Question

Consider you have the following code:
1. Why do we use double check lock, why single lock is not good enough, please provide detailed example.
2. What are the main drawbacks with this implementation? and how should I prove it?
Thanks.

 public sealed class SomeSingleton5  
{  
    private static SomeSingleton5 s_Instance = null;
    private static object s_LockObj = new Object();

    private SomeSingleton5() { }

    public static SomeSingleton5 Instance
    {
        get
        {
            if (s_Instance == null)
            {
                lock (s_LockObj)
                {
                    if (s_Instance == null)
                    {
                        s_Instance = new SomeSingleton5();
                    }
                }
            }

            return s_Instance;
        }
    }
}

Answer 1

I think the best implementation of singleton class is provided by Jon Skeet.

public sealed class Singleton
{
  private static readonly Singleton instance = new Singleton();
  public static Singleton Instance { get { return instance; } }
  static Singleton() {}
  private Singleton() {}
}

Singleton by Jon Skeet Clarification