System.String and Thread Safety
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Hello, Is it necessary to ensure thread-safety for an instance of the
System.Stringclass? I understand thatStrings are immutable types, but I was unsure if this necessarily means that an instance of the System.String class is thread-safe for multiple readers and multiple writers. In my application, severalStrings which belong to an instance of a class will be both accessed and changed from multiple threads. I can easily put a lock on them and make sure the operations do not clash, but if I do not have to waste time waiting for the lock because the type is naturally thread-safe then of course I do not want to. I understand that this may be a simple question, but for some reason I cannot find a clear universal answer to the question. Thanks in advance! Sincerely, Alexander WisemanAs strings are immutable, you can't change a string. What happens when you assign a new value to a string variable, is that the new value is a separate string object, and the reference to that object replaces the reference to the previous value. As reading and writing references are atomic, there is no need to use a lock. When you read the string value, you are copying the reference to the string. You can then use that reference to access the string that was the value at the moment you read the reference, without fear of anything happening to that string. --- b { font-weight: normal; }
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Alexander Wiseman wrote:
That being said, my original confusion remains, because it seems like, from the MSDN documentation, there is nothing required on the part of the user of the System.String class to ensure thread safety. What little I have read concerning "immutability" of the string type would support this. On the other hand, if I am doing multiple write operations and multiple read operations, is there going to be a problem?
Since strings are immutable, they are thread safe. What happens when you perform a write operation on a string is that a new string is created as a result; the original string remains unchanged.
Thanks for your reply. Let me just make sure I have this clear with a quick example. Let us say that I have two threads, A and B, both of which have access to
String someStr = "My String". Now let's say that the processor gives a time-slice to thread A, which then reads the string. Then the processor gives a time-slice to thread B, which then changes the string from "My String" to "Your String". If I understand you correctly, then because strings are immutable, no matter when the processor decides to temporarily stop processing on thread A, nevertheless thread A will still see the value "My String", even though thread B may have "changed" it already. Is that correct? Thanks again for your help! Sincerely, Alexander Wiseman -- modified at 17:48 Tuesday 13th June, 2006 [EDIT] Just read another poster's reply, which answers this scenario in the affirmative. [/EDIT] -
As strings are immutable, you can't change a string. What happens when you assign a new value to a string variable, is that the new value is a separate string object, and the reference to that object replaces the reference to the previous value. As reading and writing references are atomic, there is no need to use a lock. When you read the string value, you are copying the reference to the string. You can then use that reference to access the string that was the value at the moment you read the reference, without fear of anything happening to that string. --- b { font-weight: normal; }
Thanks for your reply Guffa. That makes perfect sense! I think the thing that I was missing was that reading and writing references are atomic. Sincerely, Alexander Wiseman
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Alexander Wiseman wrote:
In my application, several Strings which belong to an instance of a class will be both accessed and changed from multiple threads. I can easily put a lock on them and make sure the operations do not clash, but if I do not have to waste time waiting for the lock because the type is naturally thread-safe then of course I do not want to.
When you say that the strings belong to an instance of a class and are accessed and changed, are you talking about string properties? -- modified at 17:44 Tuesday 13th June, 2006
Thanks for your help, Leslie. Actually I think the question was simpler than that - Guffa's post below answers it. Sincerely, Alexander Wiseman
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Thanks for your reply. Let me just make sure I have this clear with a quick example. Let us say that I have two threads, A and B, both of which have access to
String someStr = "My String". Now let's say that the processor gives a time-slice to thread A, which then reads the string. Then the processor gives a time-slice to thread B, which then changes the string from "My String" to "Your String". If I understand you correctly, then because strings are immutable, no matter when the processor decides to temporarily stop processing on thread A, nevertheless thread A will still see the value "My String", even though thread B may have "changed" it already. Is that correct? Thanks again for your help! Sincerely, Alexander Wiseman -- modified at 17:48 Tuesday 13th June, 2006 [EDIT] Just read another poster's reply, which answers this scenario in the affirmative. [/EDIT]Alexander Wiseman wrote:
Let me just make sure I have this clear with a quick example. Let us say that I have two threads, A and B, both of which have access to String someStr = "My String". Now let's say that the processor gives a time-slice to thread A, which then reads the string. Then the processor gives a time-slice to thread B, which then changes the string from "My String" to "Your String". If I understand you correctly, then because strings are immutable, no matter when the processor decides to temporarily stop processing on thread A, nevertheless thread A will still see the value "My String", even though thread B may have "changed" it already. Is that correct?
