I have a small class I threw together to implement a quick logger. I composed it with a private System.IO.StreamWriter which is instantiated in the constructor. Because the way I'm using is prevents me from implementing a using block, I added a finalizer and called the StreamWriter's Dispose() method in it. However, when executing, that finalizer throws an exception because the StreamWriter has already been disposed.
System.ObjectDisposedException - Cannot access a closed file.
I'm confused how this happened and I'm wondering if this means I don't need to worry about cleaning up the StreamWriter. Here is my class:
public class TextFileLogger : AbstractTextLogger
{
private const string LogPath = #"C:\";
private const string LogFileName = "MyFile.log.txt";
private readonly StreamWriter _writer;
public TextFileLogger()
{
_writer = new StreamWriter($"{LogPath}{LogFileName}", true);
_writer.AutoFlush = true;
}
~TextFileLogger()
{
_writer.Dispose();
}
protected override void WriteLine(string line)
{
_writer.WriteLine(line);
}
}
The only things you are allowed to access in a finalizer are objects that are rooted (like static variables) or objects that derive from CriticalFinalizerObject.
The problem is the finalizer is not deterministic, it is allowed to run in any order it feels like. The problem you ran in to is because the text writer was finalized before your class.
You need to either just "hope for the best" and let the writer's finalizer to do the work or you need to refactor your code so your class is disposeable itself and that calls the stream writer's dispose method.
Related
I've found out some logger class and I am not so sure whether this class is really thread safe. True is that the Log method implements lock but access to class itself by multi threads will not raise problems? It consider me becase TextWriter tw is static so it means class has only one copy of tw and during each thread will access it can raise problems or not? BTW please take a look do you have any other objections here?:
public static class Logger
{
static readonly TextWriter tw;
private static readonly object _syncObject = new object();
static Logger(string SPath)
{
tw = TextWriter.Synchronized(File.AppendText(SPath + "\\Log.txt"));
}
public static void Write(string logMessage)
{
try
{
Log(logMessage, tw);
}
catch (IOException e)
{
tw.Close();
}
}
public static void Log(string logMessage, TextWriter w) {
// only one thread can own this lock, so other threads
// entering this method will wait here until lock is
// available.
lock(_syncObject) {
w.WriteLine("{0} {1}", DateTime.Now.ToLongTimeString(),
DateTime.Now.ToLongDateString());
w.WriteLine(" :");
w.WriteLine(" :{0}", logMessage);
w.WriteLine("-------------------------------");
// Update the underlying file.
w.Flush();
}
}
}
Second version for further dicsuss purposes:
public static class Logger
{
static StreamWriter sw;
private static readonly object _syncObject = new object();
static Logger(string SPath)
{
lock(_syncObject)
{
StreamWriter sw = File.AppendText(SPath));
}
}
....
It is thread safe because every path uses the Log static method which uses a lock on the static _syncObject. This means that every thread that attempts to Log(..) will block if somebody else is logging.
However, if you call the Log method directly rather than the Write method, and you pass in a TextWriter that is being used by other threads, then that's not safe.
First of all, TextWriter is an abstract class, so the real instance might be any derived class. The documentation on TextWriter doesn't guarantee thread-safety of TextWriter implementation:
Any instance members are not guaranteed to be thread safe.
There is no guarantee that external lock will bring the thread-safety. For example, the implementation may use thread-local storage for its internal data. TextWriter.Flush may or may not help, depending on the implementation (but you never know which implementation comes into Log method and whether this implementation was e. g. created under the same lock).
This all is about Log method.
The usage of Write, on the contrary, seems to be good: it uses TextWriter.Synchronized, which is thread-safe, according to the documentation:
Creates a thread-safe wrapper around the specified TextWriter.
But take into account, that thread-safety of the synchronized text writer may or may not be the thread safety you need: if you are making to consecutive calls to Log, the results are not guaranteed to be consecutive in the output file.
