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// In software engineering, the singleton pattern is a design pattern used to implement the mathematical 
// concept of a singleton, by restricting the instantiation of a class to one object. This is useful when 
// exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized 
// to systems that operate more efficiently when only one object exists, or that restrict the instantiation 
// to a certain number of objects.
 
public sealed class MySingleton
{
    private static MySingleton instance;
    private static readonly Object sync = new object();
 
    private MySingleton()
    {
        // initialize members here
    }
 
    public static MySingleton Instance
    {
        get
        {
            if (instance == null)
            {
                lock (sync)
                {
                    if (instance == null)
                        instance = new MySingleton();
                }
            }
 
            return instance;
        }
    }
    
    public void SayHello()
    {
        Console.WriteLine("Hello!");
    }
    
}