using System;

using System.Collections.Generic;

using System.Reflection;

using System.Text.Json;

​

public class Program

{

    public static void Main()

    {

        var persons = new List<Person>

        {

            new Person{ Id = 1, Name = "Alice", Birthday = new DateTime(2002, 3, 17), Job = "Dentist", Salary = 30_000 },

            new Person{ Id = 2, Name = "Bob", Birthday = new DateTime(2003, 3, 17), Job = "Doctor", Salary = 32_000 },

            new Person{ Id = 3, Name = "Carol", Birthday = new DateTime(2004, 3, 17), Job = "Trainer", Salary = 33_000 },

            new Person{ Id = 4, Name = "Dennis", Birthday = new DateTime(2003, 3, 17), Job = "Developer", Salary = 30_000 },

            new Person{ Id = 5, Name = "Alice", Birthday = new DateTime(2004, 3, 17), Job = "Scientist", Salary = 33_000 },

        };

        Console.WriteLine(JsonSerializer.Serialize(GetCommonProperties(new[] { persons[0], persons[4] })));

        Console.WriteLine(JsonSerializer.Serialize(GetCommonProperties(new[] { persons[1], persons[3] })));

        Console.WriteLine(JsonSerializer.Serialize(GetCommonProperties(new[] { persons[2], persons[4] })));

    }

​

    private static T GetCommonProperties<T>(IEnumerable<T> source) where T : new()

    {

        var first = true;

        var common = new T();

        var props = typeof(T).GetProperties();

​

        foreach (var item in source)

        {

            if (first)

            {

                first = false;

​

                foreach (var prop in props)

                {

                    var value = prop.GetValue(item, null);

                    prop.SetValue(common, value);

                }

            }

            else

            {

                foreach (var prop in props)

                {

                    var itemValue = prop.GetValue(item, null);

                    var commonValue = prop.GetValue(common, null);

​

                    if ((dynamic)itemValue != (dynamic)commonValue)

                    {

                        prop.SetValue(common, GetDefault(prop.PropertyType));

                    }

​

                }

            }

        }

​

        return common;

    }

    private static object GetDefault(Type t)

    {

        return typeof(Program)

            .GetMethod(nameof(GetDefaultValue), BindingFlags.NonPublic | BindingFlags.Static)

            .MakeGenericMethod(t)

            .Invoke(null, null);

    }

​

    private static T GetDefaultValue<T>()

    {

        return default;

    }

}

​

public class Person

{

    public DateTime Birthday { get; set; }

    public int Id { get; set; }

    public string Job { get; set; }

    public string Name { get; set; }

    public int Salary { get; set; }

}