Is a C# Singleton with a Static Constructor Thread-Safe?

Singleton thread safety issues in C# static constructor

This article explores whether C# singleton classes implemented using static constructors are thread-safe, and how to ensure that only one instance exists in a multi-threaded environment.

Initial construction

Static constructors are executed only once in the application domain, before the class is instantiated or the static members are accessed. Therefore, the initial construction of a singleton instance is thread-safe.

Follow-up visit

However, it is critical to ensure that access after initial construction is thread-safe. The provided implementation allows multiple threads to access the singleton instance simultaneously, potentially causing problems.

Synchronization mechanism

To solve this problem, the implementation can be enhanced using a synchronization mechanism. One way is to add a static mutex like this:

<code class="language-c#">public class Singleton
{
    private static Singleton instance;
    // 添加静态互斥锁以同步实例的使用。
    private static System.Threading.Mutex mutex;
    private Singleton() { }
    static Singleton()
    {
        instance = new Singleton();
        mutex = new System.Threading.Mutex();
    }

    public static Singleton Acquire()
    {
        // 阻塞当前线程，直到它获得对互斥锁的访问权。
        mutex.WaitOne();
        return instance;
    }

    // 释放互斥锁，允许其他线程访问实例。
    public static void Release()
    {
        mutex.ReleaseMutex();
    }
}</code>

How to use

In order to safely access a singleton instance, a thread must first acquire a mutex using the Acquire() method. Once completed, the Release() method will unlock the mutex, allowing other threads to access the instance.

Conclusion

By adding a synchronization mechanism, the singleton implementation becomes thread-safe, eliminating potential concurrency issues.

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