inheritance - Your object's family tree - virtual and override

Head First C#:
A Learner's Guide to Real-World Programming
with C# and .NET
Kindle版

開発環境

Head First C Sharp: A Learner’s Guide to Real-World Programming with C Sharp and .NET (Andrew Stellman(著)、Jennifer Greene(著)、O’Reilly Media)の Chapter 6(inheritance - Your object’s family tree)、p.301(mini sharpen your pencil)の解答を求めてみる。

コード

Program.cs


SafeOwner owner = new SafeOwner();
Safe safe = new Safe();
JewelThief jewelThief = new JewelThief();
// Thank you for returning my precious jewels!
jewelThief.OpenSafe(safe, owner);
// Console.ReadKey(true);

class Safe
{
    private string contents = "precious jewels";
    private string safeCombination = "12345";
    public string Open(string combination)
    {
        if (combination == safeCombination)
        {
            return contents;
        }
        return "";
    }
    public void PickLock(LockSmith lockpicker)
    {
        lockpicker.Combination = safeCombination;
    }
}
class SafeOwner
{
    private string valuables = "";
    public void ReceiveContents(string safeContents)
    {
        valuables = safeContents;
        Console.WriteLine($"Thank you for returning my {valuables}!");
    }
}
class LockSmith
{
    public void OpenSafe(Safe safe, SafeOwner owner)
    {
        safe.PickLock(this);
        string safeContents = safe.Open(Combination);
        ReturnContents(safeContents, owner);
    }
    public string Combination { private get; set; }
    protected void ReturnContents(string safeContents, SafeOwner owner)
    {
        owner.ReceiveContents(safeContents);
    }
}
class JewelThief : LockSmith
{
    private string stolenJewels;
    protected void ReturnContents(string safeContents, SafeOwner owner)
    {
        stolenJewels = safeContents;
        Console.WriteLine($"I'm stealing the jewels! I stole: {stolenJewels}");
    }
}

入出力結果(Terminal, Zsh)

% dotnet run
/…/ConsoleApp5/Program.cs(52,20): warning CS0108: 'JewelThief.ReturnContents(string, SafeOwner)' hides inherited member 'LockSmith.ReturnContents(string, SafeOwner)'. Use the new keyword if hiding was intended. [/…/ConsoleApp5/ConsoleApp5.csproj]
/…/ConsoleApp5/Program.cs(43,19): warning CS8618: Non-nullable property 'Combination' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [/…/ConsoleApp5/ConsoleApp5.csproj]
/…/ConsoleApp5/Program.cs(51,20): warning CS8618: Non-nullable field 'stolenJewels' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/…/ConsoleApp5/ConsoleApp5.csproj]
Thank you for returning my precious jewels!
%