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Are static methods Thread-Safe? RRS feed

  • Question

  • Hi all,

     

    Wonder if someone could shed some light on this one?

     

    Lets say I have a static method like this:

    Code Snippet
    DoSomething(object x, int y, string s) {
     ..do something only with x,y,s and with local variables
     ..absolutely no reference to any shared variables
    }

    So this function is, what do they call, re-entrant?  It's self-sufficent and makes no references to any variables that may be modified by someone else. The function operates only on params passed to it - and those params are not shared either.

    Now - would this method require locking under any circumstance provided it may be called by different threads?

    What I'm trying to get at is - let's say the body is

    Code Snippet
    DoSomething(object x, int y, string s) {
    line 1: do something with x and y
    line 2: do something with s
    line 3: do something with x and y based on some value of S
    }

     

    Is it possible caller-1 is at line-3 and caller-2 is at line-2 and thereby screws up results for caller-1? I don't think this should happen, but I'm not sure how the internals of static methods work within the CLR so i'm not sure at all?!

    Any ideas appreciated...

    NozFx
    Wednesday, May 2, 2007 10:09 AM

Answers

  • Check out this web site (Threading in C#), which covers most of the threading topics to help with the concepts. Good Luck.
    Wednesday, May 2, 2007 11:16 PM
    Moderator
  • It's worth pointing out too, that the 'Object x' argument is a ref to an object, that object may well be shared, the same goes for the string arg too. In both cases you are simply passing references (pointers) to objects not copies of the objects.

     

    How do you know that when that static function is executing, some other thread is not manipulating x?

     

     

     

     

    Thursday, May 3, 2007 8:01 PM
  • No, it's not thread-safe.

     

    You're free to do whatever you want to x, which could be changed by an external thread.  y is a value type and s is immutable so nothing can change their values externally; but x can change externally.

     

    Just because a method is static does not mean it is thread-safe.

    Thursday, May 3, 2007 8:58 PM
    Moderator
  • Hi,

      I think based upon the assumption the OP made about the call:

     

    "The function operates only on params passed to it - and those params are not shared either."

     

      This is thread safe as long as that assumption holds true.  Agreed on the static does not mean it is thread safe comment.

     

     

    Mark.

    Thursday, May 3, 2007 9:09 PM

All replies

  • It is thread safe.
    Wednesday, May 2, 2007 11:12 AM
  • Check out this web site (Threading in C#), which covers most of the threading topics to help with the concepts. Good Luck.
    Wednesday, May 2, 2007 11:16 PM
    Moderator
  • It's worth pointing out too, that the 'Object x' argument is a ref to an object, that object may well be shared, the same goes for the string arg too. In both cases you are simply passing references (pointers) to objects not copies of the objects.

     

    How do you know that when that static function is executing, some other thread is not manipulating x?

     

     

     

     

    Thursday, May 3, 2007 8:01 PM
  • No, it's not thread-safe.

     

    You're free to do whatever you want to x, which could be changed by an external thread.  y is a value type and s is immutable so nothing can change their values externally; but x can change externally.

     

    Just because a method is static does not mean it is thread-safe.

    Thursday, May 3, 2007 8:58 PM
    Moderator
  • Hi,

      I think based upon the assumption the OP made about the call:

     

    "The function operates only on params passed to it - and those params are not shared either."

     

      This is thread safe as long as that assumption holds true.  Agreed on the static does not mean it is thread safe comment.

     

     

    Mark.

    Thursday, May 3, 2007 9:09 PM
  • Object x is "shared" ... Yes and No???

    Here is another scenario. If Thread 1 created object x and Thread 2 created another instance of object x, and they both call this static method passing their own instance of object x, what will happen when the method internally uses or even modifies object x? Will the static method modify each instance on object x independently of each other?

    Wednesday, February 4, 2009 8:13 PM
  • ajdiaz said:

    Object x is "shared" ... Yes and No???

    Here is another scenario. If Thread 1 created object x and Thread 2 created another instance of object x, and they both call this static method passing their own instance of object x, what will happen when the method internally uses or even modifies object x? Will the static method modify each instance on object x independently of each other?



    The two would be thread-safe from each other, but not necessarily from outside forces that might modify your object x (from either thread). 
    David Morton - http://blog.davemorton.net/
    Wednesday, February 4, 2009 8:24 PM
    Moderator
  • David M Morton said:

    ajdiaz said:

    Object x is "shared" ... Yes and No???

    Here is another scenario. If Thread 1 created object x and Thread 2 created another instance of object x, and they both call this static method passing their own instance of object x, what will happen when the method internally uses or even modifies object x? Will the static method modify each instance on object x independently of each other?



    The two would be thread-safe from each other, but not necessarily from outside forces that might modify your object x (from either thread). 
    David Morton - http://blog.davemorton.net/



    Excellent response David
    John Grove - TFD Group, Senior Software Engineer, EI Division, http://www.tfdg.com
    Wednesday, February 4, 2009 11:51 PM
  • 1. Static methods in .NET are always thread safe only if you are dealing with local varible scope inside the static method body.

    2. If you are delaing with any global variable inside static method it requires to put lock in case your global variables are changing the state. It is to be noted that it is applicable for static method and instance method both.

    Cheers :) . Happy programming.


    • Edited by Pramendra Singh Wednesday, October 9, 2013 1:32 PM modified message
    Wednesday, October 9, 2013 1:30 PM