Practice

Example 1. An abstract account with interest accrual

Create an abstract Account class with an abstract method for monthly interest accrual and two implementations: a savings account (interest on a positive balance) and a credit card (interest on debt). Simulate three months.

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

Account[] accounts =
[
    new SavingsAccount("UA-S1", 50_000m, annualRate: 0.12m),
    new CreditCardAccount("UA-C7", -8_000m, annualRate: 0.36m),
    new CreditCardAccount("UA-C8", 2_500m, annualRate: 0.36m),
];

for (int month = 1; month <= 3; month++)
{
    foreach (Account a in accounts)
    {
        a.CloseMonth();
    }
}

foreach (Account a in accounts)
{
    Console.WriteLine($"{a.Number}: {a.Balance,12:N2} UAH "
        + $"({a.GetType().Name})");
}

abstract class Account(string number, decimal balance)
{
    public string Number { get; } = number;
    public decimal Balance { get; protected set; } = balance;

    // Each account type calculates monthly interest in its own way.
    protected abstract decimal MonthlyInterest();

    public void CloseMonth() =>
        Balance += Math.Round(MonthlyInterest(), 2);
}

class SavingsAccount(string number, decimal balance,
    decimal annualRate) : Account(number, balance)
{
    // Interest accrues on a positive balance.
    protected override decimal MonthlyInterest() =>
        Balance > 0 ? Balance * annualRate / 12 : 0;
}

class CreditCardAccount(string number, decimal balance,
    decimal annualRate) : Account(number, balance)
{
    // Interest is charged only on debt.
    protected override decimal MonthlyInterest() =>
        Balance < 0 ? Balance * annualRate / 12 : 0;
}

The CloseMonth method is the same for all accounts and calls the abstract MonthlyInterest, so each account accrues interest according to its own rule. The MonthlyInterest method has the protected modifier: it is needed only for the account’s internal work. The Balance property with protected set is changed only by methods of the hierarchy. Output:

UA-S1:    51 515,05 UAH (SavingsAccount)
UA-C7:    -8 741,82 UAH (CreditCardAccount)
UA-C8:     2 500,00 UAH (CreditCardAccount)

Example 2. Explicit implementation of two interfaces

Create a SalesSummary class that implements the IPrintable and ILoggable interfaces with the same Print method, but in different ways: a frame for the printer and a timestamped line for the log. Show how the implementations are called.

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

var report = new SalesSummary(104_391m);

report.Print();                     // a regular class method
((IPrintable)report).Print();       // for printing on a printer
((ILoggable)report).Print();        // for the log

ILoggable log = report;
log.Print();

interface IPrintable
{
    void Print();
}

interface ILoggable
{
    void Print();
}

class SalesSummary(decimal total) : IPrintable, ILoggable
{
    public void Print() =>
        Console.WriteLine($"Sales: {total:N2} UAH");

    // Explicit implementation: accessible only through an interface variable.
    void IPrintable.Print()
    {
        Console.WriteLine("┌──────────────────────────┐");
        Console.WriteLine($"│ SALES   {total,14:N2} ₴ │");
        Console.WriteLine("└──────────────────────────┘");
    }

    void ILoggable.Print() =>
        Console.WriteLine($"2026-09-16 12:00:00 INFO sales={total}");
}

The class’s public Print method is not related to the interfaces and is called through a class variable. The explicit implementations IPrintable.Print and ILoggable.Print have no access modifier and are accessible only through a variable of the corresponding interface or a cast. Output:

Sales: 104 391,00 UAH
┌──────────────────────────┐
│ SALES       104 391,00 ₴ │
└──────────────────────────┘
2026-09-16 12:00:00 INFO sales=104391
2026-09-16 12:00:00 INFO sales=104391

Example 3. A managed resource with IDisposable

Create a DeviceSession class that models a connection to a device and releases it in the Dispose method. Show the using statement, releasing the resource when an exception occurs, and protection against using a closed object.

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

// using statement: Dispose is called at the end of the block.
using (var session = new DeviceSession("Scanner"))
{
    session.Send("SCAN 300dpi");
}

// Dispose is called even if an exception occurs.
try
{
    using var printer = new DeviceSession("Printer");
    printer.Send("PRINT page 1");
    printer.Send("");                   // invalid command
    printer.Send("PRINT page 2");       // not executed
}
catch (ArgumentException ex)
{
    Console.WriteLine($"Error: empty argument {ex.ParamName}");
}

var closed = new DeviceSession("Plotter");
closed.Dispose();
closed.Dispose();                        // a repeated call is safe
try
{
    closed.Send("PLOT");
}
catch (ObjectDisposedException ex)
{
    Console.WriteLine($"Error: object {ex.ObjectName} is already closed");
}

class DeviceSession : IDisposable
{
    private readonly string device;
    private bool disposed;

    public DeviceSession(string device)
    {
        this.device = device;
        Console.WriteLine($"[{device}] connection opened");
    }

    public void Send(string command)
    {
        ObjectDisposedException.ThrowIf(disposed, this);
        ArgumentException.ThrowIfNullOrWhiteSpace(command);
        Console.WriteLine($"[{device}] → {command}");
    }

    public void Dispose()
    {
        if (disposed)
        {
            return;
        }
        disposed = true;
        Console.WriteLine($"[{device}] connection closed");
    }
}

In the first using block, the connection is closed after leaving the braces. In the second, the using var declaration closes the printer at the end of the try block even after an exception: the closing message is printed before the handler’s message. The Dispose method remembers its state and is safe to call repeatedly, and after closing, the Send method throws ObjectDisposedException using the ObjectDisposedException.ThrowIf helper. Output:

[Scanner] connection opened
[Scanner] → SCAN 300dpi
[Scanner] connection closed
[Printer] connection opened
[Printer] → PRINT page 1
[Printer] connection closed
Error: empty argument command
[Plotter] connection opened
[Plotter] connection closed
Error: object DeviceSession is already closed