F:\work\C#>HelloWorld.exe
My country is Russia
Count of objects:  1 

Unhandled Exception: System.ApplicationException: Only one instance!!!
   at HelloWorld..ctor()
   at HelloWorld.Main()

using System;

public class HelloWorld
{
 private static String myCountry =  "Russia" ;
 private static int Counter;

 public HelloWorld()
 {
  myCountry =  "Russia" ;
  if (Counter ==  1 )
  {
   throw new ApplicationException( "Only one instance!!!" );
  }

  Counter++;
 }

 public static int getCounter
 {
  get
  {
   return Counter;
  }
 }

 public String Country
 {
  get
  {
   return myCountry;
  }
 }

 public static void  Main()
 {
  Console.WriteLine( "My country is {0 }",myCountry);
  HelloWorld hw1 = new HelloWorld();
  Console.WriteLine( "Count of objects: {0 }",HelloWorld.getCounter);
  HelloWorld hw2 = new HelloWorld();
  Console.WriteLine( "Count of objects: {0 }",HelloWorld.getCounter);
 }
}

 /*
 Что-бы нельзя было создать обьект обычным способом - 
конструктор обьявляем как private. Но он виден внутри класса, поэтому
для создания обьекта создаём специальный public static метод GetObject.
Компилируем как csc /target:library HelloWorld.cs
*/ 

using System;

public class HelloWorld
{
 private static String myCountry =  "Russia" ;
 private static int Counter =  0 ;

 private HelloWorld()
 {
  Counter++;
 }

 public static HelloWorld GetObject
 {
   get
   {
    if (Counter ==  0 )
    {
     HelloWorld hw = new HelloWorld();
     return hw;
    }
    else
    {
     Console.WriteLine( "Only one instance!!!" );
     return null;
    }
   }
 }

 public static int getCounter
 {
  get
  {
   return Counter;
  }
 }

 public String Country
 {
  get
  {
   return myCountry;
  }
 }
}

 /*
Создаём отдельный класс для тестирования 
Компилируем как csc /reference:HelloWorld.dll Test.cs
*/ 

using System;

public class Test
{
 public static void  Main()
 {
  HelloWorld hw1 = HelloWorld.GetObject;
  Console.WriteLine( "Count of objects: {0 }",HelloWorld.getCounter);
  HelloWorld hw2 = HelloWorld.GetObject;
  Console.WriteLine( "Count of objects: {0 }",HelloWorld.getCounter);
 }
}

F:\work\C#>Test.exe
Count of objects:  1 
Only one instance!!!
Count of objects:  1 

F:\work\C#>csc /reference:HelloWorld.dll Test.cs
Microsoft (R) Visual C# .NET Compiler version  7 . 00 . 9466 
for Microsoft (R) .NET Framework version  1 . 0 . 3705 
Copyright (C) Microsoft Corporation  2001 . All rights reserved.

Test.cs( 17 , 21 ): error CS0122: 'HelloWorld.HelloWorld()' is inaccessible due to its 
protection level

...........
   HelloWorld hw3 = new HelloWorld();
   Console.WriteLine( "Count of objects: {0 }",HelloWorld.getCounter);
...........

using System;

public class One
{

    public static void  Main()
    {

      Console.WriteLine(myGenerator.OneObject.ItsMe);
      // Два экз. класса  AloneInTheDark
      AloneInTheDark a1 = new AloneInTheDark();
      AloneInTheDark a2 = new AloneInTheDark();
      // Два экз. класса One
      One o1 = new One();
      One o2 = new One();
      // Здесь создать обьект класса myGenerator не получится создать вообще
      // так у него public конструктора
      myGenerator mg1 = new myGenerator();
    }

    public class myGenerator
    {
        public static AloneInTheDark OneObject = new AloneInTheDark();
        private myGenerator(){}
    }

    public class AloneInTheDark
    {
        public string ItsMe =  "Here I am" ;
    }

}

F:\work\C#>csc One.cs
Microsoft (R) Visual C# .NET Compiler version  7 . 00 . 9466 
for Microsoft (R) .NET Framework version  1 . 0 . 3705 
Copyright (C) Microsoft Corporation  2001 . All rights reserved.

One.cs( 17 , 25 ): error CS0122: 'One.myGenerator.myGenerator()' is inaccessible due 
to its
        protection level