DRAG DROP
You are developing an application that includes a class named Warehouse. The Warehouse class includes a static property named Inventory.
The Warehouse class is defined by the following code segment. (Line numbers are included for reference only.)
You have the following requirements:
• Initialize the _inventory field to an Inventory instance.
• Initialize the _inventory field only once.
• Ensure that the application code acquires a lock only when the _inventory object must be instantiated.
You need to meet the requirements.
Which three code segments should you insert in sequence at line 09? (To answer, move the appropriate code segments from the list of code segments to the answer area and arrange them in the correct order.)
Answer: 
Explanation:
After taking a lock you must check once again the _inventory field to be sure that other threads didn’t instantiated it in the meantime.
First, you check if the inventory is null, if so, you lock it to avoid other threads to change it.
Second, you check again for the null, as in the tiny millisecond between check for null and locking could another thread get it.
Finally you create the instance and release the lock.