I have a property
public ObservableCollection<string> Name
{
get
{
return _nameCache;
}
}
_nameCache is updated by multiple threads in other class methods. The updates are guarded by a lock. The question is: should I use the same lock around my return statement? Will not using a lock lead to a race condition?
It depends on what you mean by updated.
If you mean that the reference is modified, i.e. _nameCache = newvalue;, then as Mark has said, yes you should (with the same lock) and, no, you won't get a race condition.
If, however, you mean that items are added and removed to the instance referenced by _nameCache, then you won't need to lock on return (since the reference itself never changes). However, you will have to be careful how you read the collection after retrieving it - ideally you should then use the same lock before calling any of its methods.
Either that, or you can use the event model to be notified of new items etc if all you need to do is to track changes - since the events will be raised on the thread that currently has the lock to the collection.
If this is not suitable (because you're always getting elements via an indexer or whatever), then you could always return a copy of the ObservableCollection through this property - i.e. return new ObservableCollection<string>(_nameCache);. This will make the return value of the property short-lived, but leaves any caller free to enumerate and index without any fears of state corruption from other threads.
Yes, you should. If you don't add a lock there the newest value of _nameCache might not be returned.
public ObservableCollection<string> Name
{
get
{
lock (_yourLockObject)
{
return _nameCache;
}
}
}
Yes you can use the same lock object. I assmume you have a variable declared such as the following for the lock: private object _lock = new object(); If so you can use the _lock object as in the follwoing block of code.
public ObservableCollection<string> Name
{
get
{
lock(_lock)
{
return _nameCache;
}
}
}
If you did not implement the lock in the situation where multiple threads are apptempting to access _nameCache you might receive a value that was not within the context of the current call to this propery. So yes its mandatory to implement syncronization (the lock) if multiple threads are accessing the same class memeber such as the propery you outlined above.
Enjoy!
Related
I'm currently working on a multithreaded project. Mainly for learning purposes.
My part in this project is to write a server application.
But I got to a point where this whole locking of variables is a bit confusing because I don't know exactly how it works and when/where I have to use it.
Lets say I have a class ClientHandler that contains a List<Client> clientList.
ClientHandler has a property ClientList that returns clientList:
private List<Client> clientList;
public List<Client> ClientList
{
get { lock (lockObject) return clientList; }
set { lock (lockObject) clientList = value; }
}
NetworkHandler runs a second Thread that works with this List<>.
In this network Thread the retrieved List<> is locked with another object while in use.
My question is how this locking works.
If I lock this List<> in the network Thread (with a different object than ClientHandler) is it thread-safe? So in general is it true that if you lock a variable with whatever object, is it locked for everyone else trying to access it?
I just want to be sure that the List<> cannot be altered while it is processed in another Thread.
I am assuming that you want the List itself to be passed around and manipulated on separate threads via Add/Remove/Clear/foreach methods, but not the objects themselves.
If this is the case then the List itself needs to implement an internal locking mechanism on each operation (Add, Remove, etc). The List creates the lockObject, not you.
Obviously a List<T> can't do this, so you need to either derive from it or implement your own IList/ICollection class, or you can just use a collection from the System.Collections.Concurrent namespace which is designed for this purpose already, such as ConcurrentDictionary.
If you want to make the objects inside the collection accessible by multiple threads then you have to make those objects thread safe... which is a completely separate step.
For a brief explanation of how the lock keyword works:
When you lock an object by specifying lock (someObject) { }, everything inside the code block will only be executed when every other instance of a lock on that same object is not executing.
At the start of the lock code block it sets a thread-safe flag somewhere that says "I'm reserving this object", and at the end of the lock block it says "I am no longer reserving this object." At the start of the lock block if it tries to lock an object but that object is already locked then it will wait indefinitely until it can successfully obtain a lock, blocking the calling thread in the process.
In your example you are using an inline code block:
get { lock (lockObject) return clientList; }
Which is equivalent to:
get
{
lock (lockObject)
{ // Lock starts here
return clientList;
} // Lock ends here
}
So if you access that property then the object is unlocked as soon as its given to the caller of the property. Then the caller can go ahead and call the Add() method on that List and internally it will access the inner collection with no locking mechanism, so it won't be thread safe.
