using System;

public class Program

{

    public static void Main()

    {

        var user = new User { FirstName = "Thiago", LastName = "Lunardi" };

        Console.WriteLine( user.FullName() ); // Thiago Lunardi

    }

}

​

public class User 

{

    public string FirstName {get;set;}

    public string LastName {get;set;}

}

​

public static class UserExtensions

{

    public static string FullName(this User user)

        => $"{user.FirstName} {user.LastName}";

}

​