In a FileWriter class that will be used by different threads, I currently use a ReaderWriterLockSlim to prevent errors occurring when two threads try to write to the file at the same time like this:
(1)
public class FileWriter
{
private ReaderWriterLockSlim readerWriterLock = new ReaderWriterLockSlim();
public void WriteToFile(string message)
{
try
{
this.readerWriterLock.EnterWriteLock();
// the writing happens here
}
finally
{
this.readerWriterLockSlim.ExitWriteLock();
}
}
}
which does work. But after that, I read that ReaderWriterLockSlim implements IDisposable, and so I wondered whether
(2)
public class FileWriter
{
public void WriteToFile(string message)
{
using (ReaderWriterLockSlim readerWriterLockSlim = new ReaderWriterLockSlim())
{
readerWriterLockSlim.EnterWriteLock();
// the writing happens here
readerWriterLockSlim.ExitWriteLock();
}
}
}
would be the "better" approach and whether it might introduce some new drawbacks. My gut feeling tells me that I probably shouldn't create a new ReaderWriterLockSlim every time the method is called but rather only once as in (2).
Sadly though, it does not work (it's as if I hadn't even used a lock) and so I figured that (2) can't be correct.
But then again, why would ReaderWriterLockSlim implement IDisposable if it wasn't planned to be used as in (2)?
What is the correct usage of ReaderWriterLockSlim?
Not every IDisposable is directly used in a using statement. The other pattern is if you have an IDisposable in one of your classes fields (and your class "owns" that object1), then your class should also implement IDisposable and clean up its disposables during Dispose.
You're correct that your re-write is wrong, since every caller is using a different lock object and so no exclusion is happening.
See Dispose Pattern:
DO implement the Basic Dispose Pattern on types containing instances of disposable types. See the Basic Dispose Pattern section for details on the basic pattern.
1Thanks to Dirk's comment for the important caveat. If you're in a situation where you're not trying to control an object's lifetime, it's not appropriate to Dispose of such objects
Related
It is a little bit confusing. In C# for multithread managing we have mutex and we have lock and in addition I found such lock RAII implementation
public class ReaderWriterLockSlim_ScopedLockRead : IDisposable
{
ReaderWriterLockSlim m_myLock;
public ReaderWriterLockSlim_ScopedLockRead(ReaderWriterLockSlim myLock)
{
m_myLock = myLock;
m_myLock.EnterReadLock();
}
public void Dispose()
{
m_myLock.ExitReadLock();
}
}
public class ReaderWriterLockSlim_ScopedLockWrite : IDisposable
{
ReaderWriterLockSlim m_myLock;
public ReaderWriterLockSlim_ScopedLockWrite(ReaderWriterLockSlim myLock)
{
m_myLock = myLock;
m_myLock.EnterWriteLock();
}
public void Dispose()
{
m_myLock.ExitWriteLock();
}
}
}
I would like to understand the difference between them, as for me mutex is a first multithreading managing implementation you need to call mutex.lock() and then don't forget to call mutex.release() it is usually not so suitable to call mutex.release() because you can get an error at the middle of execution, so here we have lock(obj){} as far as I see it is kind of RAII object with the same behavior but if you get error at the middle under the hood it will call mutex.release() and all nice.
But what about the last custom implementaion that I posted? It looks like the same with lock(obj){}, just with a difference that we have read and write behavior, like in write state it is possible that a few threads get accesses to method and with read state just one by one...
Am I right here?
So for locking it's important that every lock that is acquired is also released (no matter if the code it was locking had any exceptions). So normally, no matter what lock you use, it'll look something like this:
myLock.MyAcquireMethod();
try
{
//...
}
finally
{
myLock.MyReleaseMethod();
}
Now for the Monitor locking mechanism in c# they have a keyword to make it easier: lock.
which basically wraps the acquiring and releasing in one lock code-block.
So this:
lock(myObj)
{
//...
}
Is just a more convenient way of writing this:
Monitor.Enter(myObj);
try
{
//...
}
finally
{
Monitor.Exit(myObj);
}
Sadly for the other locks (and because Monitor has it's limitations we don't always want to use it) we don't have such a handy short way of doing the whole thing, and to solve that the ReaderWriterLockSlim_ScopedLockRead wrapper implements IDisposable that gives you this try finally mechanism (using also guarantees that Dispose() is called on the IDisposable no matter if the code ran to completion or an exception occurred.