Instead, the List needs to call lock everytime it accesses the internal fields, such as the _innerList field, like so:
public void Add(object item)
{
lock (_innerList)
{
_innerList.Add(item);
}
}
I have a lightly used dictionary which is hardly ever going to be read or updated since the individual items raise events and return their results with their event args. In fact the thread is always going to be updated with the same thread. I was thinking about adding a simple lock just to be safe. I was wondering if I can just place the lock in the get accessor. Does this work?
Dictionary<string,Indicator> indicators = new Dictionary<string,Indicator>();
Dictionary<string, Indicator> Indicators
{
get
{
lock (indicators)
{
return indicators;
}
}
}
public void AddIndicator(Indicator i)
{
lock (indicators)
{
indicators.Add(i.Name, i);
}
}
That doesn't do anything particularly useful, no.
In particular, if you have:
x = foo.Indicators["blah"]
then the indexer will be executed without the thread holding the lock... so it's not thread-safe. Think of the above code like this:
Dictionary<string, Indicator> indicators = foo.Indicators;
// By now, your property getter has completed, and the lock has been released...
x = indicators["blah"];
Do you ever need to do anything with the collection other than access it via the indexer? If not, you might want to just replace the property with a method:
public Indicator GetIndicator(string name)
{
lock (indicators)
{
return indicators[name];
}
}
(You may want to use TryGetValue instead, etc - it depends on what you're trying to achieve.)
Personally I'd prefer to use a reference to a privately-owned-and-otherwise-unused lock object rather than locking on the collection reference, but that's a separate matter.
As mentioned elsewhere, ConcurrentDictionary is your friend if you're using .NET 4, but of course it's not available prior to that :(
Other than Jon's input, I'll say don't lock the collection indicators itself anyway, from MSDN:
Use caution when locking on instances,
for example lock(this) in C# or
SyncLock(Me) in Visual Basic. If other
code in your application, external to
the type, takes a lock on the object,
deadlocks could occur.
It is recommended to use a dedicated object instance to lock onto. There are other places where this is covered with more details and reasons why - even here on SO, should you care to search for the information when you have time.
Alternatively, you could use ConcurrentDictionary which handles the thread safety for you.
Short answer: YES.
Why shouldn't that work, but as mention by Jon, it does not lock as intended when using indexes?
I have written the following code:
static readonly object failedTestLock = new object();
public static Dictionary<string, Exception> FailedTests
{
get
{
lock (failedTestLock)
{
return _failedTests;
}
}
set
{
lock (failedTestLock)
{
_failedTests = value;
}
}
}
public void RunTest(string testName)
{
try
{
//Run a test
}
catch (Exception exception)
{
// ?? Is this correct / threadsafe?
FailedTests.Add(testName, exception);
}
}
QUESTION:
Is this a correct manner to safely add the failed test to the Dictionary?
Is this threadsafe?
Is FailedTests.Add called INSIDE the lock or OUTSIDE the lock?
Can you explain why this is correct/threadsafe or why not?
Thanks in advance
The fundamental problem with the code above is that it only locks access to _failedTests when a thread is getting the dictionary or setting it. Only one thread can get a reference to the dictionary at a time, but once a thread has a reference to the dictionary, it can read and manipulate it without being constrained by locks.
Is this a correct manner to safely add
the failed test to the Dictionary?
No, not if two threads are trying to add to the dictionary at the same time. Nor if you expect reads and writes to happen in a particular order.
Is this threadsafe?
It depends what you mean by threadsafe, but no, not by any reasonable definition.
Is FailedTests.Add called INSIDE the
lock or OUTSIDE the lock?
The dictionary retrieval (the get accessor) happens inside a lock. This code calls Add after releasing the lock.
Can you explain why this is correct/threadsafe or why not?
If multiple threads operate on your dictionary at the same time, you can't predict the order in which those threads will change its contents and you can't control when reads will occur.
This is not thread-safe access to a dictionary, because only the property access that returns the dictionary object is thread-safe, but you are not synchronizing the call to the Add method. Consider using ConcurrentDictionary<string,Exception> in this case, or synchronize calls to Add manually.
I don;t thinks this is threadsafe, because the lock is kept only in the very brief moment where the pointer to the collection is returned. When you Add to the collection there is no lock so if two threads try to add at the same time you'll get a nasty error.
So you should lock around the FailedTest.Add code.
