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c#.netmultithreadingstaticfield

ThreadSafety and static fields - confusion?

I read some articles and did some testings/investigating and I think there's NOT accurate conculsion (due to incorrect wording maybe )

Ok . few investigations :

There was this guy who asked this question :

Why are static fields generally considered threadsafe?

Well , all of the repliers said : No. they are not.

Eric also said :

You have it backwards.Because static fields and methods are likely to be accessed from multiple threads, it is a good programming practice to do the work to ensure that they are threadsafe.

(this is pretty close to my claim which i'll ask here later)

But I also found here that Hans said :

enter image description here

And oh boy — this other guy said :

Re-entrancy is still a problem, though. static is not thread-safe, and even static readonly is not re-entrant-safe.

Ok , i'm confused.

p.s. I'm not talking about this kind of situation : 

public class A
 {
  public static List<int> LST = new List<int>();
 }

List<> is not thread safe so I don't care who holds it. BUT ->

Let's look at this 

public class A
 {
  public static int Five=5;
 }

What is not thread safe about this code ?

I can't see how Five could not always be 5 :

enter image description here

As far as I know - Five could never be 0( initial value) and it would always be five no matter how many threads are accessing it !

NOW , I DO AGREE that If I was writing a code which USES this Five field , then I'd have to watch out for changed value !

Example :

if (Five < array.Length) return array[five]; //can throw an IndexOutOfBoundsException if five is modified

With this - I Agree !

But (imho) it is incorrect to say ( as a rule) that static fields are not thread safe !

Their usage has to be treated as if it could change via another thread.

(Also, obviously - Static fields which reference a non-threadsafe object like List<> - surely not going to be thread safe)

What i'm trying to say here is that static fields are not thread safe for specific scenarios , and not as a rule !

Question :

  • Can someone please shed light ? I'm sure I miss something here ( or am I not ? ) is it true that static fields are not always non-thread safe ( as a rule)

  • And what about this readonly which makes static field threadsafe (all it does it make it one time initialize without reassigned ) ? does it make it thread safe or does it not ? (and what about re-entrent ?)

Solution

  • is it true that static fields are not always non-thread safe?

    static shouldn't be in the argument itself. static has nothing to do with thread safety. It just allows the field to be the part of "Type" itself rather than "Instance".

    what about this readonly which makes static field threadsafe ? does it make it thread safe or does it not ?

    Answer depends on what do you meant by thread safe? Thread safe is more general term in which everybody has their own meaning, If you can be more precise in what do you mean by thread safe, It will be easy to answer.