So instead of:
m_myLock.EnterWriteLock();
try
{
//...
}
finally
{
m_myLock.ExitWriteLock();
}
You're now able to do this:
using(new ReaderWriterLockSlim_ScopedLockRead(m_myLock))
{
//...
}
Hope this answers your question!
As a bonus a warning on the Monitor class of c#. This locking mechanism is re-entrant on a thread level. Meaning the thread holding the lock is allowed to acquire it multiple times (though it also has to release it multiple times), which allows you to do something like this:
private readonly object _myLock = new object();
public void MyLockedMethod1()
{
lock(_myLock)
{
MyLockedMethod2();
}
}
public void MyLockedMethod2()
{
lock(_myLock)
{
//...
}
}
So no matter if MyLockedMethod2 is called directly or through MyLockedMethod1 (that might need the lock for other stuff as well) you can have thread-safety.
However these days a lot of people use async/await where a method can be continued on a different thread, which can break the Monitor if the thread that acquired it is not the thread releasing it, so I advise you not to use something like this:
public async Task MyLockedMethod()
{
lock(_myLock)
{
await MyAsyncMethod();
}
}
Anyway there is a lot of documentation about this if you would like to learn more.
This is not it at all. The Read Writer lock is an implementation used in a specific context.
All can read at any time without one blocking the other, but blocking anyone who wants to write.
None can read or write when there is even one writing at that time
It is exactly as wikipedia describes it here and non-specific for C# or any other language. This is just the C# flavor of a ReadWriter lock
is a synchronization primitive that solves one of the readers–writers problems. An RW lock allows concurrent access for read-only operations, while write operations require exclusive access.
Check Microsoft docs here for more information
I have a singleton that uses the "static readonly T Instance = new T();" pattern. However, I ran into a case where T is disposable, and actually needs to be disposed for unit tests. How can I modify this pattern to support a disposable singleton?
The interface I would like is something like:
var x = Foo.Instance;
var y = Foo.Instance; // x == y
...
x.Release(); // this causes the next Foo.Instance to return a fresh object
// also, it assumes no further operations on x/y will be performed.
Note - the pattern has to be thread-safe, of course.
Edit - for the purpose of production code, this is a true singleton. The thing is that it locks some files, and so for cleanup in unit tests we have to dispose it.
I would also prefer a pattern that can be reused, if possible.
At that point I don't think I'd really consider it to be a singleton any more, to be honest.
In particular, if a client uses a singleton they're really not going to expect that they have to dispose of it, and they'd be surprised if someone else did.
What's your production code going to do?
EDIT: If you really, really need this for unit tests and only for unit tests (which sounds questionable in terms of design, to be frank) then you could always fiddle with the field using reflection. It would be nicer to work out whether it should really be a singleton or whether it should really be disposable though - the two very rarely go together.
Mark Release as internal and use the InternalsVisibleTo attribute to expose it only to your unit testing assembly. You can either do that, or if you're wary someone in your own assembly will call it, you can mark it as private and access it using reflection.
Use a finalizer in your singleton that calls the Dispose method on the singleton instance.
In production code, only the unloading of an AppDomain will cause the disposal of the singleton. In the testing code, you can initiate a call to Release yourself.
Singletons should not be Disposable. Period. If someone calls Dispose prematurely, your application is screwed until it restarts.
public class Foo : IDisposable
{ [ThreadStatic] static Foo _instance = null;
private Foo() {IsReleased = false;}
public static Foo Instance
{ get
{ if (_instance == null) _instance = new Foo();
return _instance;
}
}
public void Release()
{ IsReleased = true;
Foo._instance = null;
}
void IDisposable.Dispose() { Release(); }
public bool IsReleased { get; private set;}
}
If the class implements IDisposable (as you imply it does) then just call x.Dispose()
You could use a nested lazy singleton (See here) with some simple modifications:
public sealed class Singleton : IDisposable
{
Singleton()
{
}
public static Singleton Instance
{
get
{
if (!Nested.released)
return Nested.instance;
else
throw new ObjectDisposedException();
}
}
public void Dispose()
{
disposed = true;
// Do release stuff here
}
private bool disposed = false;
class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton();
}
}
Remember to throw ObjectDisposedException in all public methods/properties of the object if it has been disposed.
