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Is this method thread-safe? It seems as though it isn't...

public static void Foo(string _str, Guid _id)
{
    _str = _str + _id.ToString();

    /*
        Do Stuff 
    */

    return 
}
Adam
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  • Related posts - [C# : What if a static method is called from multiple threads?](https://stackoverflow.com/q/3037637/465053) & [Are static methods thread safe](https://stackoverflow.com/q/1090650/465053) – RBT Jun 28 '18 at 00:08

2 Answers2

23

The parameters are, in this case, two immutable values. Within a method, only a single thread is operating on the set of parameters, as multiple threads calling the method will each have their own stack and execution context, which means each thread has their own independent set of parameters and local variables, so no other thread can impact those variables.

As such, this is completely thread safe in relation to those two variables.

Note that, parameters passed by ref are not necessarily thread safe, as that would potentially allow a single variable to be shared among two or more threads, which would require synchronization.

Also, if you pass a reference type instance that isn't immutable (ie: a custom class) as a parameter, the internal state of that class would need synchronization, as it could potentially be used by more than one thread. The reference itself would be thread safe, as it's passed as a copy (unless passed using ref).

Reed Copsey
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  • Do you mean "within a static method"? If that is the case, then thank you! I understand – Adam Jun 21 '13 at 19:19
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    @Adam It actually doesn't matter if the method is static or not - the same issues apply (in regards to the parameter list). – Reed Copsey Jun 21 '13 at 19:21
  • @Reed Ahh, I see. But if the second line were ``newStr = ...`` where ``newStr`` was a private property, then it would cause threading issues. Correct? – Adam Jun 21 '13 at 19:23
  • @BenVoight Yes, this is what I was getting at. Thank you for the clarification. – Adam Jun 21 '13 at 19:23
  • @Adam Yes - if you're working on state that's *shared*, then it could be a problem. – Reed Copsey Jun 21 '13 at 19:25
  • ... and the only way to impact any local variable from outside the method is if its address is stored or passed outside the method. – Ben Voigt Jun 21 '13 at 19:25
  • Comprehending a 'thread unsafe' vulnerability in case of 'shared' data is easy. I want to ask, if 10 threads call a static class' static method, each passing a reference type, say a Person object, and let the method is responsible for incrementing the person's age by 1, then what will happen? – Vaibhav Jul 25 '14 at 18:01
  • @Vaibhav I'd consider that an issue with mutating input state, though - specifically something I said was avoided here. – Reed Copsey Jul 25 '14 at 18:30
  • @ReedCopsey - "issue with mutating input state", please excuse my ignorance but I don't think I get the point you are trying to make here. Also, I do not understand if you mean something 'you said' was avoided in your or my comment! – Vaibhav Jul 25 '14 at 18:39
  • @Vaibhav My answer starts with mentioning that the parameters are both immutable - your example is basically saying "if you pass something that is mutable, it can be bad" – Reed Copsey Jul 25 '14 at 19:03
  • @ReedCopsey - "if you pass something that is mutable..", my understanding is simply banking on the fact that each calling thread will have its local stack variables and a copy of the method itself. Hence, should passing a mutable object really matter ('bad')? Edit: I must mention that only the calling thread is the sole owner of the mutable argument passed and there is no processing/operations performed on that object outside of the static or (may be) the calling function. – Vaibhav Jul 28 '14 at 07:30
5

The parameters themselves are by definition thread-safe. It does not matter whether the method is static or not.

They could however be references to other data and that is not automatically thread-safe.

Your example uses a value type and an immutable reference types so this particular case is OK.

H H
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  • (caveat: `System.String` isn't truly immutable. Thanks to Microsoft not implementing const-correctness in .NET.) – Ben Voigt Jun 21 '13 at 19:26
  • May I ask what do you really mean by 'const-correctness' here? Is it a pointer to const implementation internally? I know that every time we reassign a string var, it uses a new memory location instead of overwriting old. – Vaibhav Jul 25 '14 at 18:14
  • @BenVoigt what do you mean? As far as I know [string are immutable](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/strings/#:~:text=String%20objects%20are%20immutable%3A%20they,in%20a%20new%20string%20object.). Each string is copied everytime using optimization techniques if two strings are the same – Krusty Aug 05 '20 at 14:57
  • @Krusty: They are not actually immutable, only intended to be immutable. Either `fixed (char* p = str)` or passing the string through p/invoke can change the contents. This is one reason that string interning is not performed "everytime" but only for literals or if you specifically request it. – Ben Voigt Aug 05 '20 at 14:58