C#

LibraryClass lc = new LibraryClass();
lc.ShowLC ("some string", this.listBox1);

// Class1.cs
using System;

public class Class1
{
    public event EventHandler<MyEventArgs> MyEvent;

    // Method for demo only
    public void PerformMyMethod(string item)
    {
        OnMyEvent(new MyEventArgs(item));
    }

    protected virtual void OnMyEvent(MyEventArgs e)
    {
        EventHandler<MyEventArgs> eh = MyEvent;
        if (eh != null)
            eh(this, e);
    }
}

// MyEventArgs.cs
public class MyEventArgs : EventArgs
{
    private string _MyString;

    public MyEventArgs(string myString)
    {
        _MyString = myString;
    }

    public String MyString
    {
        get { return _MyString; }
    }
}

// Form1.cs
using System;
using System.Windows.Forms;

public partial class Form1 : Form
{
    Class1 class1;

    public Form1()
    {
        InitializeComponent();
        class1 = new Class1();
        class1.MyEvent += new EventHandler<MyEventArgs>(class1_MyEvent);
        class1.PerformMyMethod("Hello World");
    }

    void class1_MyEvent(object sender, MyEventArgs e)
    {
        MessageBox.Show(string.Format(
            "{0} from {1}!", e.MyString, sender));
    }
}