using System;
using System.Globalization;
using System.Reflection;
 
public abstract class SingletonBase<t> where T : class
{
  /// <summary>
  /// A protected constructor which is accessible only to the sub classes.
  /// </summary>
  protected SingletonBase() { }
 
  /// <summary>
  /// Gets the singleton instance of this class.
  /// </summary>
  public static T Instance
  {
    get { return SingletonFactory.Instance; }
  }
 
  /// <summary>
  /// The singleton class factory to create the singleton instance.
  /// </summary>
  class SingletonFactory
  {
    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static SingletonFactory() { }
 
    // Prevent the compiler from generating a default constructor.
    SingletonFactory() { }
 
	internal static readonly T Instance = GetInstance();
 
    static T GetInstance()
    {
      var theType = typeof(T);
 
      T inst;
 
      try
      {
        inst = (T)theType
          .InvokeMember(theType.Name,
            BindingFlags.CreateInstance | BindingFlags.Instance
            | BindingFlags.NonPublic,
            null, null, null,
            CultureInfo.InvariantCulture);
      }
      catch (MissingMethodException ex)
      {
        throw new TypeLoadException(string.Format(
          CultureInfo.CurrentCulture,
          "The type '{0}' must have a private constructor to " +
          "be used in the Singleton pattern.", theType.FullName)
          , ex);
      }
 
      return inst;
    }
  }
}

// Must seal the class so no sub-classes can be defined.
public sealed class SequenceGeneratorSingleton
  : SingletonBase<sequenceGeneratorSingleton>
{
  // Must have a private constructor so no instance can be created externally.
  SequenceGeneratorSingleton()
  {
    _number = 0;
  }
 
  private int _number;
 
  public int GetSequenceNumber()
  {
	return _number++;
  }
}

class Program
{
  static void Main(string[] args)
  {
    Console.WriteLine("Sequence: " + SequenceGeneratorSingleton.Instance
	  .GetSequenceNumber().ToString());  // Print "Sequence: 0"
 
    Console.WriteLine("Sequence: " + SequenceGeneratorSingleton.Instance
	  .GetSequenceNumber().ToString());  // Print "Sequence: 1"
 
    Console.WriteLine("Sequence: " + SequenceGeneratorSingleton.Instance
	  .GetSequenceNumber().ToString());  // Print "Sequence: 2"
  }
}