C# Thread safe code

Question

Is this code thread safe?

DoStuff is called in a new thread using Task.

Task.Factory.StartNew(() => DoStuff());

private void DoStuff() { List<SalesRecord> salesRecords = new List<SalesRecord>(); SalesRecord salesRecord1 = new SalesRecord {Amount = 10.0, Sales = 1}; SalesRecord salesRecord2 = new SalesRecord {Amount = 15.0, Sales = 1}; SalesRecord salesRecord3 = new SalesRecord {Amount = 1.0, Sales = 2}; salesRecords.Add(salesRecord1); salesRecords.Add(salesRecord2); salesRecords.Add(salesRecord3); SalesRecord result = Util.SumSales(salesRecords); }

A struct just to store data:

public struct SalesRecord { public uint Sales; public double Amount; }

stuff

public static class Util { public static SalesRecord SumSales(List<SalesRecord> records) { SalesRecord result = new SalesRecord(); result.Amount = records.FindAll(record => (record.Sales == 1)).Sum(record => record.Amount); result.Sales = 1; return result; } }

I'm not actually seeing any multithreading happening, so I would guess from the code you provided that yes, it is thread-safe. Unless you're accessing things between different threads, this code shouldn't be a problem. — Josh L.
– Josh L., Commented Apr 24, 2015 at 14:32
I see nowhere you actually call DoStuff - would be nice to see where it's used to make an educated decision. — Der Kommissar
– Der Kommissar, Commented Apr 24, 2015 at 14:33
@user1766169 it doesn't matter as long as everything is done inside the same thread. This is single-threaded code. — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Apr 24, 2015 at 14:33
I don't see static properties and you operate with only local variable, there is no possibility to have multithreading problem outside DoStuff. This makes DoStuff thread-safe. — Sinatr
– Sinatr, Commented Apr 24, 2015 at 14:35

Sriram Sakthivel · Accepted Answer · 2015-04-24 14:36:33Z

9

Is this code thread safe?

Yes, it is. This code doesn't use any shared state. By definition code which doesn't uses any shared state is thread-safe.

You can call DoStuff concurrently in any number of threads without any problem. That said, DoStuff method isn't very useful though.

answered Apr 24, 2015 at 14:36

Sriram Sakthivel

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2 Comments

user1766169 Over a year ago

Would 'DoStuff' still be thread safe if it returned 'salesRecords'? @SriramSakthivel

Sriram Sakthivel Over a year ago

@user1766169 Yes, it will be. But that isn't still useful. Thread safety is a concern only when you access same instance/static member from two threads. But your code doesn't and thus it is thread-safe.

Community · Accepted Answer · 2017-05-23 11:59:26Z

4

Your code looks fine. Even if DoStuff is being launched via Task, I don't see anywhere in your code where you're dealing with class variables, they're all local variables.

Reference: C# : What if a static method is called from multiple threads?

edited May 23, 2017 at 11:59

CommunityBot

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answered Apr 24, 2015 at 14:36

Shar1er80

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3 Comments

Shar1er80 Over a year ago

@PeterSchneider Only if it's a class/shared state variable, which it is not. It's a local variable to the thread. If the OP were to launch another DoStuff(), via task again, there would be another local List to that class.

Panagiotis Kanavos Over a year ago

@PeterSchneider wrong wrong. It is called from a single thread, on an instance that is local to the method it is defined in. No other thread can access this instance

Peter - Reinstate Monica Over a year ago

As I said in another comment, I didn't read the method carefully enough. (But as an excuse I concur with @SiriamSa that DoStuff() doesn't do anything which is misleading to the reader. In fact, probably no code needs to be generated for it so that thread safety is easy to show.)

Josh L. · Accepted Answer · 2015-04-24 14:53:09Z

Posted for those trying to figure out why it's thread-safe even with a static method

In this particular case, access to Util.SumSales(salesRecords) is thread safe as it is a thread-local method (the thread that calls this method provides it's own copy of the data to the method and data access in that thread is exclusive to the thread that called it).

When you call the following code:

Task.Factory.StartNew(() => DoStuff());

You assign a new thread to handle DoStuff(). Everything that happens in DoStuff (unless you introduce variables that could be manipulated outside of it) is exclusive to that new thread.

I assume you'll do more with what you get from DoStuff(). In that case, you should store your results in Concurrent Collections.

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