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I have small helper classes. The classes are used internally to make sure that certain types of string always have the appropriate format (mail addresses, phone numbers, ids, urls, keys, colors, ...).
I wanted to put them into using blocks, to be able to recycle variable names:
using(Id id = ids.First())
{
Container container = containers.getById(id);
...
}
foreach(Id id in ids.Skip(1))
{
Container container = containers.getById(id);
...
}
As I did this, Visual Studio asked me to mark these classes as Disposable, which I did, but I am not sure what to do with the method stub. Let's take a "Mail Address" class as an example:
public class MailAddress : IEquatable<MailAddress>, IDisposable
{
const string MAILADDRESSPATTERN = ...
protected string _address;
public MailAddress(string address)
{
if (address == null) throw new ArgumentNullException("address");
if (!Regex.IsMatch(address, MAILADDRESSPATTERN)) throw new ArgumentException("address");
this._address = address.ToLower();
}
bool IEquatable<MailAddress>.Equals(MailAddress other)
...
public override int GetHashCode()
...
...
...
public override string ToString()
...
public void Dispose()
{
throw new NotImplementedException();
}
}
What exactly would the Dispose function have to do in such a class? What do I have to dispose of, and what would the Garbage Collector do automatically?
Up to now, I didn't call Dispose on that class anywhere, and it seemed to work fine. Now, that the class is Disposable, do I have to add calls to Dispose throughout the code?
Don't do this, you are abusing the language and the Disposable interface / pattern.
IDisposable is there for very specific reasons, among which are deterministic release of unmanaged resources or when your class owns disposable references.
It is most certainly not a means to create local scope so you can reuse variable names. If you need to do that simply do:
{
Id id = ids.First())
Container container = containers.getById(id);
...
}
The compiler forces you to implement the IDiposable interface in your Id class, because you use it with using. The sole and only purpose of using is to call IDisposable.Dispose after you leave its block. So you can only use it on IDisposable implementations.
The purpose of the IDisposable is to clean up external resources when the object is not used anymore. That includes closing files, disconneting from databases, returning windows handles and much, much more.
Since you don't seem to have anthing to clean up, you have no need for the usingstatement. In those cases when you have to use using (for example you are using one method for different types of objects where some have external resources), you still have to implement the Dispose method to satisfy the interface, but you can leave it empty.
If your class is disposable, you indeed should dispose it whenever you don't need it anymore, even if Dispose does nothing. If you or someone else adds cleanup code to Dispose later, you should rely on it being executed.
Your Dispose method has no purpose at all since you don't seem to have any unmanaged resources. Dispose is only useful in the case you want to manually dispose those resources.
All managed resources are garbage-collected by the GC, so no need for any action on your side. Removing the IDisposable on your class seems to be the appropriate action. That would require to remove the useless using too, which you can exchange for simple brackets:
{
Id id = ids.First();
Container container = containers.getById(id);
...
}
Not sure why you are using the using keyword. The using keyword guaratees a call to Dispose() when the execution leaves the scope of the using. The object inside the using brackets is therefore required to be an IDisposable. If you don't have any resources you need to clean up, there is no need to wrap your object in a using. Since it is not a great idea to use destructors in .net, the logic you would put in a destructor is often placed in the Dispose() method. This allows controlled resource management when combined with the using keyword.
When a derived class overrides Dispose should the base class Dipose be called first before the derived class disposes of any local resources?
The reason I ask is because someone on our team is saying yes for every scenario, I'm trying to work out if this is good idea or just 'cargo cult' programming, I don't have a strong view either way.
Before:
public override void Dispose()
{
base.Dispose();
if (!_isDisposed)
{
_isDisposed = true;
if (_foo != null)
{
_foo.Dispose();
}
}
}
After:
public override void Dispose()
{
if (!_isDisposed)
{
_isDisposed = true;
if (_foo != null)
{
_foo.Dispose();
}
}
base.Dispose();
}
Note: I'm not looking for how to implement the basic dispose pattern but more clarification from peoples experiences.
