We Keep Coding

Is this example thread safe?

Suppose I have singleton class that acts as a data cache. Multiple threads will read from the cache, and a single thread will periodically refresh it. It looks something like this:
public sealed class DataStore
{
public static DataStore Instance { get { return _instance; } }
public Dictionary<Foo, Bar> FooBar { get; private set; }
static DataStore() { }
private DataStore() { }
public void Refresh() {
FooBar = GetFooBarFromDB();
}
private static readonly DataStore _instance = new DataStore();
}
My question is essentially, is it safe to Refresh() while other threads may be accessing FooBar? Do I need to be using locks, or are my getting and setting operations atomic? Do I need to explicitly declare volatile fields to back up my properties?
P.S., If someone can think of a more descriptive title for this question, I would gladly welcome it.
Edit: Fixed my example to correct obviously non-atomic code.

Yes, in cases like that you need explicit synchronization, because another thread could get FooBar and start reading it before you have finished writing.
If you do this, however,
public void Refresh() {
var tmp = new Dictionary<Foo, Bar>();
// Fill out FooBar from DB
FooBar = tmp;
}
then you wouldn't need to add explicit synchronization, because the switch from one reference over to the other reference is atomic.
Of course there is an implicit assumption here that there is no writing outside the Refresh method.
EDIT: You also should switch from an auto-implemented property to a manually implemented property with a backing variable declared with volatile modifier.

Your example is not thread-safe. Dictionary is not a thread-safe class and any thread could be reading while Refresh is being executed. You can either put a lock around or use one of the thread-safe classes like ConcurrentDictionary.

Because you are exposing the dictionary publicly then you run into more issues with the code you have written around access to the methods on the dictionary itself. As #Icarus has pointed out you should use ConcurrentDictionary but I would argue that any form of locking around the instance will not help you.
You can easily get one thread adding to the collection while another is iterating over it.
EDIT
What I am saying.. never expose a static Dictionary or any other collection type. Always use the concurrent version

Well, we agree that your current code is not thread-safe.
So, you must use synchronization features, because FooBar is your critical section.
If you let it be public, you are expecting that people outside the DataStore class will act accordingly. However, this is a poor design decision.
So, I would suggest you to wrap everything into your current class, with something like this: What's the best way of implementing a thread-safe Dictionary?

rationale behind lock inside lock?

I am reviewing an example code in a book and came across the following code(simplified).
In the code, when Subscribe(T subscriber) is called, the thread enters into a lock section.
and then, when code inside the lock calls AddToSubscribers(T subscriber) method, the method has another lock. why is this second lock necessary?
public abstract class SubscriptionManager<T> where T : class
{
private static List<T> subscribers;
private static void AddToSubscribers(T subscriber)
{
lock (typeof(SubscriptionManager<T>))
{
if (subscribers.Contains(subscriber))
return;
subscribers.Add(subscriber);
}
}
public void Subscribe(T subscriber)
{
lock (typeof(SubscriptionManager<T>))
{
AddToSubscribers(subscriber);
}
}
}

In that context, it isn't; however, since locks are re-entrant that can be useful to ensure that any other caller of AddToSubscribers observes the lock. Actually, for that reason I'd say "remove it from Subscribe and just let AddToSubscribers do the locking".
However! A lock on a Type is pretty dangerous. A field would be safer:
// assuming static is correct
private static readonly object syncLock = new object();
and lock(syncLock). Depending on when subscribers is assigned, you might also get away with lock(subscribers) (and no extra field).
I should also note that having an instance method add to static state is pretty.... unusual; IMO Subscribe should be a static method, since it has nothing to do with the current instance.

In the code you posted, it isn't necessary. But then again, the code you posted is incomplete - for example the subscribers list is never initialized.
Locking on typeof(SubscriptionManager) is probably not a good idea either - locking on the subscribers field would be better - but would require the subscribers field to be initialized, e.g.
private static List<T> subscribers = new List<T>();

You probably should read near that sample and see what book talks about.
For that particular case - no, second lock is unnecessary.
Note: The sample is dangerous since it locks on public object (type). Normally one locks on special private object so external code is not able to mistakenly introduce deadlocks by mistakenly locking on the same object.

I also faced a situation once where I had to use nested Lock.
My case was, the function of the second lock maybe called from elsewhere, since it was a static function. However, for your case it won't be necessary since each data member belongs to an Instance and not static..

C# Using block usage with static objects

I'm adding some code into a using block in a C# program. I'm sort of stuffing my app, which previously was a standalone into an existing body of code, so I need to do a bit of messing around to get it to fit properly. What it's ending up looking like is the following:
public class WrapperForMyOldApp
{
public static ItemThatINeed item1;
public static ItemThatINeed item2;
public WrapperForMyOldApp ()
{
item1 = new ItemThatINeed();
item2 = new ItemThatINeed();
}
public static go()
{
// some stuff that i need to do with items 1 and 2
}
}
public class MainProgram
{
.
.
.
public void MethodThatNeedsToMakeUseOfMyApp ()
{
....
using (WrapperForMyOldApp oldAPp = new WrapperForMyOldApp())
{
WrapperForMyOldApp.go();
}
}
}
Alright, so the question here is: Have I now crippled the effects of the using block and/or created any peculiar side effects that might adversely effect the class MainProgram? I believe that the Wrapper object and it's contents will be Disposed and execution will continue as expected but is there anything I need to be aware of that I'm overlooking?
thanks!