What you're doing here isn't changing the string. You are changing the reference to the string. When you do this:
string someString = "My String";
And then do this:
someString = "Your String";
You've taken the reference to "My String" and changed it to point (sorry for the C++ nomenclature) to "Your String"; That's not the same as actually changing the string. And as far as I know, changing what a reference, erm, references is not an intrinsically thread safe operation, but I could be wrong. Changing the actual string would be doing something like this:
string someString = "My String";
// Just making this method up for the example.
someString.Concatenate(" and Your String");// Output: "My String and Your String"
Console.WriteLine(someString);Now, the above code isn't possible with an immutable string. Rather the operation for concatenating a string creates a new string:
string someString = "My String";
string anotherString = someString.Concatenate(" and Your String");
// Output: My String"
Console.WriteLine(someString);
// Output: My String and Your String"
Console.WriteLine(anotherString);Again, I'm not sure if the string class actually has a "Concatenate" method, but the principle is the same. When doing a write operation on a string, a new string is created as a result leaving the original unchanged. [EDIT]Apparently, changing what an object references is an atomic operation, so it is thread safe.[/EDIT] -- modified at 18:00 Tuesday 13th June, 2006
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As strings are immutable, you can't change a string. What happens when you assign a new value to a string variable, is that the new value is a separate string object, and the reference to that object replaces the reference to the previous value. As reading and writing references are atomic, there is no need to use a lock. When you read the string value, you are copying the reference to the string. You can then use that reference to access the string that was the value at the moment you read the reference, without fear of anything happening to that string. --- b { font-weight: normal; }
Guffa wrote:
As reading and writing references are atomic, there is no need to use a lock.
Oh, that's good to know! I had guessed that it wasn't but glad to learn that I was wrong.
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Guffa wrote:
As reading and writing references are atomic, there is no need to use a lock.
Oh, that's good to know! I had guessed that it wasn't but glad to learn that I was wrong.
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Guffa wrote:
As reading and writing references are atomic, there is no need to use a lock.
Oh, that's good to know! I had guessed that it wasn't but glad to learn that I was wrong.
It's kind of misleading though. For instance:
if(someVolatileVariable != null)
{
someVolatileVariable.DoSomething(); // Buggy! (in a multithreaded environement, anyways)
}That is buggy multithreaded code! The reason is that another thread could potentially set someVolatileVariable to null after the current thread checks whether it is not null. A correct version of that would be:
Foo someVar = someVolatileVariable;
if(someVar != null)
{
someVar.DoSomething();
}Tech, life, family, faith: Give me a visit. I'm currently blogging about: Goof around music jam with my brothers (with video) The apostle Paul, modernly speaking: Epistles of Paul Judah Himango
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As strings are immutable, you can't change a string. What happens when you assign a new value to a string variable, is that the new value is a separate string object, and the reference to that object replaces the reference to the previous value. As reading and writing references are atomic, there is no need to use a lock. When you read the string value, you are copying the reference to the string. You can then use that reference to access the string that was the value at the moment you read the reference, without fear of anything happening to that string. --- b { font-weight: normal; }
Reading and writing references may be atomic, but that doesn't solve much; a thread may need to read a variable, but may be seeing old/dirty data depending on the processor architecture. That's why we have methods like Thread.VolatileRead, Thread.VolatileWrite, and the volatile keyword.
Tech, life, family, faith: Give me a visit. I'm currently blogging about: Goof around music jam with my brothers (with video) The apostle Paul, modernly speaking: Epistles of Paul Judah Himango
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It's kind of misleading though. For instance:
if(someVolatileVariable != null)
{
someVolatileVariable.DoSomething(); // Buggy! (in a multithreaded environement, anyways)
}That is buggy multithreaded code! The reason is that another thread could potentially set someVolatileVariable to null after the current thread checks whether it is not null. A correct version of that would be:
Foo someVar = someVolatileVariable;
if(someVar != null)
{
someVar.DoSomething();
}Tech, life, family, faith: Give me a visit. I'm currently blogging about: Goof around music jam with my brothers (with video) The apostle Paul, modernly speaking: Epistles of Paul Judah Himango
Good point. Events work the same way: Bad:
if(WorkCompleted != null)
{
WorkCompleted(this, EventArgs.Empty);
}Good:
EventHandler handler = WorkCompleted;
if(handler != null)
{
handler(this, EventArgs.Empty);
}