"It depends"
You can't have a hard-and-fast rule for this, because the order you need to call dispose will depend on the implementation of the classes.
Sometimes you might want it at the start, sometimes at the end, and sometimes in the middle. Most of the time, it probably won't matter.
Generally speaking, people seem to call it first (in the absence of any other reason to call it at a different time).
One reason to lean towards calling it first is that then the derived class has a chance to do special stuff afterwards if it needs to.
You should think about the consequences of the different approaches. In most cases, the IDisposable objects owned by the base class will be independent of those owned by the derived class, and the order of disposal won't matter, provided none of the Dispose methods throws an Exception (*).
If it's critical to release an IDisposable resource, you could consider using try/finally to ensure all Dispose methods are called - but this isn't universally done. For example, the System.Component.Container class manages and disposes multiple IDisposable objects, but makes no attempt to ensure they are all disposed if an exception is thrown.
(*) It's perfectly legitimate for Dispose to throw: for example a FileStream might attempt to flush data to a network resource that's no longer available during Dispose.
Dispose object in the reverse order to create. So firstly is created base class, so it should be dispose in the end, and so on...
Say I have the following:
public abstract class ControlLimitBase : IDisposable
{
}
public abstract class UpperAlarmLimit : ControlLimitBase
{
}
public class CdsUpperAlarmLimit : UpperAlarmLimit
{
}
Two Questions:
1.
I'm a little confused on when my IDisposable members would actually get called. Would they get called when an instance of CdsUpperAlarmLimit goes out of scope?
2.
How would I handle disposing of objects created in the CdsUpperAlarmLimit class? Should this also derive from IDisposable?
Dispose() is never called automatically - it depends on how the code is actually used.
1.) Dispose() is called when you specifically call Dispose():
myAlarm.Dispose();
2.) Dispose() is called at the end of a using block using an instance of your type.
using(var myAlarm = new CdsUpperAlarmLimit())
{
}
The using block is syntactic sugar for a try/finally block with a call to Dispose() on the object "being used" in the finally block.
No, IDisposable won't be called just automatically. You'd normally call Dispose with a using statement, like this:
using (ControlLimitBase limit = new UpperAlarmLimit())
{
// Code using the limit
}
This is effectively a try/finally block, so Dispose will be called however you leave the block.
CdsUpperAlarmLimit already implements IDisposable indirectly. If you follow the normal pattern for implementing IDisposable in non-sealed classes, you'll override void Dispose(bool disposing) and dispose your composed resources there.
Note that the garbage collector does not call Dispose itself - although it can call a finalizer. You should rarely use a finalizer unless you have a direct handle on unmanaged resources though.
To be honest, I usually find it's worth trying to change the design to avoid needing to keep hold of unmanaged resources in classes - implementing IDisposable properly in the general case is frankly a pain. It's not so bad if your class is sealed (no need for the extra method; just implement the Dispose() method) - but it still means your clients need to be aware of it, so that they can use an appropriate using statement.
IDisposable has one member, Dispose().
This is called when you choose to call it. Most typically that's done for you by the framework with the using block syntactic sugar.
I'm a little confused on when my IDisposable members would actually get called. Would they get called when an instance of CdsUpperAlarmLimit goes out of scope?
No. Its get called when you use using construct as:
using(var inst = new CdsUpperAlarmLimit())
{
//...
}//<-------- here inst.Dispose() gets called.
But it doesn't get called if you write this:
{
var inst = new CdsUpperAlarmLimit();
//...
}//<-------- here inst.Dispose() does NOT get called.
However, you can write this as well:
var inst = new CdsUpperAlarmLimit();
using( inst )
{
//...
}//<-------- here inst.Dispose() gets called.
The best practice recommend when you implement Dispose() method in non sealed class you should have a virtual method for your derived classes to override.
