C#

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

public partial class FormParent : Form
{
    private ListBox listBox1;
    private Button button1;
    public FormParent()
    {
        InitializeComponent();
        Width = 430;
        listBox1 = new ListBox();
        listBox1.Location = new Point(12, 12);
        listBox1.Size = new Size(400, 200);
        Controls.Add(listBox1);
        button1 = new Button();
        button1.Click += new EventHandler(button1_Click);
        button1.Location = new Point(12, 224);
        button1.Text = "Open &Child";
        Controls.Add(button1);
    }
    void button1_Click(object sender, EventArgs e)
    {
        FormChild child = new FormChild();
        child.StatusUpdate += new EventHandler<StatusEventArgs>(child_StatusUpdate);
        child.Show();
    }
    void child_StatusUpdate(object sender, StatusEventArgs e)
    {
        listBox1.Items.Add(sender.ToString() + ": " + e.Message);
    }
}

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

public partial class FormChild : Form
{
    public event EventHandler<StatusEventArgs> StatusUpdate;
    private Button button1;
    public FormChild()
    {
        InitializeComponent();
        button1 = new Button();
        button1.Click += new EventHandler(button1_Click);
        button1.Location = new Point(12, 12);
        button1.Width = 100;
        button1.Text = "Open &Grandchild";
        Controls.Add(button1);
    }
    void button1_Click(object sender, EventArgs e)
    {
        FormGrandChild grandchild = new FormGrandChild();
        grandchild.StatusUpdate += new EventHandler<StatusEventArgs>(grandchild_StatusUpdate);
        grandchild.Show();
        OnStatusUpdate(this, new StatusEventArgs("Grandchild created"));
    }
    void grandchild_StatusUpdate(object sender, StatusEventArgs e)
    {
        OnStatusUpdate(sender, e);
    }
    //Using the sender in a parameter here  so the originating object can be passed instead of this.
    protected virtual void OnStatusUpdate(object sender, StatusEventArgs e)
    {
        EventHandler<StatusEventArgs> eh = StatusUpdate;
        if (eh != null)
            eh(sender, e);
    }
}

//FormGrandChild
using System;
using System.Drawing;
using System.Windows.Forms;

public partial class FormGrandChild : Form
{
    public event EventHandler<StatusEventArgs> StatusUpdate;
    private Button button1;
    public FormGrandChild()
    {
        InitializeComponent();
        button1 = new Button();
        button1.Click += new EventHandler(button1_Click);
        button1.Location = new Point(12, 12);
        button1.Width = 100;
        button1.Text = "&Send Status";
        Controls.Add(button1);
    }
    void button1_Click(object sender, EventArgs e)
    {
        OnStatusUpdate(new StatusEventArgs("Button Clicked"));
    }
    protected virtual void OnStatusUpdate(StatusEventArgs e)
    {
        EventHandler<StatusEventArgs> eh = StatusUpdate;
        if (eh != null)
            eh(this, e);
    }
}

//StatusEventArgs
using System;

public class StatusEventArgs : EventArgs
{
    private string m_Message;

    public StatusEventArgs(string message)
    {
        m_Message = message;
    }
    public string Message
    {
        get { return m_Message; }
    }
}

//Form1
using System;
using System.Windows.Forms;
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        Singleton.Instance.SingletonEvent += new EventHandler(Instance_SingletonEvent);
        Form2 frm2 = new Form2();
        frm2.Show();
    }

    void Instance_SingletonEvent(object sender, EventArgs e)
    {
        //Update list / text box / whatever
        Console.WriteLine("Singleton Event");
    }
}

//Form2
using System;
using System.Windows.Forms;

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        Singleton.Instance.SingletonMethod();
    }
}

//Singleton
using System;

public class Singleton
{
    // Using empty event args but can use custom - see previous suggestion for pattern
    public event EventHandler SingletonEvent;
    static readonly Singleton instance = new Singleton();
    static Singleton() { }
    Singleton() { }
    public static Singleton Instance
    {
        get { return instance; }
    }
    public void SingletonMethod()
    {
        OnSingletonEvent(new EventArgs());
    }
    protected virtual void OnSingletonEvent(EventArgs e)
    {
        EventHandler eh = SingletonEvent;
        if (eh != null)
            eh(this, e);
    }
}

public interface IMainForm
{
    void UpdateTextBoxText(string text);
    ....
}

public class MainForm : Form, IMainForm
{
    public void UpdateTextBoxText(string text){
       // update textbox here
    }

    void button_click(object sender,EventArgs args)
    {
         ASeparateClass a = new ASeparateClass(this);
         // call the methods on this class to do processing
    }

}

public class ASeparateClass
{
    IMainForm form;
    public ASeparateClass(IMainForm form){   // This is called dependency injection
        this.form = form;
    }
    
    // Other methods in this class. When you need to update the control, 
    // call form.UpdateTextBoxText()
}

[System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "GetWindowLong")]<br />
        public static extern Int32 GetWindowLong(Int32 hWnd, Int32 nIndex);