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Is it safe to access multiple indexes of a C# List concurrently?

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2

1

I've got a list full of structs, which I want to iterate through and alter concurrently.

The code is conceptually as follows:

Parallel.For(0, pointsList.Count(), i=> pointsList[i] = DoThing(pointsList[i]));

I'm neither adding to nor removing from the list, only accessing and mutating its items.

I imagine this is fine, but thought I should check: is this OK as is, or do I need to use Lock somewhere for fear that I mess up the list object??

c# multithreading list

share|improve this question

edited Mar 26 at 4:08

Jack

asked Mar 26 at 3:19

JackJack

11399 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It is safe to perform multiple read operations on a List<T>, but issues can occur if the collection is modified while it's being read.
  
  – PetSerAl
  Mar 26 at 3:23
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of Can a List<t> be accessed by multiple threads? or Parallel.ForEach on List<Object> Thread Safety
  
  – BACON
  Mar 26 at 3:23
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi @PetSerAl. This is indeed a relevant link, which I have already looked up. However, the page was not explicit though about exactly which operations would cause which issues. For example, I would expect adding, inserting, removing, and clearing a list all to cause issues, I would possibly think that simply accessing list elements is fine
  
  – Jack
  Mar 26 at 3:28
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi @BACON, thanks for the response: this question is different to the one you linked, which is premised on the idea that "the list will be locked during a changes", whereas I'm asking if this is really neccesary.
  
  – Jack
  Mar 26 at 3:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The link from @PetSerAl tells you all you need to know. Reading the list is fine, modifying it is not.
  
  – DavidG
  Mar 26 at 3:36
  

  
  
  
  

 | 
show 2 more comments

2

1

I've got a list full of structs, which I want to iterate through and alter concurrently.

The code is conceptually as follows:

Parallel.For(0, pointsList.Count(), i=> pointsList[i] = DoThing(pointsList[i]));

I'm neither adding to nor removing from the list, only accessing and mutating its items.

I imagine this is fine, but thought I should check: is this OK as is, or do I need to use Lock somewhere for fear that I mess up the list object??

c# multithreading list

share|improve this question

edited Mar 26 at 4:08

Jack

asked Mar 26 at 3:19

JackJack

11399 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It is safe to perform multiple read operations on a List<T>, but issues can occur if the collection is modified while it's being read.
  
  – PetSerAl
  Mar 26 at 3:23
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of Can a List<t> be accessed by multiple threads? or Parallel.ForEach on List<Object> Thread Safety
  
  – BACON
  Mar 26 at 3:23
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi @PetSerAl. This is indeed a relevant link, which I have already looked up. However, the page was not explicit though about exactly which operations would cause which issues. For example, I would expect adding, inserting, removing, and clearing a list all to cause issues, I would possibly think that simply accessing list elements is fine
  
  – Jack
  Mar 26 at 3:28
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi @BACON, thanks for the response: this question is different to the one you linked, which is premised on the idea that "the list will be locked during a changes", whereas I'm asking if this is really neccesary.
  
  – Jack
  Mar 26 at 3:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The link from @PetSerAl tells you all you need to know. Reading the list is fine, modifying it is not.
  
  – DavidG
  Mar 26 at 3:36
  

  
  
  
  

 | 
show 2 more comments

I've got a list full of structs, which I want to iterate through and alter concurrently.

The code is conceptually as follows:

Parallel.For(0, pointsList.Count(), i=> pointsList[i] = DoThing(pointsList[i]));

I'm neither adding to nor removing from the list, only accessing and mutating its items.

I imagine this is fine, but thought I should check: is this OK as is, or do I need to use Lock somewhere for fear that I mess up the list object??

c# multithreading list

share|improve this question

edited Mar 26 at 4:08

Jack

asked Mar 26 at 3:19

JackJack

11399 bronze badges

Parallel.For(0, pointsList.Count(), i=> pointsList[i] = DoThing(pointsList[i]));

share|improve this question

edited Mar 26 at 4:08

Jack

asked Mar 26 at 3:19

JackJack

11399 bronze badges

share|improve this question

edited Mar 26 at 4:08

Jack

asked Mar 26 at 3:19

JackJack

11399 bronze badges

asked Mar 26 at 3:19

JackJack

11399 bronze badges

asked Mar 26 at 3:19

JackJack

11399 bronze badges

1 Answer
1

active

oldest

votes

4

It is not guaranteed to be safe. Changing an element in the list increments the list's private _version field, which is how various threads can tell if the list has been modified. If any other thread attempts to enumerate the list or uses a method like List<T>.ForEach() then there could potentially be an issue. It sounds like it would be extremely rare, which isn't necessarily a good thing-- in fact, it makes for the most maddening type of defect.

If you want to be safe, create a new list instead of changing the existing one. This is a common "functional" approach to threading issues that avoids locks.

var newList = pointsList.AsParallel().Select( item => DoThing(item) );

When you're done, you can always replace the old list if you want.

pointsList = newList.ToList();

share|improve this answer

edited Mar 26 at 3:45

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Good point on it bumping the version number. I didn't expect it to do that but it does. The relevant source code is here / here / here.
  
  – BACON
  Mar 26 at 3:45
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Oh yeah, it looks like incrementing _version in parallel could result in a dirty write. This is only relevant for when the list is being altered or used as an IEnumerable however, neither of which I am doing. Furthermore, if I did perform any of the above operations on pointsList later on, the residual value inside _version wouldn't actually have any detrimental affect. However, the thought process here is complex enough I think I'd want to save the next guy from having to think about it, and so I think I'll avoid assigning to the list in parallel, as suggested.
  