Read more on Dispose pattern here http://www.codeproject.com/KB/cs/idisposable.aspx
when using an IDisposable object, it's always good to use it this way:
using(var disposable = new DisposableObject())
{
// do you stuff with disposable
}
After the using block has been run, the Dispose method will be called on the IDisposable object. Otherwise you would need to call Dispose manually.
When someone calls .Dispose on it.
No, it already implements it through inheritance.
IDisposable is implemented when you want to indicate that your resource has dependencies that must be explicitly unloaded and cleaned up. As such, IDisposable is never called automatically (like with Garbage Collection).
Generally, to handle IDisposables, you should wrap their usage in a using block
using(var x = new CdsUpperAlarmLimit()) { ... }
this compiles to:
CdsUpperAlarmLimit x = null;
try
{
x = new CdsUpperAlarmLimit();
...
}
finally
{
x.Dispose();
}
So, back to topic, if your type, CdsUpperAlarmLimit, is implementing IDisposable, it's saying to the world: "I have stuff that must be disposed". Common reasons for this would be:
CdsUpperAlarmLimit keeps some OTHER IDisposable resources (like FileStreams, ObjectContexts, Timers, etc.) and when CdsUpperAlarmLimit is done being used, it needs to make sure the FileStreams, ObjectContexts, Timers, etc. also get Dispose called.
CdsUpperAlarmLimit is using unmanaged resources or memory and must clean up when it's done or there will be a memory leak
I have a class hierarchy, each member of which may create IDisposable objects.
I added a List<IDisposable> property to the base class in this hierarchy, to which I add any disposable objects on creation. The root Dispose method iterates through this list and calls Dispose for each item in its list and clears the list. In the application, I explicitly call the top object's Dispose method, causing disposal to cascade through the hierarchy.
This works, but is there a better way? Am I unwittingly duplicating some functionality already present in the framework?
(Note - the objects in question have a lifetime that precludes just wrapping them in a using block or disposing of them in the same methodwhere they are created.)
Edit
Just for clarification - I'm only keeping those objects around that need to be kept. Some are disposed of in the same method where they are created, but many are used in such a way that this isn't possible.
No that is correct. IDisposable is designed to free up unmanaged resources and should be called as soon as possible after you have finished with the instance. It's a common misconception that this is unnecessary, or that the finailizer will do this automatically when the object is garbage collected. It does not.
The correct pattern for IDisposable is here, and included below for quick reference.
public class Resource : IDisposable
{
// Dispose() calls Dispose(true)
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// NOTE: Leave out the finalizer altogether if this class doesn't
// own unmanaged resources itself, but leave the other methods
// exactly as they are.
~Resource()
{
// Finalizer calls Dispose(false)
Dispose(false);
}
// The bulk of the clean-up code is implemented in Dispose(bool)
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// free managed resources
}
// free native resources here if there are any
}
}
So long as you implement the disposable pattern correctly (as described here), this method is fine.
As far as I know, only using statements have special support for IDisposable - there isn't anything in the framework that replicates what you are doing.
If you're talking about arbitrary IDisposable objects, I don't believe it exists.
The System.ComponentModel.Container class implements cascading Dispose, but requires the elements to implement IComponent. If you control your IDisposable objects you could make them implement IComponent - it only requires implementing a single property Site that can return null.
Sounds like a situation where the visitor pattern could be appropriate. Although I never understand the claim that it extends your classes and leaves them unchanged, because I only know examples where classes should have an AcceptVisitor method or the like. BTW, it's not a pattern I like because it is complex and tends to clutter code.
If one object will create many other IDisposable objects and maintain ownership of them throughout its existence, the pattern you describe can be a good one. It may be enhanced by having your class implement method "T RegDispos<T>(T thing) where T:IDisposable;" which will add a disposable to the list and return it. One can thus take care of the creation and cleanup of a resource in the same statement, by replacing a statement like "someField = someDisposType.CreateThing();" with "someField = RegDispos(someDisposType.CreateThing());".
