Docs: Are static constructors thread-safe?

[michael-hawker]

Would be nice if this article could call out thread-safety aspects of static constructors. Thanks!

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Document Details

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• ID: aa3ed1c5-1f79-9234-beec-b5771c3fb3c0
• Version Independent ID: ac4f6361-70a7-fb43-101e-c12044f60896
• Content: Static Constructors - C# Programming Guide
• Content Source: docs/csharp/programming-guide/classes-and-structs/static-constructors.md
• Product: dotnet-csharp
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

/cc @jcouv to check this answer.

My interpretation of the C# spec section on static constructors, it states:

The static constructor for a closed class type executes at most once in a given application domain.

That would seem to imply that only thread will invoke the static constructor.

However, there are no restrictions on the code you write inside a static constructor. You could write code that starts new threads in the constructor. That would require your own thread synchronization.

[michael-hawker]

Yeah, this is a similar discussion #10244, there's always unsafe things you can do anywhere in code, but it's always nice to know when you don't need an extra layer of mutexing around things or not or can use the inherit constructs in the language. :)

[BillWagner]

I'll propose the following text based on the above comments:

The runtime calls a static constructor no more than once in a single application domain. Furthermore, that call is made in a locked region based on the specific type of the class. No additional locking mechanisms are needed in the body of a static constructor.

Please vote up or down, with additional suggestions.

[EugeneKramer]

As it's executed under a lock, might also be worth noting what to avoid:

Static constructor deadlocks:

So, to avoid the risk of deadlocks, avoid blocking the current thread in static constructors and initializers: don’t wait on tasks, threads, wait handles or events, don’t acquire locks, and don’t execute blocking parallel operations like parallel loops, Parallel.Invoke and PLINQ queries.