I am having trouble understanding the way XmlWriter works in C#. Take the following code as if it hypothetically was used somewhere.
StringBuilder builder = new StringBuilder();
XmlWriterSettings settings = new XmlWriterSettings();
settings.Encoding = new UTF8Encoding(false);
settings.ConformanceLevel = ConformanceLevel.Document;
settings.Indent = true;
XmlWriter writer = XmlWriter.Create(builder, settings);
// Do stuff
writer.Close();
Since XmlWriter is not used within an using statement, could this potentially result in an OutOfMemoryException due to it not being properly disposed?
Ultimately, the purpose of the Dispose() in this case is to allow the XmlWriter to assume ownership of whatever it is writing to - for example, if you create an XmlWriter over a Stream, calling Dispose() on the XmlWriter can (by default) flush the xml writer and then call Dispose() on the stream. This makes it easy to pass an XmlWriter to APIs without also having to pass them a chain of other objects they need to dispose when they're done (it could, for example, be an XmlWriter talking to a CompressionStream talking to a SslStream talking to a NetworkStream, etc).
In the general case, the purpose of the Dispose() on the final end thing is to close the underlying resource (which could be a file, a socket, a pipe, etc)
In this specific case, you're talking to a StringBuilder. The Dispose() here is basically a no-op, as there is no external resource. It'll just be collected by the GC either way, at some point in the future. As such, no: there is no memory leak problem here; the GC can see what you're doing.
So: in this case it won't make a functional difference, but: it is good practice to get into the habit of calling Dispose() (usually via using) when that is part of the API, as in many cases this is really, really important.
Its good to have using block, where it ensures calling the Dispose on any object that implements IDisposable.
using (XmlWriter writer = XmlWriter.Create(builder, settings))
{
//do stuff
}
The Dispose method for XmlWriter looks like-
protected virtual void Dispose(bool disposing)
{
if (this.WriteState != WriteState.Closed)
{
try
{
this.Close();
}
catch
{
}
}
}
It could potentially cause memory issues if there would be an exception in
// Do stuff
In this scenario, the Close method would not be executed.
Using statement is a syntactic sugar around try-finally blocks. It calls Dispose in finally block even there was an exception
XmlWriter.Dispose calls Close method behind the scene
according to c# docs
The using statement ensures that Dispose is called even if an exception occurs within the using block. You can achieve the same result by putting the object inside a try block and then calling Dispose in a finally block; in fact, this is how the using statement is translated by the compiler. The code example earlier expands to the following code at compile time (note the extra curly braces to create the limited scope for the object):
{
var font1 = new Font("Arial", 10.0f);
try
{
byte charset = font1.GdiCharSet;
}
finally
{
if (font1 != null)
((IDisposable)font1).Dispose();
}
}
and close function is do the same thing
This method calls Dispose, specifying true to release all resources. You do not have to specifically call the Close method. Instead, ensure that every Stream object is properly disposed. You can declare Stream objects within a using block (or Using block in Visual Basic) to ensure that the stream and all of its resources are disposed, or you can explicitly call the Dispose method.
Related
I am attempting to follow this solution here to mirror my Console output to a log file as well, however, i noticed that the output to file gets cut off, so the full console output is not completely outputted in the file. Why is that? Is TextWriter limited to certain amount of lines?
private TextWriter txtMirror = new StreamWriter("mirror.txt");
// Write text
private void Log(string strText)
{
Console.WriteLine(strText);
txtMirror.WriteLine(strText);
}
p.s. the reason im using this solution is because I have Console.Writeline in functions as well that i call in the main(). so if i was to use this solution instead, i would have to open a using statement everywhere i have a Console.WriteLine()...which seems redundant
You can use the AutoFlush property System_IO_StreamWriter_Flush
Flushing the stream will not flush its underlying encoder unless you explicitly call Flush or Close. Setting AutoFlush to true means that data will be flushed from the buffer to the stream after each write operation, but the encoder state will not be flushed.
By the way, if you are instantiating your logger class many times, you are going to have many StreamWriter objects. Make sure you dispose them as per documentation
This type implements the IDisposable interface. When you have finished using the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/catch block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section in the IDisposable interface topic.
Disposing the objects, makes sure that flush is called and any buffered information is written to the underlying object.
