6

Let's say I have a static class with a static method.

Multiple threads can call this static method concurrently.

Is there a potential for a race condition under these circumstances:

a - if the method depends only on local variables
b - if the method depends on local variables and member fields
cdeszaq
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Kevin Meredith
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  • Please edit your question to provide some pseudo code or samples to provide more information. Thanks. – Gray Mar 20 '12 at 17:45
  • None of that has any effect one way or the other. The only thing that matters is whether multiple threads use the same object. – Ben Voigt Mar 20 '12 at 17:46
  • Each method call has its own copy of the local variables, so I'm 99% sure that using only local variables will not create a race condition. Using member fields will, however. – leviathanbadger Mar 20 '12 at 17:46
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    It's not obvious what your "a" and "b" scenarios mean. Can you elaborate with some code that demonstrates your problem? – Paul Turner Mar 20 '12 at 17:46
  • @BenVoigt - Since it is a static method, if it is accessing member fields, those fields must also be static. Your saying "using the same object" would imply "using the same instance of an object", however we are dealing with static calls. – Stefan H Mar 20 '12 at 17:49
  • @Stefan: Nonsense. A static method can access members of an object passed in as a parameter, as well as objects which are static members of some type, or objects which are global variables. Whether the method is static has no connection to thread safety. – Ben Voigt Mar 20 '12 at 18:10
  • @StefanH: I see this bizarrely false claim all the time. Can you explain whyyou believe the falsehood that a static method cannot access instance fields? A static method can access *any* field of the type, including private fields. I am curious as to why people believe clearly false things; is it a problem with the documentation, or the language design, or what? – Eric Lippert Mar 20 '12 at 18:13
  • @EricLippert: It is a problem with the average quality of computer science professors. Hard to say whether the professors share this false concept of static member functions, or they know better but are inexact in their statements in lecture. It certainly isn't C# specific, this claim is made frequently in the context of C++ and Java as well, where it is equally wrong. – Ben Voigt Mar 20 '12 at 18:14
  • @BenVoigt: Well, the supposition there is that the typical C# programmer comes from a CS theory background. Though many do, a large population of C# programmers have no formal training in computer science. Besides which, computer science professors shouldn't be teaching *programming C#* (or Java or whatever) in the first place; they should be teaching *computer science*. – Eric Lippert Mar 20 '12 at 18:18
  • @EricLippert: The difference between the theory of computer science education and the practice of computer science education... And the problem propagates from the CS courses to those who have never taken a CS course, because those who learn programming "unconventionally" tend to trust the answers they get from formally trained CS graduates. – Ben Voigt Mar 20 '12 at 18:21
  • @EricLippert I was under the impression that accessing a non-static field from a static context was not possible. – Stefan H Mar 20 '12 at 18:32
  • @BenVoigt The example the OP gave for scenario B was not that it was manipulating an object that was passed into the method, but rather that it was accessing variables that are local to the method, as well as static members. Since the class is static, I think it does have a bearing on thread safety. I completely agree with all the possible ways that a static method could manipulate an object outside of the class, however that is not the scenario the OP laid out. – Stefan H Mar 20 '12 at 18:39
  • Let me further explain my reasoning, that I'm talking about instance fields in the class that the static method is defined in. In the OP's example, it is a static class and therefore has no instance members. I do realize that accessing instance members of a different object is entirely permissible from a static context. – Stefan H Mar 20 '12 at 18:49
  • @StefanH: Why do you talk about "members of a different object"? Different from what? It's a static method, there is no `this` object. Accessing instance members of ANY object is permissible from static context, subject to accessibility checking. – Ben Voigt Mar 20 '12 at 19:13
  • @BenVoigt Yes, I am well aware that there is no this object and accessing instance members of any object is permissible from a static context. However if the class were not static, static methods could not access instance members of the object that the static method is being called on. The point that I was trying to make is that your saying "None of that has any effect one way or the other. The only thing that matters is whether multiple threads use the same object." is not entirely accurate, because the fact that the members are static mean that separate threads will use the same objects. – Stefan H Mar 20 '12 at 19:45
  • @Stefan: It makes absolutely no sense to say "the object that the static method is being called on". A static method is not called on an object. A static method is only associated with a type. – Ben Voigt Mar 20 '12 at 20:04
  • @BenVoigt Sorry, that was bad wording on my part. I should have said the class that the static method is called on. Other than that though, the rest of my point stands. – Stefan H Mar 20 '12 at 20:06
  • @Stefan: Even so, [different threads do not necessarily use the same object (even though the object is stored in a static data member)](http://msdn.microsoft.com/en-us/library/system.threadstaticattribute.aspx). – Ben Voigt Mar 20 '12 at 20:13
  • @BenVoigt Sorry, I made the assumption that someone who is asking about thread safety with static members is not explicitly calling out those members as ThreadStatic. My bad. – Stefan H Mar 20 '12 at 20:15
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    let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/9109/discussion-between-ben-voigt-and-stefan-h) – Ben Voigt Mar 20 '12 at 20:16

3 Answers3

25

Let's say I have a static class with a static method. Multiple threads can call this static method concurrently.

OK.

Is there a potential for a race condition under these circumstances: a - if the method depends only on local variables

Yes, there are potential race conditions.

b - if the method depends on local variables and member fields

Yes, there are potential race conditions.