You should also, provide a finalizer method for the object, in case Dispose doesn't get called. See how to correctly implement IDisposable here.
For unit tests you could use a "manual" instance (but you would need a way to instantiate the object).
In your case, probably you should better use the factory pattern (abstract/method - whichever is the best for your case), combined with a singleton.
If you want to test if the singleton has disposed properly of the used objects (in unit test), then use the Factory method, otherwise use the singleton pattern.
By the way, if you don't have access to the singleton source code or you are not allowed to modify it, you would better wrap it to another singleton, and provide all the logic from the new one (more like a proxy). It sounds like overkill, but it could be a viable solution.
Also, in order to be able to control the access to it, provide a factory, and let the clients get the new object only if the object hasn't been disposed.
Another option to make a disposable Singleton is to use SandCastle's [Singleton] atribute for your class, then Castle framework takes care of disposing all disposable Singleton objects
This question already has answers here:
Is it abusive to use IDisposable and "using" as a means for getting "scoped behavior" for exception safety?
(12 answers)
Closed 9 years ago.
We all know that the using statement is really good for resources you want to cleanup in a timely manner, such as an open file or database connection.
I was wondering if it would be considered a good thing to use the statement in cases where resources cleanup isn't the goal of the Dispose() method, but rather resetting to a previous state.
For example, a class that allows a using statement to wrap a procedure that takes a noticeable amount of time and changes the Cursor to a waiting state.
class CursorHelper : IDisposable
{
readonly Cursor _previousState;
public CursorHelper(Cursor newState)
{
_previousState = Cursor.Current;
Cursor.Current = newState;
}
public void Dispose()
{
Cursor.Current = _previousState;
}
}
Then the class can be used as such, without worrying about reverting the Cursor when your are done.
public void TimeIntensiveMethod()
{
using (CursorHelper ch = new CursorHelper(Cursors.WaitCursor))
{
// something that takes a long time to complete
}
}
Is this an appropriate use of the using statement?
There are certainly precedents for (ab)using the using statement in this way, for example FormExtensions.BeginForm in the ASP.NET MVC framework. This renders a <form> closing tag when it's disposed, and it's main purpose is to enable a more terse syntax in MVC views. The Dispose method attempts to render the closing tag even if an exception is thrown, which is slightly odd: if an exception is thrown while rendering a form, you probably don't want to attempt to render the end tag.
Another example is the (now deprecated) NDC.Push method in the log4net framework, which returns an IDisposable whose purpose is to pop the context.
Some purists would say it's an abuse, I suggest you form your own judgement on a case-by-case basis.
Personally I don't see anything wrong with your example for rendering an hourglass cursor.
The discussion linked in a comment by #I4V has some interesting opinions - including arguments against this type of "abuse" from the ubiquitous Jon Skeet.
In reality using is simply syntactical sugar for try/finally, so why don't you just do plain old try/finally like below...
try
{
// do some work
}
finally
{
// reset to some previous state
}
imho implementing the Dispose method to reset to some state would be very misleading, especially if you have consumers for your code.
I am opposed to this and believe it to be an abuse. I also think that RAII in C++ is a terrible idea. I am aware that I am in a minority on both positions.
This question is a duplicate. For detailed reasons on why I think this is an unwarranted abuse of the using statement, see: https://stackoverflow.com/a/2103158/88656
No, it isn't appropriate use of using and/or Dispose. The pattern has a very clear use ("Defines a method to release allocated resources."), and this isn't it. Any future developers using this code would look upon it with the contempt warranted for such evil.
If you want this kind of behaviour then implement events and expose them, calling code can subscribe to them and manage the cursor, if needs be, otherwise the cursor should be manageable by general parameters and maybe using Begin and End methods (although such naming conventions are generally reserved for asynchronous implementations of methods, you get the picture) - hacking this way doesn't actually buy you anything.
I my opinion it makes sense to use using-Disposable thing in a way other than just disposing objects. Of course it depends on the context and usage. If leads to a more readable code then it is ok.