Example:
// Write text
private void Log(string strText)
{
Console.WriteLine(strText);
using (StreamWriter txtMirror = new StreamWriter("mirror.txt")) {
txtMirror.WriteLine(strText);
}
}
I have the following logger-like class pattern:
public class DisposableClassWithStream : IDisposable
{
public DisposableClassWithStream()
{
stream = new FileStream("/tmp/file", FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite);
writer = new StreamWriter(stream);
}
public void WriteLine(string s)
{
writer.WriteLine(s);
}
~DisposableClassWithStream()
{
Dispose();
}
private readonly object disposableLock = new object();
private bool isDisposed;
public void Dispose()
{
lock (disposableLock)
{
if (!isDisposed)
{
writer.Close();
stream.Close();
GC.SuppressFinalize(this);
isDisposed = true;
}
}
}
private FileStream stream;
private TextWriter writer;
}
and a very simple code using this class:
public static void Main(string[] args)
{
var t = new DisposableClassWithStream();
t.WriteLine("Test");
}
The code throws (nondeterministically), both on .net and mono, ObjectDisposedException caused by method Close of object writer as it tries to flush buffer to stream which is already disposed.
I understand that the reason is that GC finalizes stream before writer. How can I change the class pattern to be sure that stream is not disposed before the writer?
I tired to use GC.SuppressFinalize(writer) in the constructor, but I'm not sure if it is not too hacky.
Edit:
I would like to address the question of having finalizer in the first place. As mentioned at the beginning of the question, the class is used as a logger and I want to assure that all lines from writer are flushed to the hard disk before closing the process.
Do not create finalizer, unless your IDisposable implementation really works with unmanaged resources. FileStream and StreamWriter are managed resources.
Moreover, since SafeHandle was introduced, it is hard to imagine use-case, when any unmanaged resource couldn't be wrapped into managed SafeHandle. Do not follow blindly IDisposable implemetation from MSDN - it's OK, but it is intended for the case, when your type operates both managed and unmanaged resources.
Remove finalizer at all, and throw away GC.SuppressFinalize(this); from Dispose:
public void Dispose()
{
lock (disposableLock)
{
if (!isDisposed)
{
writer.Close();
stream.Close();
isDisposed = true;
}
}
}
Update.
Finalizers are intended for unmanaged resource cleanup.
You may consider finalizer as a place to close file handle, network socket, etc. This isn't a place for any application logic. In general, managed object during finalization resides in unusable state - there's no guarantee, that any of its IDisposables (like stream or writer in your sample) are not finalized already.
If you want to be sure, that particular log message is flushed into file, than write it and call writer.Flush(). Otherwise, if immediate flushing is undesirable for you, make sure, that you're calling dispose for the logger on application shutdown. Also note, that you can't protect from process termination, so don't be too paranoid with your logger.
Managed objects should not be cleaned up in the finalizer. The finalizer should only used for cleaning up unmanaged resources.
Rewrite your code as follows to dispose of managed resources when you are done with the streams.
public static void Main(string[] args)
{
using (var t = new DisposableClassWithStream())
{
t.WriteLine("Test");
}
}
And be sure to check out the Dispose Pattern article on MSDN
When a finalizer runs, most managed objects to which it holds references will meet one of the following criteria:
-1- Not care about cleanup, in which case the finalizer should do nothing with them.
-2- Be unable to perform cleanup in a thread-safe manner when invoked in the context of finalizer cleanup, in which case the finalizer should do nothing with them.
-3- Have already cleaned themselves up using their own finalizer, in which case the finalizer should do nothing with them.
-4- Have a Finalize method which hasn't run yet, but is scheduled to run at first opportunity, in which case the finalizer for the class holding the reference should do nothing with them.
Different objects will meet different criteria, and it may be difficult to know which of those criteria a particular object might meet, but for the vast majority of objects will meet any of the criteria, the required handling is the same: don't do anything with them in the finalizer.
There are a few rare scenarios involving things like weak events where finalizers may be able to do something useful with managed objects, but in nearly all such cases, the only classes which should have finalizers are those whose sole purpose is to managed the finalization cleanup of other intimately-related objects. If one doesn't understand all the wrinkles of finalization, including the differences between short and long weak references and how they interact with the finalizer, any finalizer one tries to write will likely do more harm than good.