If your class doesn't expose a public constructor (requires outsiders use factory methods), and if you're either using vb.net or are willing to use thread-static fields, you can even combine initialization and cleanup with declaration (e.g. "var someField = RegDispos(someDisposType.CreateThing());"). For this to be safe, the constructor must be invoked within a try/catch or try/finally block that can call Dispose on created sub-objects if construction fails. Because field initializers in C# don't have access to constructor parameters (a weakness in the language, IMHO) the only way to implement such a pattern is to have a factory method create the list and put it into a thread-static variable which can then be read by a static RegDispos method.
This question already has answers here:
What are the uses of "using" in C#?
(29 answers)
Closed 8 years ago.
Are there particular instances where I should (or shouldn't?) be using "using" blocks:
using(SomeType t = new SomeType()){
...
}
Some objects need some action to be taken when you have finished with them. Usually this is because the object uses some kind of resource that needs to be disposed of. For example, if you have a file object of class File, and this object opens a file from the file system, the file in the file system will need to be closed again.
If you just left the file object, and forgot to call file.Close() it wouldn't be cleaned up until the Garbage Collector (GC) ran and worked out nothing was still using the file object. When the Garbage Collector runs should be left to the Common Language Runtime (CLR) to decide. If the GC doesn't run for quite a while after you have finished with the file, the file could remain open potentially for a long time. This can pose a big problem if there are many file objects, or if something wants to open a file, but can't because the file object you left is still hanging around.
To solve this problem, C# has the IDisposable interface. This has one method called Dispose. Classes that require some cleanup implement this Dispose method. This gives you a standard way for cleaning up any objects that use resources. There are a lot of classes that need to have Dispose called. The problem with this is that code gets covered with calls to Dispose, and they are tricky to follow because the place where you new'ed the object and call Dispose to clean it up are different. So, you had to look around the code a lot and be very careful to check there were calls to Dispose in the right place.
To solve this problem C# introduced the 'using' keyword. You can put a 'using' keyword around where you new an object, and this ensures Dispose will be called on it for you. It guarantees that Dispose will be called whatever happens... even if there is an exception thrown within the body of the using statement.
So, you should use 'using' when you want to be sure an object that allocates resources will be cleaned up.
using can only be used for objects that are declared on the stack, i.e. in a function. It doesn't work for objects that are declared as members of a class. For them, you have to call Dispose yourself. You may have to implement Dispose in your class so that in can call Dispose on any member objects it has that require it.
Common objects that need using called on them are: Files, Database connections, Graphics objects such as Pen and Brush.
Sometimes it is also used when you want two operations to happen together. For example if you want to write a log statement when a block of code is entered and when it exits you could write a log class that you could use like this:
using( Log log = new Log("Doing stuff") )
{
// Stuff
}
The constructor for the log class could be made to write out the message, and the Dispose method could also write it out. Implement the finalizer (~Log) to assert if the Dispose method doesn't get called to ensure the 'using' is remembered around the 'new Log'.
When the SomeType class implements IDisposable.
Use using whenever the type implements IDisposable, unless you're going to wrap it in a try/catch block anyway, then you might as well (depending on what look you prefer) use a finally block.
I see plenty of other answers indicated when you should have a using statement. I want to address when specifically should not have a using statement:
If you need to use your object outside of the scope of the current function, don't have a using block. Good example are a factory method that returns a database connection or a method that needs to return a datareader. In either of those cases if you create your object with a using statement it would be disposed before the method returned, and therefore not usable outside the method.
Now, you still want to be sure that those objects are disposed, so you still might want a using statement somewhere. Just don't include it in the method where the object is actually created. Instead, you can wrap the function call itself in a using statement.
When SomeType implements IDisposable.
That is a clue to you the developer that SomeType uses unmanaged resources that need to be cleaned up.