I have had used it in a unit of work & repository pattern implementation like:
public class UnitOfWork: IDisposable {
// this is thread-safe in actual implementation
private static Stack<UnitOfWork> _uowStack = new Stack<UnitOfWork>();
public static UnitOfWork Current {get { return _uowStack.Peek(); }}
public UnitOfWork() {
_uowStack.Push(this);
}
public void Dispose() {
_ouwStack.Pop();
}
public void SaveChanges() {
// do some db operations
}
}
public class Repository {
public void DoSomething(Entity entity) {
// Do Some db operations
UnitOfWork.Current.SaveChanges();
}
}
With this implementation it is guaranteed that nested operations will use their corresponding UnitOfWork without passing parameter. Usage is like.
using (new UnitOfWork())
{
var repo1 = new UserRepository();
// Do some user operation
using (new UnitOfWork())
{
var repo2 = new AccountingRepository();
// do some accounting
}
var repo3 = new CrmRepository();
// do some crm operations
}
In this sample repo1 and repo3 use the same UnitOfWork while repo2 uses a different repository. What reader reads is "using new unit of work" and it makes a lot of sense.
In C#, if a class, such as a manager class, does not have resources, is there any benefit to having it : IDisposable?
Simple example:
public interface IBoxManager
{
int addBox(Box b);
}
public class BoxManager : IBoxManager
{
public int addBox(Box b)
{
using(dataContext db = new dataContext()){
db.Boxes.add(b);
db.SaveChanges();
}
return b.id;
}
}
Will there be any benefit in memory use when using BoxManager if it also implements IDisposable? public class BoxManager : IBoxManager , IDisposable
For example:
BoxManager bm = new BoxManager();
bm.add(myBox);
bm.dispose();//is there benefit to doing this?
There are only 2 reasons for implementing IDisposable on a type
The type contains native resources which must be freed when the type is no longer used
The type contains fields of type IDisposable
If neither of these are true then don't implement IDisposable
EDIT
Several people have mentioned that IDisposable is a nice way to implement begin / end or bookended operations. While that's not the original intent of IDisposable it does provide for a very nice pattern.
class Operation {
class Helper : IDisposable {
internal Operation Operation;
public void Dispose() {
Operation.EndOperation();
}
}
public IDisposable BeginOperation() {
...
return new Helper() { Operation = this };
}
private void EndOperation() {
...
}
}
Note: Another interesting way to implement this pattern is with lambdas. Instead of giving an IDisposable back to the user and hoping they don't forget to call Dispose have them give you a lambda in which they can execute the operation and you close out the operation
public void BeginOperation(Action action) {
BeginOperationCore();
try {
action();
} finally {
EndOperation();
}
}
There won't be a scrap of difference between the disposable and non-disposable version if you don't explicitly make use of the Dispose() method.
While your code wouldn't benefit from implementing IDisposable, I can't agree with other opinions here that state that IDisposable is only meant to (directly or indirectly) free native resources. IDisposable can be used whenever the object needs to perform clean up task at the end of it's lifetime span. It's extremely useful together with using.
A very popular example: in ASP.NET MVC Html.BeginForm returns an IDisposable. On creation, the object opens the tag, when Dispose is called it closes it. No native resources involved, yet still a good use of IDisposable.
No, there will be no benefit if you don't do something useful like releasing unmanaged resources that your class might hold in the Dispose method.
One major point of confusion, which may not be applicable in your case but arises often, is what exactly constitutes a "resource". From the perspective of an object, an unmanaged resource is something which an outside entity () is "doing"(*) on its behalf, which that outside entity will keep doing--to the detriment of other entitites--until told to stop. For example, if an object opens a file, the machine which hosts the file may grant that object exclusive access, denying everyone else in the universe a chance to use it unless or until it gets notified that the exclusive access isn't needed anymore.
(*) which could be anything, anywhere; possibly not even on the same computer.
(**) or some way in which the the behavior or state of an outside entity is altered
If an outside entity is doing something on behalf of an object which is abandoned and disappears without first letting the entity know its services are no longer required, the outside entity will have no way of knowing that it should stop acting on behalf of the object which no longer exists. IDisposable provides one way of avoiding this problem by providing a standard means of notifying objects when their services are not required. An object whose services are no longer required will generally not need to ask any further favors from any other entities, and will thus be able to request that any entities that had been acting on its behalf should stop doing so.