If a class has an instance field that implements IDisposable then the containing class implements IDisposable and class that fields Dispose method from within its Dispose method.
public class A : IDisposable
{
public System.Drawing.Font font = new Font("Arial", 10.0f);
public void Dispose()
{
font.Dispose()
}
}
(I know I didn't do the dispose pattern correctly, but for sample code should be good enough)
If the field is a static field though where should the call to the field's Dispose be?
public class B
{
public static System.Drawing.Font font = new Font("Arial", 10.0f);
}
I could make class B implement IDisposable and have that call font.Dispose but if B.font is used again later on that would cause problems. As well as you'd have to remember that dispise accessing a static method you need to create an instance just to call Dispose.
I could also make a static Dispose method but then users have to remember to call Dispose and have to make sure they're the last user of it in the program.
Static fields are initialised when the type is loaded.
Therefore it logically it makes sense to dispose the object assigned to the static field when the containing type is unloaded.
However, types are not unloaded. There may be some exotic complication here around AppDomains, but I suspect that doesn't apply in your case.
Therefore I wouldn't dispose the instance, otherwise you will have a publicly available instance of an object that is unfit for use.
If the field is static, then maybe the intention is to have it last for the complete duration of the application? Because then it will only need to be disposed of when the application shuts down. And that will happen one way or the other, by itself-
If you plan to reassign the static field to different IDisposable objects several times during the lifetime of the application, then of course you would want to dispose the old object when you reassign. Maybe you could use a property for that? Not that I have thought a lot about it, but something like:
// private - don't write to this field from outside the property setter
static Font font = new Font("Arial", 10.0f));
public static Font Font
{
get
{
return font;
}
set
{
var oldFont = font;
if (oldFont != null)
oldFont.Dispose();
font = value;
}
}
You dispose of it like any other object. It doesn't make a difference. A static object is just an object that is available per class. It's still an instance of something. Granted, you probably wouldn't want to do this since after you dispose of it someone can still access it and get a ObjectDisposedException exception.
static void Main(string[] args)
{
using (Test.Instance)
{
}
Thread.Sleep(TimeSpan.FromSeconds(10));
}
public class Test:IDisposable
{
public static Test Instance = new Test();
public void Dispose()
{
Console.WriteLine("Disposed");
}
}
And the output is:
Disposed
Per your updated example:
public class A
{
public static System.Drawing.Font font = new Font("Arial", 10.0f));
}
You can just do A.font.Dispose() or using(A.font)
In general I think the idea is you really wouldn't have a public static disposable field since by making it static you imply it should be around for the lifetime of the application. If it has to be disposable you should could make it lazy and make it be thread-safely re-initializable, otherwise accessing it after disposing will throw exceptions. Or you can have a hook into your application end code and safely dispose of all static disposables there. You can register your disposable on start somewhere as well. Just an idea
You can't dispose a static class because there is no instance of it
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;
// }
//}
}
assume this class:
public class Logger
{
static TextWriter fs = null;
public Logger(string path)
{
fs = File.CreateText(path);
}
public static void Log(Exception ex)
{
///do logging
}
public static void Log(string text)
{
///do logging
}
}
and I have to use this like:
Logger log = new Logger(path);
and then use Logger.Log() to log what I want. I just use one Logger.
the question is: is this a good design? to instantiate a class and then always call it's static method? any suggestion yield in better design is appreciated.
Edit based on Marc's answer:
I flush on the last line of Log and there is no need for me to read the file while it is open, the issue with file not cleanly closed is right. this class simply satisfy my requirements and there is no need to be thread safe for it. I just want to get read of the instantiation part, I should get into the SetPath you said, any suggestion for closing file?
Yes, having a constructor just for this is bad design. A static SetPath method that can only be called once (else throws an exception) would seem better. You would set the path during app-startup, etc.
Then you can either make it a static class, or a singleton if it is required to satisfy some interface-based scenario.
Next: you must add synchronisation here! That is not thread safe. If two threads attempt to log at the same time, I would expect this to collapse horribly. It doesn't need to be complex; at the simplest:
private readonly object syncLock = new object();
public static void Log(string value) {
lock(syncLock) {
//...
}
}
(but note that this may incur some blocking costs; which can be improved with more sophisticated code - see below)
There are existing logging libraries that will think of lots more issues - file partitioning, async (to stop your code being blocked by IO), batching, etc; why not just use one of them? In particular, at te moment your file will not be cleanly closed at app-exit, doesn't flush regularly, and will keep the file locked most of the time. Not good.
