I have the following code:
A disposable class that contains two disposable members.
One of them is being initialized using new() method, and the other using static factory method.
I Also have static code analisys rules, with CA2213 as error.
public class DisposableClass : IDisposable
{
private WebClient m_DisposableMember1;
public WebClient DisposableMember1
{
get
{
if (m_DisposableMember1 == null)
{
m_DisposableMember1 = new WebClient();
}
return m_DisposableMember1;
}
}
private WebClient m_DisposableMember2;
public WebClient DisposableMember2
{
get
{
if (m_DisposableMember2 == null)
{
m_DisposableMember2 = Factory.Create();
}
return m_DisposableMember2;
}
}
#region Finalize/Dispose Pattern
private bool m_IsDisposed = false;
//Implement IDisposable.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
~DisposableClass()
{
Dispose(false);
}
protected virtual void Dispose(bool disposing)
{
if (!m_IsDisposed)
{
if (disposing)
{
DisposableMember1.Dispose();
// DisposableMember2 in not disposed and not notified by fxCop
}
m_IsDisposed = true;
}
}
#endregion Finalize/Dispose Pattern
}
This is the simple factory class:
public static class Factory
{
public static WebClient Create()
{
return new WebClient();
}
}
When I call the Dispose() method of the DisposableMember1 property, I get CA2213.
When I call the Dispose() method of the m_DisposableMember1 member, I don't get this error.
More over, I don't get this error for m_DisposableMember2 (Wich was initialized using the static factory), and it is not being disposed.
Does anybody familiar with this issue? What can cause this behavior?
CA2213 is not particularly aggressive about identifying fields that ought to be disposed. Amongst other potential problems, it only considers fields that are directly assigned from a constructor invocation. Assignment of a field value via a property or from a source other than a constructor invocation do not result in the target field being included in the pool of "should be disposed" fields by the rule.
You are expecting a bit too much out of the smarts of this tool. That the property is an alias for the field is only obvious to human eyes. It requires pretty sophisticated analysis to let a tool see that. FxCop just doesn't have that kind of horse power. Your code is in fact questionable. If the property was never used by the client code then you'll create the WebClient object and immediately dispose it. Use the field instead, test for null and Dispose().
The problem with the second field is probably induced by the need for the tool to avoid unnecessary warnings. It will only complain if it can determine with absolute certainty that the field is ever initialized. Same kind of lack of smarts here, it can't tell that the factory always returns a non-null reference. Only when it sees the constructor being used in the class itself can it be sure that the field needs to be disposed.
It's a good tool, not perfect.
Related
I have a class from a third-party assembly (so I can't edit it):
public class MyClass
{
private bool _loggedIn;
public void Login() {_loggedIn = true;}
public void Logout() {
if (!_loggedIn) throw new InvalidOperationException();
_loggedIn = false;
}
}
Now, suppose I have an instance of MyClass (for which I don't know _loggedIn), and I need call LogOut. Which of the following methods of avoiding a fatal exception will generally be faster? (any other method would be fine too):
To call LogOut, and if _loggedIn == false, just catch the exception
To use reflection to check that _loggedIn == true, and only call LogOut if so
It depends on the invariants you expect to see in your application.
1. If you expect to have a lot of MyClass having different state(logged in, logged off), then it is better to avoid overhead of exception (because exception is Exceptional situation) and use some specific public IsLoggedIn property (obviously to avoid Reflection) or some TryXxxxx-like methods.
And even if you can't modify the original code no one stops you from wrapping it:
public class MyWrappedClass
{
public Boolean IsLoggedIn {get; private set;}
private MyClass m_Log;
public MyWrappedClass ()
{
this.m_Log = new MyClass();
this.IsLoggedIn = false;
}
public void Log()
{
try
{
this.m_Log.LogIn();
this.IsLoggedIn = true;
}
catch
{
this.IsLoggedIn = false;
}
}
public void LogOut()
{
try
{
this.m_Log.LogOut();
this.IsLoggedIn = false;
}
catch
{
this.IsLoggedIn = true;
}
}
}
You could even go further and implement IDisposable interface with it to avoid manual LogIn-LogOut management:
public class MyWrappedClass
{
private class LogSessionToken : IDisposable
{
private MyWrappedClass parent;
public LogSessionToken (MyWrappedClass parent)
{
parent.LogIn();
}
public void Dispose()
{
parent.LogOut();
}
}
public IDisposable LogSession()
{
return new LogSessionToken (this);
}
// ...
