C#

using System.Collections.Generic;
...
Dictionary<int,MyClass> MyDictionary = new Dictionary<int,MyClass>();
...
MyDictionary.Add(1,new MyClass(...));
MyDictionary.Add(2,new MyClass(...));
...
MyClass ClassNo1 = null;
if (MyDictionary.ContainsKey(1)) ClassNo1 = MyDictionary[1];
...

public class DataStructure
{
  public void Add(int key,MyObject data)
  {
    DataList.AddTail(new Entry(key,data));
    DataDictionary[key] = data;
  }
  public MyObject Find(key)
  {
    MyObject data = null;
    DataDictionary.TryGetValue(key,out data);
    return data;
  }
  private class Entry
  {
    public Entry(int key,MyObject data)
    {
      Key = key;
      Data = data;
    }
    public int Key;
    public MyObject Data;
  }
  private LinkedList<Entry> DataList = new LinkedList<Entry>();
  private Dictionary<int,MyObject> DataDictionary = new Dictionary<int,MyObject>();
}

class Form1 : Form
{
    ...
    Form2 RefToForm2 = new Form2();
    ...
    private void ButtonToShowForm2_Clicked(...)
    {
        RefToForm2.Show();
    }
    ...
}

class Form1 : Form
{
    ...
    private void ButtonToShowForm2_Clicked(...)
    {
        Form2 RefToForm2 = Application.OpenForms["Form2"];
        if (RefToForm2 == null) RefToForm2 = new Form2();
        RefToForm2.Show();
        RefToForm2.Activate();
    }
    ...
}