Dev Blog

Practice ​

Example 1. An animal hierarchy ​

Create an Animal base class with a virtual Speak method, the derived classes Dog and Cat, and a sealed Puppy class derived from Dog. Show the order in which constructors are called and a polymorphic method call for an array of animals.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

Animal[] zoo =
    [new Dog("Brovko"), new Cat("Murka"), new Puppy("Rex")];

foreach (Animal a in zoo)
{
    Console.WriteLine($"{a.GetType().Name,-6} {a.Name}: {a.Speak()}");
}

int dogs = 0;
foreach (Animal a in zoo)
{
    if (a is Dog)            // a Puppy is also a Dog
    {
        dogs++;
    }
}
Console.WriteLine($"Dogs: {dogs}");

class Animal
{
    public Animal(string name)
    {
        Console.WriteLine($"  Animal({name}) constructor");
        Name = name;
    }

    public string Name { get; }

    public virtual string Speak() => "...";
}

class Dog : Animal
{
    public Dog(string name) : base(name)
    {
        Console.WriteLine($"  Dog({name}) constructor");
    }

    public override string Speak() => "Woof!";
}

// A puppy is a dog; this class cannot be inherited from.
sealed class Puppy : Dog
{
    public Puppy(string name) : base(name)
    {
        Console.WriteLine($"  Puppy({name}) constructor");
    }

    public override string Speak() => base.Speak() + " (squeaky)";
}

class Cat : Animal
{
    public Cat(string name) : base(name) { }

    public sealed override string Speak() => "Meow!";
}

For the puppy, three constructors run: Animal, Dog, and Puppy, from the base class to the derived one. The Speak method in Puppy extends the Dog version by calling base.Speak(). In the Cat class, the method is overridden with the sealed override modifier: descendants of Cat can no longer change it. The a is Dog check also returns true for the puppy. Output:

  Animal(Brovko) constructor
  Dog(Brovko) constructor
  Animal(Murka) constructor
  Animal(Rex) constructor
  Dog(Rex) constructor
  Puppy(Rex) constructor
Dog    Brovko: Woof!
Cat    Murka: Meow!
Puppy  Rex: Woof! (squeaky)
Dogs: 2

Example 2. Overriding versus hiding ​

Create a Logger class with a virtual Write method and a regular Level method, and a derived TimestampLogger class that overrides Write and hides Level. Call both methods through variables of the base and derived types.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

Logger viaBase = new TimestampLogger();
TimestampLogger viaDerived = new TimestampLogger();

viaBase.Write("override: via Logger");
viaDerived.Write("override: via TimestampLogger");

viaBase.Level("new: via Logger");
viaDerived.Level("new: via TimestampLogger");

class Logger
{
    public virtual void Write(string message) =>
        Console.WriteLine($"[Logger] {message}");

    public void Level(string message) =>
        Console.WriteLine($"[INFO] {message}");
}

class TimestampLogger : Logger
{
    // Overriding: called according to the actual type of the object.
    public override void Write(string message) =>
        base.Write($"12:00:00 {message}");

    // Hiding: called according to the type of the variable.
    public new void Level(string message) =>
        Console.WriteLine($"[INFO 12:00:00] {message}");
}

For the overridden Write method, the result does not depend on the variable’s type: both calls execute the TimestampLogger version. The hidden Level method is selected by the variable’s type, so the call through the Logger variable executes the base version. Output:

[Logger] 12:00:00 override: via Logger
[Logger] 12:00:00 override: via TimestampLogger
[INFO] new: via Logger
[INFO 12:00:00] new: via TimestampLogger

Example 3. Messages with address validation ​

Create a Message → Email, Sms hierarchy in which the derived class constructors validate the recipient’s address and throw a custom InvalidRecipientException, while the cost and sending format are determined by virtual members.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

(string Kind, string To, string Text)[] requests =
[
    ("email", "[email protected]", "Tomorrow's schedule has been updated"),
    ("sms", "+380671234567", "Confirmation code: 4821"),
    ("email", "olena.knu.edu.ua", "A letter with an address error"),
    ("sms", "+38067", "Short message"),
    ("sms", "+380501112233", new string('★', 75)),
];

decimal cost = 0;
foreach (var (kind, to, text) in requests)
{
    try
    {
        Message m = kind == "email"
            ? new Email(to, text)
            : new Sms(to, text);
        cost += m.Cost;
        Console.WriteLine($"{m.Send()}  ({m.Cost:N2} UAH)");
    }
    catch (InvalidRecipientException ex)
    {
        Console.WriteLine(
            $"Rejected: {ex.Message} “{ex.Recipient}”");
    }
}
Console.WriteLine($"Sending cost: {cost:N2} UAH");

class Message
{
    protected Message(string to, string text)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(text);
        To = to;
        Text = text;
    }

    public string To { get; }
    public string Text { get; }
    public virtual decimal Cost => 0m;

    public virtual string Send() => $"→ {To}: {Text}";
}

class Email : Message
{
    public Email(string to, string text) : base(to, text)
    {
        int at = to.IndexOf('@');
        if (at <= 0 || to.IndexOf('.', at) < 0)
        {
            throw new InvalidRecipientException(
                "invalid e-mail address", to);
        }
    }

    public override string Send() => "E-mail " + base.Send();
}

sealed class Sms : Message
{
    private const int PartLength = 70;   // Unicode text: 70 characters
    private const decimal PricePerPart = 0.85m;

    public Sms(string to, string text) : base(to, text)
    {
        if (to.Length != 13 || !to.StartsWith("+380"))
        {
            throw new InvalidRecipientException(
                "invalid number", to);
        }
    }

    public int Parts => (Text.Length + PartLength - 1) / PartLength;
    public override decimal Cost => Parts * PricePerPart;

    public override string Send()
    {
        string preview = Text.Length > 20 ? Text[..20] + "…" : Text;
        return $"SMS → {To}: {preview} [{Parts} part(s)]";
    }
}

class InvalidRecipientException : Exception
{
    public InvalidRecipientException(string message, string recipient)
        : base(message)
    {
        Recipient = recipient;
    }

    public string Recipient { get; }
}

The Message constructor has the protected modifier: you cannot create a “plain” message, only an e-mail or an SMS. The Cost property for an SMS depends on the number of parts (70 Unicode characters, such as Cyrillic, per part). The (string Kind, string To, string Text)[] notation declares an array of tuples with named fields (Topic 11), and foreach (var (kind, to, text) in requests) deconstructs each tuple into variables. Output:

E-mail → [email protected]: Tomorrow's schedule has been updated  (0,00 UAH)
SMS → +380671234567: Confirmation code: 4… [1 part(s)]  (0,85 UAH)
Rejected: invalid e-mail address “olena.knu.edu.ua”
Rejected: invalid number “+38067”
SMS → +380501112233: ★★★★★★★★★★★★★★★★★★★★… [2 part(s)]  (1,70 UAH)
Sending cost: 2,55 UAH