}
And use it like
using (var logToken = wrappedInstance.LogSession)
{
// do the work.
} // No need to worry about manual LogOut
2. If you expect to use only few of MyClass in a proper fashion, then it would be a better idea to not handle exception at all - if something wrong happened then it is some programming error thus the program shall be terminated.
First, if your class doesn't expose at least a read-only property for LoggedIn, there sounds like a fairly large design flaw.
For speed, using reflection will generally be faster, particularly if you cache the FieldInfo or build a Func<bool> using System.Linq.Expressions. This is because Exceptions collect lots of debug information when thrown, including a StackTrace, which can be expensive.
As with anything, though, it is often best to test such operations, as there are sometime optimizations or other factors that may surprise you.
If the pattern if (CanFoo) Foo(); appears very much, that tends to imply very strongly that either:
A properly-written client would know when it can or cannot call Foo. The fact that a client doesn't know suggest that it's probably deficient in other ways.
The class exposing CanFoo and Foo should also expose a method which will Foo if possible and appropriate (the method should throw if unable to establish expected post-conditions, but should return silently if the post-conditions were established before the call)
In cases where a class one does not control should provide such a method but doesn't, the cleanest approach may be to write one's own wrapper method whose semantics mirror those the missing method should have had. If a later version of the class implements the missing method, changing one's code to use that implementation may be easier than refactoring lots of if (CanFoo) constructs.
BTW, I would suggest that a properly-designed class should allow calling code to indicate whether it is expecting a transition from logged-in state to logged-out state, or whether it wants to end up in logged-out state but it doesn't care how it gets there. Both kinds of semantics have perfectly legitimate uses; in cases where the first kind would be appropriate, having a LogOut method throw an exception if called on a closed session would be a good thing, but in cases where client code merely wants to ensure that it is logged out, having an EnsureLoggedOut method that could be invoked unconditionally would be cleaner than having to add extra client-side code for that purpose.
Can we use the using statement in a constructor to declare an instance of an object for later usage. For example.
public class TestClass {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
using (this._dataContext = DataClassesDataContext(connString));
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
}
You must implement IDisposable yourself and call Dispose on the data context from you Dispose method.
public class TestClass : IDisposable {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
_dataContext = new DataClassesDataContext(connString);
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
public void Dispose(){
_dataContext.Dispose();
}
}
It's not clear what you expect the using statement to do here. All it does is make sure that Dispose is called at the end of the block.
So basically you'd be creating a DataClassesDataContext (I assume you missed the new keyword...), storing a reference in a field, and then immediately disposing of it. That's not going to work well - you should get rid of the using statement, but quite possibly make your class implement IDisposable so that when the instance of TestClass is disposed, you dispose of the data context.
According to MSDN:
The using statement calls the Dispose method on the object in the
correct way, and (when you use it as shown earlier) it also causes the
object itself to go out of scope as soon as Dispose is called.
The using statement is basically syntactic sugar for try/finally.
try
{
_dataContext = new DataClassesDataContext(connString);
}
finally
{
if (_dataContext != null)
((IDisposable)dataContext).Dispose();
}
Looking at it in this way it should become obvious that datacontext is no longer in scope and therefore can't be used by other methods as you desire. To solve the problem, you should make the class implement IDisposeable.
using (this._dataContext = DataClassesDataContext(connString));
is the same as
try
{
this._dataContext = DataClassesDataContext(connString);
}
catch
{
}
finally
{
if(this._dataContext!=null)
((IDisposable)this._dataContext).Dispose();
}
So you'll get _dataContext disposed in you constructor and it will be no longer available. You should implement IDisposable interface and you'll be able to employ using statement where you want like this:
using (TestClass test = new TestClass("conn"))
{
//your code
}
If a C# derived IDisposable class constructor generates an error, how to dispose of the already fully constructed IDisposable base class?