The answers to (a) and (b) are a consequence of a more general rule, namely, there is always the potential for race conditions any time you call any method from multiple threads. For example, this program deadlocks:

class MyClass
{
  static MyClass() 
  {
    // Let's run the initialization on another thread!
    var thread = new System.Threading.Thread(Initialize);
    thread.Start();
    thread.Join();
  }

  static void Initialize() 
  { }

  static void Main() 
  { }
}

It has no fields, two methods that do absolutely nothing, and a single local variable that is only accessed on a single thread. And nevertheless, it immediately and consistently deadlocks. (Do you see why? See http://ericlippert.com/2013/01/31/the-no-lock-deadlock/ for more on this program.)

It sounds like you are looking for a guarantee that your program is threadsafe if your static methods do not access fields. There is no such guarantee. Your program is threadsafe if and only if you write it to be threadsafe.

Eric Lippert
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  • Does anything change when: `Initialize` is non-static and `Thread(new MyClass().Initialize)` ? Or `Thread((MethodInvoker)delegate { new MyClass().Initialize(); })` ? – Ben Voigt Mar 20 '12 at 18:18
  • I'm not exactly sure that this example fits in the OP's problem frame. I was under the impression that he was referring to a static method being explicitly called by multiple external threads. – Tudor Mar 20 '12 at 18:22
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    @BenVoigt: Oddly enough, no. The runtime checks to see if the static ctor is run when the Initialize method is jitted, regardless of whether it is a static or an instance method. Now you might say, but the instance Initialize method cannot be called unless there is already an instance constructed, in which case the thread that did the constructor has called the static constructor. But (1) as we've seen, the static constructor might not be done yet, and (2) what if the "this" is null? In that case there might not have been an instance created. – Eric Lippert Mar 20 '12 at 18:25
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    @Tudor: My point is simply to illustrate that *any* method called on multiple threads causes interesting threading behaviour. The original poster is asking questions about locals and fields and whatnot, looking for a guarantee that he doesn't have to think hard about thread safety in some circumstances. **There is no such circumstance**. You always have to think hard about it. – Eric Lippert Mar 20 '12 at 18:26
  • @Eric: If the second thread (again) triggers invocation of the static ctor, my intuition would be that execution of the ctor would occur on the second thread which would result in recursive thread creation. What it seems like you're saying is that static ctors are only run on the primary thread, but that thread is currently in a wait state. There seems to be some magic here where one thread's execution triggers the execution of code on another thread (the second causing the ctor to again execute on the primary). – Tergiver Mar 22 '12 at 19:45
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    @Tergiver: Roughly speaking, the static constructor runs (1) before a member is touched for the first time, and (2) runs no more than once. The second thread waits for the first thread to complete before it continue. The second thread cannot continue, because then it would be continuing to access a member *during* the static construction, which would lead to a race condition, and the second thread cannot *run the ctor itself* because *again*, now we have two threads both running the same code at the same time. The second thread therefore blocks until the first thread unlocks the cctor. – Eric Lippert Mar 22 '12 at 21:35
  • @Eric: Are you saying that the first thread obtains a lock on some cctor-calling-lock, so the second thread deadlocks because it tries to take that same lock? And here I thought I had a fairly good grasp of multi-threading.. – Tergiver Mar 22 '12 at 21:52
  • @Tergiver: Exactly right. That's how some of Jon's mechanisms for a threadsafe lazy singleton works; they take advantage of the fact that accessing the field automatically takes out a lock on your behalf. See http://csharpindepth.com/Articles/General/Singleton.aspx for details. – Eric Lippert Mar 22 '12 at 22:53
  • Dear Eric, I noticed that your performance benchmarking articles are all HTTP 404 since 3+ days now. [Your blog post](https://ericlippert.com/2013/05/14/benchmarking-mistakes-part-one/) link still works, but [the main articles on TechPro](http://tech.pro/blog/1293/c-performance-benchmark-mistakes-part-one) are all gone. Those articles were really great and I have no idea what I shall do without them. It seems they are not on archive.org either. Do you know other locations or could you re-publish them somewhere? – Thomas Weller Feb 16 '17 at 19:33
  • @ThomasWeller: Thanks for the kind words. They've been gone for some months now. TechPro must have gone out of business. They commissioned a series of articles from me, I wrote the first few entries, and then their editor said "well, I'm quitting, you should hear from my replacement in a couple of days to schedule the next entry", and I never heard from them. I was unsurprised when they vanished. I should see if I still have copies of those somewhere. – Eric Lippert Feb 16 '17 at 20:55
  • @ThomasWeller: I found them. Posted here: https://ericlippert.com/2013/05/14/benchmarking-mistakes-part-one/ Apologies for the four year delay. :) – Eric Lippert Feb 11 '21 at 16:57
5

First off, a method is simply a piece of code residing at an address. Each thread calling a method will have a copy of that method and its local variables on its own private stack. So in case a, provided there are no other catches, it should be thread-safe.

Case b depends on a lot of factors:

  • are you actually accessing those member variables?
  • how are you accessing them: only reads, reads + writes, etc.
  • what kind of member variables: data structures, single values.
  • do you have any synchronization in place?
  • etc.

Generally though, assuming you do access the class members, it should not be considered thread-safe.

Tudor
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4

A - No. Race conditions only occur when static methods are attempting to access a shared resource. The local variable will be unique to each thread calling the method.

B - Yes. Those static members will be shared by all of the threads calling that method.

Stefan H
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  • The local variable will be unique, but what about the object it refers to? That might be shared. – svick Mar 20 '12 at 20:57
  • @svick I'm assuming that by "local variable" the OP does NOT mean anything that was passed into the method, or global variables. In other words, the local variable are objects that will be created and die within the method. – Stefan H Mar 20 '12 at 21:12