"Clever" C#

Main-01.0.cs

public class Person
{
  private string _firstName;
  private string _lastName;

  public Person(string lastName)
  {
    _firstName = "None";
    _lastName = _lastName;
  }

  public string GetFirstName()
  {
    return _firstName;
  }

  public void SetFirstName(string value)
  {
    _firstName = value;
  }

  public string GetLastName()
  {
    return _lastName;
  }
}

Main-02.0.cs

public class Person
{
  private string _firstName;
  private string _lastName;

  public Person(string lastName)
  {
    _firstName = "None";
    _lastName = lastName;
  }

  public FirstName
  {
    get { return _firstName; }
    set { _firstName = value; }
  }

  public LastName
  {
    get { return _lastName; }
  }
}

Main-04.0.cs

public class Person
{
  private string _lastName;
  public Person(string lastName)
  {
    FirstName = "None";
    _lastName = lastName;
  }

  public FirstName { get; set; }
  public LastName { get { return _lastName }; }
}

Main-05.0.cs

public Person
{
  public Person(string lastName)
  {
    LastName = lastName;
  }

  public string FirstName { get; set; } = "None"
  public string LastName { get; private set; }
}

Main-12.0.cs

public class Person(string lastName)
{
    public string FirstName { get; set; } = "None";
    public string LastName { get; private set; } = lastName;
}