Example:
using(SqlConnection MyConnection = new SqlConnection("Connection string"))
{
MyConnection.Open();
//...
// 1. SQLConnection is a type that implements IDisposable
// 2. So you can use MyConnection in a using statement
// 3. When using block finishes, it calls Dispose method of
// SqlConnection class
// 4. In this case, it will probably close the connection to
// the database and dispose MyConnection object
}
You can create your own objects that implements IDisposable:
public class MyOwnObjectThatImplementsIDisposable : IDisposable
{
//... some code
public void Dispose()
{
// Put here the code you want to be executed when the
// using statement finish.
}
}
So you could use an object of MyOwnObjectThanImplementsIDisposable type in a using statement:
using(MyOwnObjectThatImplementsIDisposable MyObject = new MyOwnObjectThatImplementsIDisposable)
{
// When the statement finishes, it calls the
// code you´ve writed in Dispose method
// of MyOwnObjectThatImplementsIDisposable class
}
Hope this helps
In this context the using statement is handy for types that implement IDisposable. When the code block exits the scope of the using statement, Dispose() is called implicitly. It's a good habit when working with objects you want to dispose immediately after use.
One specific instance in which you should be careful using a using block is with a WCF Service Client.
As noted in this MSDN article, wrapping a WCF client (which does implement IDisposable) in a using block could mask any errors which result in the client being left in a faulted state (like a timeout or communication problem). Long story short, when Dispose() is called, the client's Close() method fires, but throws and error because it's in a faulted state. The original exception is then masked by the second exception. Not good.
There are various workarounds out there, including one in the MSDN article itself. Others can be found at IServiceOriented and blog.davidbarret.net.
I prefer the last method, myself.
If you want a summary rule. Anytime an object using IDisposable where you would not have a catch, use using. Using, essentially, is this pattern:
try
{
//instantiate and use object
}
finally
{
//dispose object
}
If you do not need a catch, using can save you typing, which is a good thing.
The primary rule is:
* Use USING statement when objects implements IDisposable interface.
This interface provides the Dispose method, which should release the object's resources. If this method is not invoked then the object will stay in memory as long, as CLR wants to perform garbage collection. If the programmer use the USING statement then on the end the object will be disposed, and all resources will be free.
It is very important that all resources that are no longer in use be free as soon as possible.
For more information about it just visit this link: microsoft
Maybe it is worth mentioning that underlying reason for adding “using” lo C# languge is following: some resources can be scarce enough that it doesn’t make sense to wait for GC to call IDisposable. For example, DB connections. If you use try/catch/finally you won’t end up with a dangling connection, but connection will be left hanging until GC doesn’t kick in and this can take a while (if you do not close it explicitly). IF you use "using" (excuse the pun) you will release the connection immediately even if you forgot to close it and even if some exception occured inside the using block.
Another reason, as previous post mentions, is that programmers do not always use finally to clean up. If not using finally in the case of exception you end up with leaking resources…
One situation is when you want to do something at the beginning of a code block, and then undo it at the end of the block, unconditionally (even if there is a throw).
The ctor for the disposable class that you build (and call within the using) would perform the action, and then the Dispose method would undo that action. This is typically how I use it.
Other people has mentioned about "IDisposable" already.
But one of the caveats when using "using" statement is that,
any exceptions thrown within "using" will not be caught
even thought "SomeType" will be disposed regardless.
So in the following snippet,
using (SomeType t = new SomeType()){
throw new Exception("thrown within using");
}
throw new Exception("thrown within using"); should not be disregarded.
I would also add that use a using() statement if something implements IDispose and also if that something you want to dispose of holds on to NON-MANAGED resources like database connections and file handles.
If it's a normal object with say a List<T>, where T is like a Customer object that holds names and address, then you don't need to. The garbage collector is smart enough to manage this for you. But the garbage collector WILL NOT return connections to the connection pool or close file handles.