No, this doesn't make sense. Each time a Logger is instantiated, the static TextWriter will be overwritten, which will affect all consumers of the class. If you want to keep the instance constructor then you should make the TextWriter an instance field and the methods should be instance methods.
As an alternative, you may want to consider using log4net, which will do this kind of logging work for you.
I think you should make whole class static with static property allowing you to set up the log path.
public static class Logger
{
static TextWriter fs = null;
public static string FileName
{
set
{
fs = File.CreateText(value);
}
}
public static void Log(Exception ex)
{
if(fs == null) return;
///do logging
}
public static void Log(string text)
{
if(fs == null) return;
///do logging
}
}
I asked a question earlier today, but I think I need to approach it in a different way (on top of that there was a "hang up" in regards to DataSet).
Here's a class that encapsulates the creation of a Font (in other words, it is reading data from an xml file and is creating a font, at runtime, based on what it reads from that file):
public class FontCreator
{
private Font m_TheFont = null;
public FontCreator( ... some parameters ... )
{
m_TheFont = GetTheFont();
}
public Font TheFont
{
return m_TheFont;
}
private Font GetTheFont()
{
// code, and more code, that eventually leads to:
Font f = new Font(fntFamily, fntSize, fntStyle);
return f;
}
}
The consumer of the FontCreator class looks something like:
public class TheConsumer()
{
private FontCreator m_FontCreator = null;
public TheConsumer()
{
m_FontCreator = new m_FontCreator( ... some parameters ... );
Initialize();
}
private void Initialize()
{
InitializeThis();
InitializeThat();
}
private void InitializeThis()
{
.... some code ...
SomeObject.ApplyFont(m_FontCreator.TheFont);
}
private void InitializeThat()
{
... some code ...
SomeObject.ApplyFont(m_FontCreator.TheFont);
}
}
What code do you add, and where, to ensure that "TheFont"'s Dispose method is explicitly called?
If you don't wish to maintain a reference to TheFont after it is initially used, then call it's Dispose method in your constructor, right after Initialize. If you wish to keep TheConsumer alive for a while and maintain a reference to TheFont, it gets more interesting. Two Options:
You can have TheFont's dispose method called from the Destructor of the TheConsumer object. This is not the common practice and has problems. Mainly, this is not called until garbage collection happens. Better is:
You can make the TheConsumer object itself implement IDisposable, and call TheFont.Dispose from TheConsumer.Dispose. Since TheConsumer implements IDisposable, the code that uses it should call its Dispose method.
Edit in response to harsh comment!
Yes, I should have made clear to only use 1 in addition to 2, if at all. I know all developers everywhere are supposed to notice when IDisposable is implemented, but they often don't. If the referenced managed resource might really remain around a long time and cause problems if not properly disposed, I sometimes have a safety Dispose() method call in the destructor of the object holding the reference. Is that so wrong? :)
public TheConsumer()
{
using (m_FontCreator = new m_FontCreator( ... some parameters ... ))
{
Initialize();
}
}
I am confused, if you want to quickly use the font creater object then implement IDisposable on the FontCreater and use
using(m_FontCreator = new FontCreater(....))
{
InitializeThis();
InitializeThat();
}
If you need to keep the instance of the FontCreater through the lifetime of TheConsumer, then implement IDisposable on both FontCreater and TheConsumer classes.
public class TheConsumer : IDisposable
{
void Dispose()
{
if(m_FontCreator != null)
m_FontCreator.Dispose();
}
}
then use TheConsumer class like so
using(TheConsumer consumer = new TheConsumer(....))
{
....
}
Answer 1: Avoid it. Don't keep objectsthat contain unmanaged resources around any longer than necessary.
Answer 2: If you do need the embedded fields as shown in your code, than both the FontCreator and the Consumer class need to implement IDisposable. But not a destructor (Finalizer).
The main argument for this is that FontCreator is the 'owner' of the Font and should therefore take responsibility. And the Consumer is responsible for the Creator in the same way.
As others have noted, it appears you can at least avoid the m_FontCreator field in the Consumer class. But it depends on the rest of the code, is m_FontCreator used elsewhere?