You may also want to look into concurrent collections, they might provide what you need.
Regards GJ
The call to Add() is outside the locks.
You can solve it by writing your own Add() method to replace the property.
I need to create a thread safe list of items to be added to a lucene index.
Is the following thread safe?
public sealed class IndexQueue
{
static readonly IndexQueue instance = new IndexQueue();
private List<string> items = new List<string>();
private IndexQueue() { }
public static IndexQueue Instance {
get { return instance; }
}
private object padlock = new object();
public void AddItem(string item) {
lock (padlock) {
items.Add(item);
}
}
}
Is it necessary to lock even when getting items from the internal list?
The idea is that we will then have a separate task running to grab the items from indexqueue and add them to the lucene index.
Thanks
Ben
Your implementation seems thread-safe, although you will need to lock when reading from items as well - you can not safely read if there is a concurrent Add operation. If you ever enumerate, you will need locking around that as well and that will need to live as long as the enumerator.
If you can use .net 4, I'd strongly suggest looking at the System.Collections.Concurrent namespace. It has some well tested and pretty performant collections that are thread-safe and in fact optimized around multiple-thread access.
Is it necessary to lock even when getting items from the internal list?
The List class is not thread-safe when you make modifications. It's necessary to lock if:
You use a single instance of the class from multiple threads.
The contents of the list can change while you are modifying or reading from the list.
Presumably the first is true otherwise you wouldn't be asking the question. The second is clearly true because the Add method modifies the list. So, yes, you need it.
When you add a method to your class that allows you to read back the items it is also necessary to lock, and importantly you must use the same lock object as you did in AddItem.
Yes; while retrieval is not an intrinsically unsafe operation, if you're also writing to the list, then you run the risk of retrieving in the middle of a write.
This is especially true if this will operate like a traditional queue, where a retrieval will actually remove the retrieved value from the list.
I have been reading around and am getting conflicting answers on whether I should or should not use synclock on properties.
I have a multi-threaded application that needs to get/set properties across threads on instance objects. It is currently implemented without using synclock and I have not noticed any problems so far. I am using synclock on common static methods but I'd like to implement my instance classes properly and in a thread safe way.
Any feedback would be greatly appreciated.
A good rule of thumb is that you need to lock if any of the following conditions hold true:
if any field of an object is going to be modified on more than one thread
if any modifications involve accessing more than one field
if any modifiable field is a Double, Decimal, or structured value type
if any modifications involve read-modify-write (i.e. adding to a field or setting one field with the value from another)
then you probably need to lock in every method or property that accesses those fields.
EDIT: Keep in mind that locking inside of a class is rarely sufficient -- what you need to do is make sure that things don't go wrong across the span of an entire logical operation.
As #Bevan points out, if calling code needs to access an object more than once, the client code should take out its own lock on the object for the entire duration of its work to ensure that another thread doesn't get "in between" its accesses and foul up its logic.
You also need to take care that if anything needs to take out multiple locks at once, that they always be taken in the same order. If thread 1 has a lock on instance A and tries to lock instance B, and thread 2 has a lock on instance B and tries to get a lock on instance A, both threads are stuck and unable to proceed -- you have a deadlock.
You can't make an object thread safe just by surrounding individual methods with locks. All you end up doing is serialising (slowing down) access to the object.
Consider this minor example:
var myObject = ...
var myThreadSafeList = ...
if (!myThreadSafeList.Contains(myObject))
{
myThreadSafeList.Add(myObject);
}
Even if myThreadSafeList has every method locked, this isn't threadsafe because another thread can alter the contents of the list between the calls to Contains() and Add().
In the case of this list, an additional method is needed: AddIfMissing():
var myObject = ...
var myThreadSafeList = ...
myThreadSafeList.AddIfMissing(myObject);
Only by moving the logic into the object can you surround both operations with the lock and make it safe.
Without further details, it's hard to comment futher, but I'd suggest the following:
Make all properties read-only, and allow anyone to read them at any time
Provide mutator methods that take sets of properties that get modified together, and make the changes atomically within a lock
To illustrate:
public class Person {
public string FullName { get; private set; }
public string FamilyName { get; private set; }
public string KnownAs { get; private set; }
public void SetNames( string full, string family, string known) {
lock (padLock) {
...
}
}
}