I have a very simple logging mechanism in my application which periodically writes a line to a file (a logging library would be overkill for my needs) which looks something like this:
private string logfile = #"C:\whatever.log";
public void WriteLine(string line)
{
using(FileStream fs = File.Open(logfile, FileMode.Append))
{
// Log Stuff
}
}
So any time I call that method, a new FileStream is created and disposed after logging is finished. So I considered using an already instantiated object to prevent the continuous creation of new objects:
private string logfile = #"C:\whatever.log";
private FileStream myStream = File.Open(logfile, FileMode.Append);
public void WriteLine(string line)
{
using(myStream)
{
// Log Stuff
}
}
However, the MSDN reference discourages this (last example), due to scope issues.
What does one do in that case? Is the overhead in my first example negligible?
The using statement doesn't do anything else than calling the Dispose() method of the object.
So considering your second example, after the first call to the WriteLine(string) method the filestream is disposed. So any other call, after the first one, to this Method will result in an exception.
Using the File.AppendText() method like Chris had suggested in the comment would be a way to go. But keep in mind, that using this or any other File... method will also open a stream and close and dispose it afterwards.
It will just result in less code.
The second approach does also dispose the stream every time you call WriteLine since you are also using the using-statement. MSDN discourages from this approach because the variable myStream does still "exist" even if the object is disposed. So that is more error-prone.
If you often need to use this method you should cosider to use the using "outside" or use a try-catch-finally:
var myLogger = new MyLogger();
try
{
// here is your app which calls myLogger.WriteLine(...) often
}
catch(Exception ex)
{
// log it
}
finally
{
myLogger.Dispose(); // myLogger is your Log class, dispose should call myStream.Dispose();
}
The overhead might not be negligible, but that might be beside the point.
When you are using using, the creation, acquisition of resource and the disposing of the used resources is nicely scoped. You know where it starts, where it's used, and where it's finished.
If you go for the second scenario, you know where it starts (it's when the containing class is created), but after that, you have no platform-guaranteed way to control where it's used, and where (if at all) the resources are disposed.
You can do this yourself if this is critical code, and your containing class implements the IDisposable pattern properly, but this can be tricky and not for the faint of heart :)
However, you stated in the question "a logging library would be overkill for my needs", so I think you are fine with the minimal overhead. IMHO, you should be fine with one of the ready-made File methods, like File.AppendAllText:
public void WriteLine(string line)
{
//add an enter to the end
line += Environment.NewLine;
File.AppendAllText(logfile, line);
}
or File.AppendAllLines:
public void WriteLine(string line)
{
File.AppendAllLines(logfile, new []{line});
}
I've the following code
using(MemoryStream ms = new MemoryStream())
{
//code
return 0;
}
The dispose() method is called at the end of using statement braces } right? Since I return before the end of the using statement, will the MemoryStream object be disposed properly? What happens here?
Yes, Dispose will be called. It's called as soon as the execution leaves the scope of the using block, regardless of what means it took to leave the block, be it the end of execution of the block, a return statement, or an exception.
As #Noldorin correctly points out, using a using block in code gets compiled into try/finally, with Dispose being called in the finally block. For example the following code:
using(MemoryStream ms = new MemoryStream())
{
//code
return 0;
}
effectively becomes:
MemoryStream ms = new MemoryStream();
try
{
// code
return 0;
}
finally
{
ms.Dispose();
}
So, because finally is guaranteed to execute after the try block has finished execution, regardless of its execution path, Dispose is guaranteed to be called, no matter what.
For more information, see this MSDN article.
Addendum:
Just a little caveat to add: because Dispose is guaranteed to be called, it's almost always a good idea to ensure that Dispose never throws an exception when you implement IDisposable. Unfortunately, there are some classes in the core library that do throw in certain circumstances when Dispose is called -- I'm looking at you, WCF Service Reference / Client Proxy! -- and when that happens it can be very difficult to track down the original exception if Dispose was called during an exception stack unwind, since the original exception gets swallowed in favor of the new exception generated by the Dispose call. It can be maddeningly frustrating. Or is that frustratingly maddening? One of the two. Maybe both.
using statements behave exactly like try ... finally blocks, so will always execute on any code exit paths. However, I believe they are subject to the very few and rare situations in which finally blocks are not called. One example that I can remember is if the foreground thread exits while background threads are active: all threads apart from the GC are paused, meaning finally blocks are not run.
