C#

//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);

private void listview1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
    for(int i= 0; i < listview1.Items.Count; i++)
    {
        if(listview1.Items[i].SubItems[1].Text.StartsWith(e.KeyChar.ToString().ToUpper()))
        {
            listview1.Items[i].Selected = true;
            listview1.Items[i].Focused = true;
            listview1.Items[i].EnsureVisible();
            break;
        }
    }
 
    e.handled = true;
}