C#

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        Process[] processesBefore;
        Process[] processesAfter;
        private void Form1_Load(object sender, EventArgs e)
        {
            processesBefore = Process.GetProcesses();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            processesAfter = Process.GetProcesses();
            CompareProcesses compare = new CompareProcesses();
            IEnumerable<Process> theDifference = processesAfter.Except(processesBefore, compare);
            foreach (Process p in theDifference)
            {
                listboxDifference.Items.Add(p.ProcessName);
            }
        }
    }

    public class CompareProcesses : IEqualityComparer<Process>
    {
        public bool Equals(Process x, Process y)
        {
            return (x == y);
        }

        public int GetHashCode(Process obj)
        {
            return obj.GetHashCode() ^ obj.GetHashCode();
        }
    }

private void findAndReplaceToolStripMenuItem_Click(object sender, EventArgs e)
{
    frmFindReplace find = new frmFindReplace();
    find.Clicked += new ButtonClick(find_Clicked);
    find.ShowDialog();
}

// TextEventArgs.cs
using System;

public class TextEventArgs : EventArgs
{
    private string text;

    public TextEventArgs(string text)
    {
        this.text = text;
    }

    public string Text
    {
        get { return text; }
    }
}

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

public partial class FormFindAndReplace : Form
{
    public event EventHandler<TextEventArgs> FindClicked;

    public FormFindAndReplace()
    {
        InitializeComponent();
    }

    private void buttonFind_Click(object sender, EventArgs e)
    {
        OnFindClicked(new TextEventArgs(textBox.Text));
    }

    protected virtual void OnFindClicked(TextEventArgs e)
    {
        EventHandler<TextEventArgs> eh = FindClicked;
        if (eh != null)
            eh(this, e);
    }
}

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

public partial class FormMain : Form
{
    private FormFindAndReplace formFindAndReplace = null;

    public FormMain()
    {
        InitializeComponent();
    }

    private void findAndReplaceToolStripMenuItem_Click(object sender, EventArgs e)
    {
        if (formFindAndReplace == null)
            formFindAndReplace = new FormFindAndReplace();
        formFindAndReplace.FindClicked += formFindAndReplace_FindClicked;
        formFindAndReplace.FormClosing += formFindAndReplace_FormClosing;
        formFindAndReplace.Show();
    }

    private void formFindAndReplace_FindClicked(object sender, TextEventArgs e)
    {
        // Do your thing with e.Text
        Console.WriteLine(e.Text);
    }
    private void formFindAndReplace_FormClosing(object sender, FormClosingEventArgs e)
    {
        formFindAndReplace.FormClosing -= formFindAndReplace_FormClosing;
        formFindAndReplace.FindClicked -= formFindAndReplace_FindClicked;
        formFindAndReplace = null;
    }
}

PIEBALDconsult wrote:

The Singleton pattern is more for C++ and is not of much use in C#