Does your wrapper class implement IDisposable and you're just not showing it? If it's not disposable, then you don't need the using statement at all.

In order for this to work, you'll need to have WrapperForMyOldApp implement IDisposable.
The Dispose() call in WrapperForMyOldApp would then do your cleanup.
However, static objects are typically used for objects that have a lifetime beyond the single object. In general, for this type of usage, you'd make the ItemThatINeed instances non-static, generate them in your WrapperForMyOldApp constructor, then clean them up in WrapperForMyOldApp.Dispose().
With static objects, you're potentially creating a nightmare - you're constructing the object, and then saying you want to perform the cleanup (at the end of the using block), so you Dispose() method would be cleaning up the static objects. However, if they get used again, what should happen? What is the correct behavior if you create two WrapperForMyOldApp instances in 2 threads? I would consider these issues if you want deterministic cleanup.

Well, if WrapperForMyOldApp implements IDisposable, and your Dispose() implementation can be sure to get rid of any resources, then it should work... but there can be other side effects. The code could change global (static) state, such as culture etc. It could spawn threads. All sorts of things.
It isn't a bad approach, but you need to know what the code you are encapsulating does to know whether Dispose() is going to do anything useful.

How to make a method exclusive in a multithreaded context?

I have a method which should be executed in an exclusive fashion. Basically, it's a multi threaded application where the method is invoked periodically by a timer, but which could also be manually triggered by a user action.
Let's take an example :
The timer elapses, so the method is
called. The task could take a few
seconds.
Right after, the user clicks on some
button, which should trigger the
same task : BAM. It does nothing
since the method is already running.
I used the following solution :
public void DoRecurentJob()
{
if(!Monitor.TryEnter(this.lockObject))
{
return;
}
try
{
// Do work
}
finally
{
Monitor.Exit(this.lockObject);
}
}
Where lockObject is declared like that:
private readonly object lockObject = new object();
Edit : There will be only one instance of the object which holds this method, so I updated the lock object to be non-static.
Is there a better way to do that ? Or maybe this one is just wrong for any reason ?

This looks reasonable if you are just interested in not having the method run in parallel. There's nothing to stop it from running immediately after each other, say that you pushed the button half a microsecond after the timer executed the Monitor.Exit().
And having the lock object as readonly static also make sense.

You could also use Mutex or Semaphore if you want it to work cross process (with a slight performance penalty), or if you need to set any other number than one of allowed simultaneous threads running your piece of code.
There are other signalling constructs that would work, but your example looks like it does the trick, and in a simple and straightforward manner.

Minor nit: if the lockObject variable is static, then "this.lockObject" shouldn't compile. It also feels slightly odd (and should at least be heavily documented) that although this is an instance method, it has distinctly type-wide behaviour as well. Possibly make it a static method which takes an instance as the parameter?
Does it actually use the instance data? If not, make it static. If it does, you should at least return a boolean to say whether or not you did the work with the instance - I find it hard to imagine a situation where I want some work done with a particular piece of data, but I don't care if that work isn't performed because some similar work was being performed with a different piece of data.
I think it should work, but it does feel a little odd. I'm not generally a fan of using manual locking, just because it's so easy to get wrong - but this does look okay. (You need to consider asynchronous exceptions between the "if" and the "try" but I suspect they won't be a problem - I can't remember the exact guarantees made by the CLR.)

I think Microsoft recommends using the lock statement, instead of using the Monitor class directly. It gives a cleaner layout and ensures the lock is released in all circumstances.
public class MyClass
{
// Used as a lock context
private readonly object myLock = new object();
public void DoSomeWork()
{
lock (myLock)
{
// Critical code section
}
}
}
If your application requires the lock to span all instances of MyClass you can define the lock context as a static field:
private static readonly object myLock = new object();

The code is fine, but would agree with changing the method to be static as it conveys intention better. It feels odd that all instances of a class have a method between them that runs synchronously, yet that method isn't static.
Remember you can always have the static syncronous method to be protected or private, leaving it visible only to the instances of the class.
public class MyClass
{
public void AccessResource()
{
OneAtATime(this);
}
private static void OneAtATime(MyClass instance)
{
if( !Monitor.TryEnter(lockObject) )
// ...

This is a good solution although I'm not really happy with the static lock. Right now you're not waiting for the lock so you won't get into trouble with deadlocks. But making locks too visible can easily get you in to trouble the next time you have to edit this code. Also this isn't a very scalable solution.
I usually try to make all the resources I try to protect from being accessed by multiple threads private instance variables of a class and then have a lock as a private instance variable too. That way you can instantiate multiple objects if you need to scale.