  – Jack
  Mar 26 at 4:00
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the answer John. .AsParallel() is a nifty little extension method.
  
  – Jack
  Mar 26 at 4:01
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Jack After further consideration, I reached the same conclusion as your 2nd sentence. The set indexer invalidates any enumeration operations...but why does that matter if you know there are none? It's like using a for loop to remove list items: no, you can't simultaneously access the list on another thread, but having that limitation doesn't mean that loop is "unsafe". I suppose it boils down to if you know the list won't be accessed in problematic ways then there's no problem, otherwise taking the cautious approach isn't a bad thing, either.
  
  – BACON
  Mar 26 at 19:38
  

  
  
  
  

add a comment | 

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4

It is not guaranteed to be safe. Changing an element in the list increments the list's private _version field, which is how various threads can tell if the list has been modified. If any other thread attempts to enumerate the list or uses a method like List<T>.ForEach() then there could potentially be an issue. It sounds like it would be extremely rare, which isn't necessarily a good thing-- in fact, it makes for the most maddening type of defect.

If you want to be safe, create a new list instead of changing the existing one. This is a common "functional" approach to threading issues that avoids locks.

var newList = pointsList.AsParallel().Select( item => DoThing(item) );

When you're done, you can always replace the old list if you want.

pointsList = newList.ToList();

share|improve this answer

edited Mar 26 at 3:45

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Good point on it bumping the version number. I didn't expect it to do that but it does. The relevant source code is here / here / here.
  
  – BACON
  Mar 26 at 3:45
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Oh yeah, it looks like incrementing _version in parallel could result in a dirty write. This is only relevant for when the list is being altered or used as an IEnumerable however, neither of which I am doing. Furthermore, if I did perform any of the above operations on pointsList later on, the residual value inside _version wouldn't actually have any detrimental affect. However, the thought process here is complex enough I think I'd want to save the next guy from having to think about it, and so I think I'll avoid assigning to the list in parallel, as suggested.
  
  – Jack
  Mar 26 at 4:00
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the answer John. .AsParallel() is a nifty little extension method.
  
  – Jack
  Mar 26 at 4:01
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Jack After further consideration, I reached the same conclusion as your 2nd sentence. The set indexer invalidates any enumeration operations...but why does that matter if you know there are none? It's like using a for loop to remove list items: no, you can't simultaneously access the list on another thread, but having that limitation doesn't mean that loop is "unsafe". I suppose it boils down to if you know the list won't be accessed in problematic ways then there's no problem, otherwise taking the cautious approach isn't a bad thing, either.
  
  – BACON
  Mar 26 at 19:38
  

  
  
  
  

add a comment | 

4

It is not guaranteed to be safe. Changing an element in the list increments the list's private _version field, which is how various threads can tell if the list has been modified. If any other thread attempts to enumerate the list or uses a method like List<T>.ForEach() then there could potentially be an issue. It sounds like it would be extremely rare, which isn't necessarily a good thing-- in fact, it makes for the most maddening type of defect.

If you want to be safe, create a new list instead of changing the existing one. This is a common "functional" approach to threading issues that avoids locks.

var newList = pointsList.AsParallel().Select( item => DoThing(item) );

When you're done, you can always replace the old list if you want.

pointsList = newList.ToList();

share|improve this answer

edited Mar 26 at 3:45

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Good point on it bumping the version number. I didn't expect it to do that but it does. The relevant source code is here / here / here.
  
  – BACON
  Mar 26 at 3:45
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Oh yeah, it looks like incrementing _version in parallel could result in a dirty write. This is only relevant for when the list is being altered or used as an IEnumerable however, neither of which I am doing. Furthermore, if I did perform any of the above operations on pointsList later on, the residual value inside _version wouldn't actually have any detrimental affect. However, the thought process here is complex enough I think I'd want to save the next guy from having to think about it, and so I think I'll avoid assigning to the list in parallel, as suggested.
  
  – Jack
  Mar 26 at 4:00
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the answer John. .AsParallel() is a nifty little extension method.
  
  – Jack
  Mar 26 at 4:01
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Jack After further consideration, I reached the same conclusion as your 2nd sentence. The set indexer invalidates any enumeration operations...but why does that matter if you know there are none? It's like using a for loop to remove list items: no, you can't simultaneously access the list on another thread, but having that limitation doesn't mean that loop is "unsafe". I suppose it boils down to if you know the list won't be accessed in problematic ways then there's no problem, otherwise taking the cautious approach isn't a bad thing, either.
  
  – BACON
  Mar 26 at 19:38
  

  
  
  
  

add a comment | 

It is not guaranteed to be safe. Changing an element in the list increments the list's private _version field, which is how various threads can tell if the list has been modified. If any other thread attempts to enumerate the list or uses a method like List<T>.ForEach() then there could potentially be an issue. It sounds like it would be extremely rare, which isn't necessarily a good thing-- in fact, it makes for the most maddening type of defect.

If you want to be safe, create a new list instead of changing the existing one. This is a common "functional" approach to threading issues that avoids locks.

var newList = pointsList.AsParallel().Select( item => DoThing(item) );

When you're done, you can always replace the old list if you want.

pointsList = newList.ToList();

share|improve this answer

edited Mar 26 at 3:45

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

var newList = pointsList.AsParallel().Select( item => DoThing(item) );

pointsList = newList.ToList();

share|improve this answer

edited Mar 26 at 3:45

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges

answered Mar 26 at 3:42

John WuJohn Wu

33.5k44 gold badges2727 silver badges5656 bronze badges