C#

public class Singleton : Component
{
   private static SingletonInternal instance = null;
   private static int instancecount = 0;

   private class SingletonInternal : IDisposable
   {
      //just the normal "instance" implementation

      public void SomeMethod()
      {
      }

      public void Dispose()
      {
         // free unmanaged resources
      }
   }

   public Singleton()
   {
      if (instance == null)
         instance = new SingletonInternal();
      instancecount++;
   }

   public void SomeMethod()
   {
      instance.SomeMethod();
   }
   
   protected override void Dispose(bool disposing)
   {
      if (!this.disposed)
      {
         if (disposing)
         {
            // free managed resources
         }
         instancecount--;
         if (instancecount == 0)
            instance.Dispose();

         this.disposed = true;
      }
   }
}

using System;<br />
using System.Windows.Forms;<br />
namespace myApp<br />
{<br />
	// activeScreen must be public (this is call by several classes)<br />
	public ???? activeScreen <br />
<br />
	public class myApp : System.Windows.Forms.Form<br />
	{<br />
		public mYApp() {<br />
			// value=1 must be created Screen1 object<br />
			// value=2 must be created Screen2 object<br />
<br />
		<br />
			// Variable condition<br />
			int value = 1;<br />
<br />
			switch(value) {<br />
				case 1:<br />
					// How create "Screen1" and asign to activeScreen ????<br />
					// Remember activeScreen must be access by another clasess<br />
					Console.WriteLine(activeScreen.MyMethod());<br />
					break;		<br />
<br />
				case 2:<br />
					// How create "Screen2" and asign to activeScreen ????<br />
					Console.WriteLine(activeScreen.MyMethod());<br />
					break;<br />
			}		<br />
		}<br />
	}<br />
}

using System;<br />
using System.Windows.Forms;<br />
namespace myApp<br />
{<br />
	public class Screen2 : System.Windows.Forms.UserControl<br />
	{<br />
		public Screen2()<br />
		{<br />
		<br />
		}<br />
		public int MyMethod() {<br />
			int value = 2;<br />
			return value;<br />
		}<br />
	}<br />
}

using System;<br />
using System.Windows.Forms;<br />
namespace myApp<br />
{<br />
	public class Screen1 : System.Windows.Forms.UserControl<br />
	{<br />
		public Screen1()<br />
		{<br />
		<br />
		}<br />
		public int MyMethod() {<br />
			int value = 1;<br />
			return value;<br />
		}<br />
	}<br />
}