnumerable<Person> result = persons;

        if(!String.IsNullOrEmpty(person.FirstName))

        {

            result = result.Where(x => x.FirstName == person.FirstName);

        }

        if(!String.IsNullOrEmpty(person.LastName))

        {

            result = result.Where(x => x.LastName == person.LastName);

        }

        if(person.Age.HasValue)

        {

            result = result.Where(x => x.Age == person.Age);

        }

        return result.ToList();

    }

}

​

public class Person

{

    public int Id { get; set; }

​

    public string FirstName { get; set; }

​

    public string LastName { get; set; }

​

    public int? Age { get; set; }

}

​

​