Since all fields in the class hierarchy are initialized before any constructor executes, is it safe for the derived constructor to call base.Dispose()? It violates the rule of not calling virtual methods until object is fully constructed, but I can't think of another way to do it, and my searching hasn't turned up anything about this scenario.
My view is that constructors should be light, not relying on external resources/etc that might throw an exception. The constructor should do enough to verify Dispose() can be safely called. Consider using containment instead of inheritance, or having a factory method do the work that might throw.
All derived class constructors must call Dispose on the object being constructed if they exit via an exception. Further, it's very difficult to write leak-proof classes if field initializers construct IDisposable instances or can fail. Too bad, since requiring objects to be declared in one place, initialized in a second, and cleaned up in a third is not exactly a recipe for coherent code.
I would suggest that the best pattern is probably something like:
class foo : baseFoo , IDisposable
{
foo () : baseFoo
{
bool ok = false;
try
{
do_stuff();
ok = true; // Only after last thing that can cause failure
}
finally
{
if (!ok)
Dispose();
}
}
}
Note that C++/CLI automatically implements that pattern, and can also automatically handle cleanup of IDisposable fields. Too bad the language seems like a pain in other ways.
PS--With relatively few exceptions, mostly revolving around predictable-cost shared immutable objects (e.g. brushes, fonts, small bitmaps, etc.), code which relies upon Finalize to clean up objects is broken. If an IDisposable is created, it must be disposed unless the code which created it has particular knowledge about the consequences of deferring disposal to a finalizer.
For Managed resources, this should not be a problem, they will get Garbage Collected. For Unmanaged resources, make sure you have a Finalizer defined for your object, that will ensure the Unmanaged resources are cleaned up.
Also, throwing exceptions from a constructor is considered very bad manners, It's better to provide a Factory Method to do the construction and error handling or to equip your object with an Initialize method that will throw the actual exception. That way the construction always succeeds and you're not left with these type of issues.
Correct, Dispose isn't called by the Garbage Collector, but a Finalizer is, which in turn needs to call Dispose. It's a more expensive technique, except when used correctly. I didn't say so in my answer, did I ;).
You can call into the GC to force a collection run and you can wait for all pending finalizers. It's still better to not put the exception generating code in the construstor or by placing a try/catch around the code in the constructor to ensure Dispose is called on these files in case of an error. You can always rethrow the exception later.
This solution builds on the proposal by #supercat. Initialization of any member that might throw must be performed in the constructor's try/catch block. With that condition met an exception thrown from any constructor will correctly dispose a fully or partially constructed base or derived class.
In this test code uncomment each of the four exceptions in turn and the program will output which of the Disposable resources were not disposed properly due to the constructor throwing an exception. Then uncomment the two Dispose calls and observe that everything gets cleaned up as it should.
class DisposableResource : IDisposable
{
public DisposableResource(string id) { Id = id; }
~DisposableResource() { Console.WriteLine(Id + " wasn't disposed.\n"); }
public string Id { get; private set; }
public void Dispose() { GC.SuppressFinalize(this); }
}
class Base : IDisposable
{
public Base()
{
try
{
throw new Exception(); // Exception 1.
_baseCtorInit = new DisposableResource("_baseCtorInit");
// throw new Exception(); // Exception 2.
}
catch(Exception)
{
// Dispose(); // Uncomment to perform cleanup.
throw;
}
}
public virtual void Dispose()
{
if (_baseFieldInit != null)
{
_baseFieldInit.Dispose();
_baseFieldInit = null;
}
if (_baseCtorInit != null)
{
_baseCtorInit.Dispose();
_baseCtorInit = null;
}
}
private DisposableResource _baseFieldInit = new DisposableResource("_baseFieldInit");
private DisposableResource _baseCtorInit;
}
class Derived : Base
{
public Derived()
{
try
{
// throw new Exception(); // Exception 3.
