(http://csharppad.com/gist/f075352ee464c737ee84d5545655c7a7)

classes-properties.cs

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public class Person
{
    public string Name { get; set; }

    private int _age;
    public int Age
    {
        get { return _age; }
        set { _age = value; }
    }

    public Person(string name, int age)
    {

        Name = name;
        Age = age;
    }
}

public class ChatPerson: Person
{
    private const string DefaultMessage = "default message";
    private string _message;
    public ChatPerson(string name, int age) : base(name, age)
    {
        _message = DefaultMessage;
    }
    public ChatPerson(string message) : this("John", 42)
    {
        _message = message;
    }
}

var person = new ChatPerson("hello");