C#

public abstract class Transportation{
    public abstract void DoThat();
}
public class Air{
   ...
}
public class Vessel{
   ...
}

public abstract class Transportation
{
 
   public abstract void DoThat();
}
public class Air: Transportation
{
   ...
}

public class Vessel: Transportation 
{
   ...
}

abstract class TransportationMedium
{
    // Common transportation medium functionality and data here
    abstract string Name { get; }
    abstract void Transport();
}

class Vessel: TransportationMedium
{    
    // Extend common stuff from base
    override string Name { get { return "Vessel"; } }

    override void Transport()
    {
        // Vessel-specific transport implementation
    }
}

class Air: TransportationMedium
{
    // Extend common stuff from base
    override string Name { get { return "Air"; } }

    override void Transport()
    {
        // Air-specific transport implementation
    }
}

class Land: TransportationMedium
{
    // Extend common stuff from base
    override string Name { get { return "Land"; } }

    override void Transport()
    {
        // Land-specific transport implementation
    }
}

static class TransportationMediumFactory
{
    static TransportationMedium Construct(int typeID)
    {
        switch (typeID)
        {
            case 1: return new VesselMedium();
            case 2: return new AirMedium();
            case 3: return new LandMedium();
        }
    }

    static TransportationMedium Construct(string type)
    {
        switch (type)
        {
            case "Vessel": return new VesselMedium();
            case "Air": return new AirMedium();
            case "Land": return new LandMedium();
        }
    }
}

class Program
{
    void Main()
    {
        int mediumType = 0;
        string intput;

        Console.Write("Select medium (1=Vessel, 2=Air, 3=Land): ");
        input = Console.ReadLine();

        mediumType = Convert.ToInt32(input);

        TransportationMedium medium = TransportationMediumFactory.Construct(mediumType);

        Console.WriteLine("You chose " + medium.Name + " as your transportation medium.");
        
        medium.Transport();
    }
}

// FormParent.cs
using System;
using System.Windows.Forms;

public partial class FormParent : Form
{
    public FormParent()
    {
        InitializeComponent();
        IsMdiContainer = true;
        FormChild frmChild = new FormChild();
        frmChild.MdiParent = this;
        frmChild.ChildEvent += new EventHandler<ChildEventArgs>(frmChild_ChildEvent);
        frmChild.Show();
    }

    void frmChild_ChildEvent(object sender, ChildEventArgs e)
    {
        MessageBox.Show(e.Message);
    }
}

// FormChild.cs
using System;
using System.Windows.Forms;

public partial class FormChild : Form
{
    public event EventHandler<ChildEventArgs> ChildEvent;
    public FormChild()
    {
        InitializeComponent();
        Shown += new EventHandler(FormChild_Shown);
    }

    void FormChild_Shown(object sender, EventArgs e)
    {
        PerformChildEvent();
    }
    // Could be a property setter, a button click or anything
    // using a method for example only
    void PerformChildEvent()
    {
        OnChildEvent(new ChildEventArgs("Child Event"));
    }
    protected virtual void OnChildEvent(ChildEventArgs e)
    {
        EventHandler<ChildEventArgs> eh = ChildEvent;
        if (eh != null)
            eh(this, e);
    }
}

// ChildEventArgs.cs
public class ChildEventArgs : EventArgs
{
    private string m_Message;
    public ChildEventArgs(string message)
    {
        m_Message = message;
    }
    public string Message
    {
        get { return m_Message; }
    }
}

// Method for example only, OnChildEvent should be called directly when needed
void PerformChildEvent()
{
    OnChildEvent(EventArgs.Empty);    
}    
protected virtual void OnChildEvent(EventArgs e)    
{        
    EventHandler eh = ChildEvent;
    if (eh != null)
        eh(this, e);
}

protected override void OnFormClosing(FormClosingEventArgs e)
{
    // do your thing before event
    base.OnFormClosing(e);
    // do your thing after event
}

protected override void OnFormClosing(FormClosingEventArgs e)
        {
            orderedBooks = GenerateBookList();
            
            CreateOrderEventArgs2 args2 = new CreateOrderEventArgs2(orderedBooks);
            base.OnFormClosing(args2);
        }

public partial class Form1 : Form
{
    public new event EventHandler<CreateOrderEventArgs> FormClosing;
    private List<BookOrder> orderedBooks;
    public Form1()
    {
        InitializeComponent();
    }
    List<BookOrder> GenerateBookList()
    {
        List<BookOrder> result = new List<BookOrder>();
        // do your stuff
        return result;
    }
    protected override void OnFormClosing(FormClosingEventArgs e)
    {
        EventHandler<CreateOrderEventArgs> eh = FormClosing;
        if (eh != null)
        {
            orderedBooks = GenerateBookList();
            CreateOrderEventArgs createOrderEventArgs = new CreateOrderEventArgs(
                e, orderedBooks);
            eh(this, createOrderEventArgs);
        }
    }
}
public class CreateOrderEventArgs : CancelEventArgs
{
    private CloseReason m_CloseReason;
    private List<BookOrder> m_OrderedBooks;
    public CreateOrderEventArgs(
        FormClosingEventArgs e, List<BookOrder> orderedBooks)
    {
        m_CloseReason = e.CloseReason;
        Cancel = e.Cancel;
        m_OrderedBooks = orderedBooks;
    }
    public CloseReason CloseReason
    {
        get { return m_CloseReason; }
    }
    public List<BookOrder> OrderedBooks
    {
        get { return m_OrderedBooks; }
    }
}
public class BookOrder
{
    // your stuff here
}