_derivedCtorInit = new DisposableResource("_derivedCtorInit");
// throw new Exception();
}
catch (Exception)
{
// Dispose(); // Uncomment to perform cleanup.
throw;
}
}
public override void Dispose()
{
if (_derivedFieldInit != null)
{
_derivedFieldInit.Dispose();
_derivedFieldInit = null;
}
if (_derivedCtorInit != null)
{
_derivedCtorInit.Dispose();
_derivedCtorInit = null;
}
base.Dispose();
}
private DisposableResource _derivedFieldInit = new DisposableResource("_derivedFieldInit");
private DisposableResource _derivedCtorInit;
}
class Program
{
static void Main(string[] args)
{
try
{
Derived d = new Derived();
}
catch (Exception)
{
Console.WriteLine("Caught Exception.\n");
}
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
Console.WriteLine("\n\nPress any key to continue...\n");
Console.ReadKey(false);
}
}
I've seen this error on other posts, but not for this exact situation.
I have two classes that do the same thing with a MessageQueue. Because of that, I abstracted the creation and disposal of the queue to a helper class. I'm getting this error, and I can't see how the queue can be disposed more than once.
Object 'messageQueue' can be disposed more than once in method 'MsmqHelper.DisposeQueue(MessageQueue)'
In one of the classes, this is how the queue is used:
private MessageQueue _messageQueue;
Then, in the constructor of the class:
this._messageQueue = MsmqHelper.InitializeQueue();
Not that it really matters, but for completeness, here is where the queue is used:
this._messageQueue.Send(workflowCreated);
And here are the Dispose methods:
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if (disposing == false) { return; }
MsmqHelper.DisposeQueue(this._messageQueue);
}
And this is the code in the helper class that actually calls Dispose():
public static void DisposeQueue(MessageQueue messageQueue)
{
if (messageQueue != null)
{
messageQueue.Close();
messageQueue.Dispose();
messageQueue = null;
}
}
Where is it possible for the queue to be disposed more than once in this situation?
** Edit **
I thought it would be nice to add my comments, in the conversation below, here. It's a good summary, along with the accepted answer:
I think I get it now. The messageQueue method parameter has nothing to do with the original (this._messageQueue) reference to the object. So checking messageQueue for null, and setting it to null, does no good. The caller could still pass in its variable (this._messageQueue) even after being disposed. Hence, being able to be disposed more than once.
By the way, even setting the caller's variable (this._messageQueue) to null, in the calling method, doesn't help. The problem exists solely in MsmqHelper.DisposeQueue(). So the answer is to pass by ref or simply don't call DisposeQueue() and do it all in the calling method.
** Edit 2 **
After trying this, I get the same error. I simply don't get it.
public static void DisposeQueue(ref MessageQueue messageQueue)
{
if (messageQueue == null) { return; }
messageQueue.Close();
messageQueue.Dispose();
messageQueue = null;
}
** Edit 3 -- Bug? **
I'm starting to think that this may be a bug. If I comment messageQueue.Dispose(), the error goes away. HOWEVER, I can call messageQueue.Close() and messageQueue.Dispose() together in the calling method. Go figure. I think I'm just going to make these same calls from the calling methods, or only call Close() or Dispose() instead of both.
Close frees all resources of the MessageQueue object. See the documentation here. The error is most likely generated in CA because it sees that the execution path of Close also calls Dispose.
From the documentation:
public void ReceiveMessage()
{
// Connect to the a on the local computer.
MessageQueue myQueue = new MessageQueue(".\\myQueue");
// Set the formatter to indicate body contains an Order.
myQueue.Formatter = new XmlMessageFormatter(new Type[]
{typeof(String)});
try
{
// Receive and format the message.
Message myMessage1 = myQueue.Receive();
Message myMessage2 = myQueue.Receive();
}
catch (MessageQueueException)
{
// Handle sources of any MessageQueueException.