To allow for the case where an object gets abandoned without IDisposable.Dispose() having been called first, the system allows objects to register a "failsafe" cleanup method called Finalize(). Because for whatever reason, the creators of C# don't like the term Finalize(), the language requires the use of a construct called a "destructor" which does much the same thing. Note that in general, Finalize() will mask rather than solve problems, and can create problems of its own, so it should be used with extreme caution if at all.
A "managed resource" is typically a name given to an object which implements IDisposable and usually, though not always, implements a finalizer.
No, if there are no (managed or unmanaged) resources there is no need for IDisposable either.
Small caveat: some people use IDisposable to clean up eventhandlers or large memory buffers but
you don't seem to use those
it's a questionable pattern anyway.
From my personal experience (confirmed with discussion and other posts here) I would say, that there could be a situations where your object use massive amount of events, or not massive amount but frequent subscriptions and unsubscription from the event which sometimes leads to that the object is not garbage collected. In this case I in Dispose unsubscribe from all events my object subscribed before.
Hope this helps.
IDisposable is also great if you want to benefit the using () {} syntax.
In a WPF project with ViewModels, I wanted to be able to temporarily disable NotifyPropertyChange events from raising. To be sure other developers will re-enable notifications, I wrote a bit of code to be able to write something like:
using (this.PreventNotifyPropertyChanges()) {
// in this block NotifyPropertyChanged won't be called when changing a property value
}
The syntax looks okay and is easily readable. For it to work, there's a bit of code to write. You will need a simple Disposable object and counter.
public class MyViewModel {
private volatile int notifyPropertylocks = 0; // number of locks
protected void NotifyPropertyChanged(string propertyName) {
if (this.notifyPropertylocks == 0) { // check the counter
this.NotifyPropertyChanged(...);
}
}
protected IDisposable PreventNotifyPropertyChanges() {
return new PropertyChangeLock(this);
}
public class PropertyChangeLock : IDisposable {
MyViewModel vm;
// creating this object will increment the lock counter
public PropertyChangeLock(MyViewModel vm) {
this.vm = vm;
this.vm.notifyPropertylocks += 1;
}
// disposing this object will decrement the lock counter
public void Dispose() {
if (this.vm != null) {
this.vm.notifyPropertylocks -= 1;
this.vm = null;
}
}
}
}
There are no resources to dispose here. I wanted a clean code with a kind of try/finally syntax. The using keyword looks better.
is there any benefit to having it : IDisposable?
It doesn't look so in your specific example, however: there is one good reason to implement IDisposable even if you don’t have any IDisposable fields: your descendants might.
This is one of the big architectural problems of IDisposable highlighted in IDisposable: What your mother never told you about resource deallocation. Basically, unless your class is sealed you need to decide whether your descendants are likely to have IDisposable members. And this isn't something you can realistically predict.
So, if your descendants are likely to have IDisposable members, that might be a good reason to implement IDisposable in the base class.
Short answer would be no. However, you can smartly use the nature of the Dispose() executement at the end of the object lifecycle. One have already gave a nice MVC example (Html.BeginForm)
I would like to point out one important aspect of IDisposable and using() {} statement. At the end of the Using statement Dispose() method is automatically called on the using context object (it must have implemented IDisposable interface, of course).
There one more reason that no one mentioned (though it's debateful if it really worth it):
The convension says that if someone uses a class that implement IDisposable, it must call its Dispose method (either explicitly or via the 'using' statement).
But what happens if V1 of a class (in your public API) didn't need IDisposable, but V2 does? Technically it doesn't break backward compatibility to add an interface to a class, but because of that convension, it is! Because old clients of your code won't call its Dispose method and may cause resources to not get freed.
The (almost) only way to avoid it is to implement IDisposable in any case you suspect that you'll need it in the future, to make sure that your clients always call your Dispose method, that some day may be really needed.
The other (and probably better) way is to implemet the lambda pattern mentioned by JaredPar above in the V2 of the API.
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.