Thread-Safe Singleton Implementation

Write a C# program to implement a thread-safe singleton class. A singleton is a design pattern that restricts the instantiation of a class to only one instance and provides a global point of access to it.

Example 1

  
Input: 

      
Logger logger1 = Logger.Instance;
      
logger1.Log("Message from thread 1");
      

      
Logger logger2 = Logger.Instance;
      
logger2.Log("Message from thread 2");
      

      
Console.WriteLine(object.ReferenceEquals(logger1, logger2));
    
  
  
Output: 

      
Initializing Logger instance...
      
Message from thread 1
      
Message from thread 2
      
True
    
  
  
Explanation: 

      
Step 1: First access to Logger.Instance creates a new instance, showing initialization message.
      
Step 2: The first logger instance logs a message from thread 1.
      
Step 3: Second access to Logger.Instance returns the existing instance (no initialization message).
      
Step 4: The second logger instance logs a message from thread 2.
      
Step 5: ReferenceEquals confirms both variables reference the same object, proving singleton behavior.
    
  

Example 2

  
Input: 

      
Task[] tasks = new Task[5];
      
for (int i = 0; i < tasks.Length; i++)
      
{
      
    int threadId = i;
      
    tasks[i] = Task.Run(() => {
      
        Logger logger = Logger.Instance;
      
        logger.Log($"Message from thread {threadId}");
      
    });
      
}
      
Task.WaitAll(tasks);
    
  
  
Output: 

      
Initializing Logger instance...
      
Message from thread 0
      
Message from thread 1
      
Message from thread 2
      
Message from thread 3
      
Message from thread 4
    
  
  
Explanation: 

      
Step 1: Create 5 parallel tasks, each accessing the Logger singleton.
      
Step 2: Only one initialization message appears, confirming the singleton is only initialized once.
      
Step 3: All tasks successfully access the same singleton instance to log their messages.
      
Step 4: Thread synchronization ensures no race conditions occur during initialization.
      
Step 5: All messages are properly logged from the 5 different threads using the same instance.
    
  

Constraints

  
Must be thread-safe under heavy concurrent access
  
Initialization should happen only once
  
Instance access time should be O(1) after initialization
  
Space Complexity: O(1) as only one instance exists
  
Must handle reentrant calls and avoid deadlocks