C#

public partial class Form1 : Form
{
  public delegate void ListBoxItemClickHandler(string itemText);

  private ListBoxItemClickHandler OnListBoxItemClick;

  public Form1()
  {
    InitializeComponent();
  }

  private void Form1_Load(object sender, EventArgs e)
  {
    listBox1.Items.Add("monkey");
    listBox1.Items.Add("nut");
    listBox1.Items.Add("car");

    Form2 frm = new Form2();
    OnListBoxItemClick += new ListBoxItemClickHandler(frm.ShowItem);
    frm.Show();
  }

  private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
  {
    if (OnListBoxItemClick != null) OnListBoxItemClick(listBox1.SelectedItem.ToString());
  }
}

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

  public void ShowItem(string itemText)
  {
    textBox1.Text = itemText;
  }
}

public interface IObservable
{
  void AddObserver(IObserver observer);
  void NotifyObservers(object obj);
  void RemoveObserver(IObserver observer);
}

public interface IObserver
{
  void Update(object obj);
}

public partial class Form1 : Form, IObservable
{
  public string ItemText { get; private set; }

  private List<IObserver> _Observers = new List<IObserver>();

  public Form1()
  {
    InitializeComponent();
  }

  private void Form1_Load(object sender, EventArgs e)
  {
    listBox1.Items.Add("monkey");
    listBox1.Items.Add("nut");
    listBox1.Items.Add("car");

    for (int k = 0; k < 3; k++)
    {
      Form2 frm = new Form2();
      frm.Text = k.ToString();
      AddObserver(frm);
      frm.FormClosed += new FormClosedEventHandler(frm_FormClosed);
      frm.Show();
    }
  }

  private void frm_FormClosed(object sender, FormClosedEventArgs e)
  {
    RemoveObserver(sender as IObserver);
  }

  private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
  {
    ItemText = listBox1.SelectedItem.ToString();
    NotifyObservers(this.listBox1);
  }

  public void AddObserver(IObserver observer)
  {
    _Observers.Add(observer);
  }

  public void NotifyObservers(object obj)
  {
    foreach (var observer in _Observers)
    {
      observer.Update(obj);
    }
  }

  public void RemoveObserver(IObserver observer)
  {
    _Observers.Remove(observer);
  }
}

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

  public void Update(object obj)
  {
    ListBox box = obj as ListBox;
    textBox1.Text = box.Text;
  }
}

public class Item : BaseNotifier {
 private string name;
 public string Name {
  get { return name; }
  set { name = value; Notify("Name"); }
 }

 public override string ToString() { return name; }
}

public class DataModel {
 public IList<Item> Items { get; private set; }

 public DataModel() {
  Items = new List<Item>();
 }
}

public class BaseNotifier : INotifyPropertyChanged {
 public event PropertyChangedEventHandler PropertyChanged;

 protected void Notify(string property) {
  var handler = PropertyChanged;
  if(null != handler) handler(this, new PropertyChangedEventArgs(property));
 }
}

DataModel dataModel = new DataModel();
dataModel.Items.Add(new Item { Name = "Item 1" });
dataModel.Items.Add(new Item { Name = "Item 2" });

class Form1 {
 // ...

 private void PopulateListBox(DataModel dataModel) {
  listBox1.Items.Clear();
  foreach(Item item in dataModel.Items) {
   listBox1.Items.Add(item);
   item1.PropertyChanged += HandleItemPropertyChanged;
  }
 }
}

private void HandleItemPropertyChanged(object sender, PropertyChangedEventArgs e) {
 Item item = (Item)sender;
 int index = listBox1.Items.IndexOf(item);
 if(index >= 0) {
  if(e.property == null || e.property == "Name") {
   int previousSelectedIndex = listBox1.SelectedIndex;
   listBox1.Items.Remove(item);
   listBox1.Items.Insert(index, item);
   listBox1.SelectedIndex = previousSelectedIndex;
  }
 }
}

class Form2 : Form {
 // ...

 private Item editingItem;
 public Item EditingItem {
  get { return editingItem; }
  set {
   textBox2.DataBindings.Clear();
   editingItem = value;
   if(null != value) {
    textBox2.DataBindings.Add("Text", value, "Name");
   }
  }
 }
}

public class ApplicationVM : BaseNotifier {
 private readonly DataModel model;
 
 public ApplicationVM(DataModel model) { this.model = model; }

 private Item selectedItem;
 public Item SelectedItem {
  get { return selectedItem; }
  set { selectedItem = value; Notify("SelectedItem"); }
 }

 public IList<Items> Items { get { return model.Items; } }
}

Form1 {
 // Attach this to ListBox.SelectedValueChanged
 private void listBoxSelectedValueChanged(object sender, EventArgs e) {
  viewModel.SelectedItem = (Item)listBox.SelectedItem;
 }
}

public Form1(ApplicationVM viewModel) {
 InitialiseComponent(); // this will already be there

 PopulateListBox(viewModel); // you will have added this above
 viewModel.PropertyChanged += (s, e) => listBox1.SelectedItem = viewModel.SelectedItem;
}

public Form2(ApplicationVM viewModel) {
 InitialiseComponent(); // this will already be there

 viewModel.PropertyChanged += (s, e) => EditingItem = viewModel.SelectedItem;
}

Chris Quinn wrote:

Take your computer into the pharmacy

object newString = "sample string";
s.add(newString);