A more declarative way of doing this is using the MethodImplOptions.Synchronized specifier on the method to which you wish to synchronize access:
[MethodImpl(MethodImplOptions.Synchronized)]
public void OneAtATime() { }
However, this method is discouraged for several reasons, most of which can be found here and here. I'm posting this so you won't feel tempted to use it. In Java, synchronized is a keyword, so it may come up when reviewing threading patterns.

We have a similar requirement, with the added requirement that if the long-running process is requested again, it should enqueue to perform another cycle after the current cycle is complete. It's similar to this:
https://codereview.stackexchange.com/questions/16150/singleton-task-running-using-tasks-await-peer-review-challenge
private queued = false;
private running = false;
private object thislock = new object();
void Enqueue() {
queued = true;
while (Dequeue()) {
try {
// do work
} finally {
running = false;
}
}
}
bool Dequeue() {
lock (thislock) {
if (running || !queued) {
return false;
}
else
{
queued = false;
running = true;
return true;
}
}
}

Advantages to Using Private Static Methods

When creating a class that has internal private methods, usually to reduce code duplication, that don't require the use of any instance fields, are there performance or memory advantages to declaring the method as static?
Example:
foreach (XmlElement element in xmlDoc.DocumentElement.SelectNodes("sample"))
{
string first = GetInnerXml(element, ".//first");
string second = GetInnerXml(element, ".//second");
string third = GetInnerXml(element, ".//third");
}
...
private static string GetInnerXml(XmlElement element, string nodeName)
{
return GetInnerXml(element, nodeName, null);
}
private static string GetInnerXml(XmlElement element, string nodeName, string defaultValue)
{
XmlNode node = element.SelectSingleNode(nodeName);
return node == null ? defaultValue : node.InnerXml;
}
Is there any advantage to declaring the GetInnerXml() methods as static? No opinion responses please, I have an opinion.

From the FxCop rule page on this:
After you mark the methods as static, the compiler will emit non-virtual call sites to these members. Emitting non-virtual call sites will prevent a check at runtime for each call that ensures that the current object pointer is non-null. This can result in a measurable performance gain for performance-sensitive code. In some cases, the failure to access the current object instance represents a correctness issue.

When I'm writing a class, most methods fall into two categories:
Methods that use/change the current instance's state.
Helper methods that don't use/change the current object's state, but help me compute values I need elsewhere.
Static methods are useful, because just by looking at its signature, you know that the calling it doesn't use or modify the current instance's state.
Take this example:
public class Library
{
private static Book findBook(List<Book> books, string title)
{
// code goes here
}
}
If an instance of library's state ever gets screwed up, and I'm trying to figure out why, I can rule out findBook as the culprit, just from its signature.
I try to communicate as much as I can with a method or function's signature, and this is an excellent way to do that.

A call to a static method generates a call instruction in Microsoft intermediate language (MSIL), whereas a call to an instance method generates a callvirt instruction, which also checks for a null object references. However, most of the time the performance difference between the two is not significant.
Source: MSDN - https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2012/79b3xss3(v=vs.110)

Yes, the compiler does not need to pass the implicit this pointer to static methods. Even if you don't use it in your instance method, it is still being passed.

It'll be slightly quicker as there is no this parameter passed (although the performance cost of calling the method is probably considerably more than this saving).
I'd say the best reason I can think of for private static methods is that it means you can't accidentally change the object (as there's no this pointer).

This forces you to remember to also declare any class-scoped members the function uses as static as well, which should save the memory of creating those items for each instance.

I very much prefer all private methods to be static unless they really can't be. I would much prefer the following:
public class MyClass
{
private readonly MyDependency _dependency;
public MyClass(MyDependency dependency)
{
_dependency = dependency;
}
public int CalculateHardStuff()
{
var intermediate = StepOne(_dependency);
return StepTwo(intermediate);
}
private static int StepOne(MyDependency dependency)
{
return dependency.GetFirst3Primes().Sum();
}
private static int StepTwo(int intermediate)
{
return (intermediate + 5)/4;
}
}
public class MyDependency
{
public IEnumerable<int> GetFirst3Primes()
{
yield return 2;
yield return 3;
yield return 5;
}
}
over every method accessing the instance field. Why is this? Because as this process of calculating becomes more complex and the class ends up with 15 private helper methods, then I REALLY want to be able to pull them out into a new class that encapsulates a subset of the steps in a semantically meaningful way.
When MyClass gets more dependencies because we need logging and also need to notify a web service (please excuse the cliche examples), then it's really helpful to easily see what methods have which dependencies.
Tools like R# lets you extract a class from a set of private static methods in a few keystrokes. Try doing it when all private helper methods are tightly coupled to the instance field and you'll see it can be quite a headache.

As has already been stated, there are many advantages to static methods. However; keep in mind that they will live on the heap for the life of the application. I recently spent a day tracking down a memory leak in a Windows Service... the leak was caused by private static methods inside a class that implemented IDisposable and was consistently called from a using statement. Each time this class was created, memory was reserved on the heap for the static methods within the class, unfortunately, when the class was disposed of, the memory for the static methods was not released. This caused the memory footprint of this service to consume the available memory of the server within a couple of days with predictable results.