Singleton Design Pattern For Multi-Threading

I create a class with singleton design pattern as below

   private static Object _lockObj = new Object();

   public static T CreateObject()
    {
        Debug.Print("Thread ID : " + Thread.CurrentThread.ManagedThreadId);
        // 'Double checked locking' pattern which avoids locking each
        // time the method is invoked
        if (_cpSng == null)
        {
            lock (_lockObj)
            {
                if (_cpSng == null)
                    _cpSng = new T();
            }
        }
        return _cpSng;
    }

as you see in comment i use double check and and object for avoid locking each time the method is invoked.

But when i use this class in multithread i have never face with an error like locking the object.

So,using double check and object is unnecessary ??

Someone can show me how can i get error without using double check and object.

Thanks.