}
// Catch other exceptions as necessary.
finally
{
// Free resources.
myQueue.Close();
}
return;
}
Close apparently will release the resources but will allow the component to reacquire them if they haven't been collected yet. It might be more prudent to open the MessageQueue object, use it, and then close it within the same call rather than open it for a period of time and closing it later because connection caching removes the overhead of opening the MessageQueue in repeated calls.
*UPDATE*
It appears that CA treats CA2202 differently for member fields versus passing the disposable object to a method, even if that method is private to the class. Regardless, according to the documentation, you should only have to call Close() or Dispose() but not both. I recommend however changing your design so that you create, use, and then close the MessageQueue object all within the scope of your message operations like the example from the documentation example demonstrates above.
Yes. This can dispose the object multiple times:
The value that this._messageQueue evaluates to does not change after invoking MsmqHelper.DisposeQueue(this._messageQueue).
Only the local parameter (named messageQueue) was assigned the value null in the DisposeQueue method. Thus the "null guard" fails to correctly guard subsequent times around. (This is because C#'s default behavior is Call-By-Value: please see the link to understand what this means in context of "passing the value of a reference to an object".)
Either take in ref or assign this._messageQueue = null in the caller.
If MessageQueue class implements IDisposable iterface, then there is no point to use Dispose method explicitly and Close() method, because in all such classes Close() method is usually is not an iterface method but rather a class method. Typically, in Dispose method all correct impementation should call Close() mehod before release managed/unmanaged resources.
Again, by impelmenting external static helper, you break the Disposable pattern. It's not the correct way to control lifetime of the object; You don't need to mess up with Disposable pattern, you can simply use it
And your code might be simplified like this:
// 1. Use static class. By the agreement, all helper classes should be static to avoid
// IDisposable inheritance, in example
public static class MsmqHelper//: IDisposable
{
//private MessageQueue _messageQueue;
//public MessageQueueHelper(bool workflowCreated)
//{
// this._messageQueue = MsmqHelper.InitializeQueue();
// this._messageQueue.Send(workflowCreated);
//}
public static SendMessage(object workflowCreated)
{
// 2. If static method in static class does not takes parameters,
// I might be better to to implicitly call the constructor?
// using(MessageQueue msmsq = MsmqHelper.InitializeQueue())
using(MessageQueue msmsq = new MessageQueue())
{
msmq.Send(workflowCreated);
msmq.Close();
// MsmqHelper.DisposeQueue(msmq);
// 3. You should explicitly call Close object to immediately release
// unmanaged resources, while managed ones will be released
// at next GC rounds, as soon as possible
}
}
//private MessageQueue _messageQueue;
//public void Dispose()
//{
// Dispose(true);
// GC.SuppressFinalize(this);
//}
//private void Dispose(bool disposing)
//{
// if (disposing == false) { return; }
//
// MsmqHelper.DisposeQueue(this._messageQueue);
//}
//public static void DisposeQueue(MessageQueue messageQueue)
//{
// if (messageQueue != null)
// {
// messageQueue.Close();
// messageQueue.Dispose();
// messageQueue = null;
// }
//}
}
Just a quick sanity check here!
If I have a static method in an instance class e.g:
public class myClass
{
public static void AMethod()
{
//do somit
}
}
Would it cause problems if I was making reference to IDisposable resources in the method body, an object context for example?
public static void AMethod()
{
ObjectContext context = new ObjectContext();
// do somit
}
By would it cause problems, I mean, would it retain the object context behind the scenes after the end of the method body due to the fact it was a static method?
The class as can be seen, isn't static, and the variable is local to the method.
I'm aware that I should be using 'using' here, just curious as to whether this particular combo of events could/would cause memory leaks.
In order to avoid any problems it is recommended to dispose IDisposable resources as soon as you have finished using them. This could happen by wrapping them in a using statement:
public static void AMethod()
{
using (ObjectContext context = new ObjectContext())
{
// do something
}
}
After leaving the scope of your method AMethod, your context object can't be used anymore so it'll be garbage collected eventually.
But as it implements IDisposable, you should use the using statement:
using (ObjectContext context = new ...)
{
// Use the context here
}