Obvious edit: they behave the same apart from the logic that lets them handle IDisposable objects, d'oh.
Bonus content: they can be stacked (where types differ):
using (SqlConnection conn = new SqlConnection("string"))
using (SqlCommand comm = new SqlCommand("", conn))
{
}
And also comma-delimited (where types are the same):
using (SqlCommand comm = new SqlCommand("", conn),
comm2 = new SqlCommand("", conn))
{
}
Your MemoryStream object will be disposed properly, no need to worry about that.
With the using statement, the object will be disposed of regardless of the completion path.
Further reading...
http://aspadvice.com/blogs/name/archive/2008/05/22/Return-Within-a-C_2300_-Using-Statement.aspx
http://csharpfeeds.com/post/8451/Return_Within_a_Csharp_Using_Statement.aspx
Take a look at your code in reflector after you compile it. You'll find that the compiler refactors the code to ensure that dispose is called on the stream.
If I have the following situation:
StreamWriter MySW = null;
try
{
Stream MyStream = new FileStream("asdf.txt");
MySW = new StreamWriter(MyStream);
MySW.Write("blah");
}
finally
{
if (MySW != null)
{
MySW.Flush();
MySW.Close();
MySW.Dispose();
}
}
Can I just call MySW.Dispose() and skip the Close even though it is provided? Are there any Stream implimentations that don't work as expected (Like CryptoStream)?
If not, then is the following just bad code:
using (StreamWriter MySW = new StreamWriter(MyStream))
{
MySW.Write("Blah");
}
Can I just call MySW.Dispose() and
skip the Close even though it is
provided?
Yes, that’s what it’s for.
Are there any Stream implementations
that don't work as expected (Like
CryptoStream)?
It is safe to assume that if an object implements IDisposable, it will dispose of itself properly.
If it doesn’t, then that would be a bug.
If not, then is the following just bad
code:
No, that code is the recommended way of dealing with objects that implement IDisposable.
More excellent information is in the accepted answer to Close and Dispose - which to call?
I used Reflector and found that System.IO.Stream.Dispose looks like this:
public void Dispose()
{
this.Close();
}
As Daniel Bruckner mentioned, Dispose() and Close() are effectively the same thing.
However Stream does NOT call Flush() when it is disposed/closed. FileStream (and I assume any other Stream with a caching mechanism) does call Flush() when disposed.
If you are extending Stream, or MemoryStream etc. you will need to implement a call to Flush() when disposed/closed if it is necessary.
All standard Streams (FileStream, CryptoStream) will attempt to flush when closed/disposed. I think you can rely on this for any Microsoft stream implementations.
As a result, Close/Dispose can throw an exception if the flush fails.
In fact IIRC there was a bug in the .NET 1.0 implementation of FileStream in that it would fail to release the file handle if the flush throws an exception. This was fixed in .NET 1.1 by adding a try/finally block to the Dispose(boolean) method.
Both StreamWriter.Dispose() and Stream.Dispose() release all resources held by the objects. Both of them close the underlying stream.
The source code of Stream.Dispose() (note that this is implementation details so don't rely on it):
public void Dispose()
{
this.Close();
}
StreamWriter.Dispose() (same as with Stream.Dispose()):
protected override void Dispose(bool disposing)
{
try
{
// Not relevant things
}
finally
{
if (this.Closable && (this.stream != null))
{
try
{
if (disposing)
{
this.stream.Close();
}
}
finally
{
// Not relevant things
}
}
}
}
Still, I usually implicitly close streams/streamwriters before disposing them - I think it looks cleaner.
For objects that need to be manually closed, every effort should be made to create the object in a using block.
//Cannot access 'stream'
using (FileStream stream = File.Open ("c:\\test.bin"))
{
//Do work on 'stream'
} // 'stream' is closed and disposed of even if there is an exception escaping this block
// Cannot access 'stream'
In this way one can never incorrectly access 'stream' out of the context of the using clause and the file is always closed.
I looked in the .net source for the Stream class, it had the following which would suggest that yes you can...
// Stream used to require that all cleanup logic went into Close(),
// which was thought up before we invented IDisposable. However, we
// need to follow the IDisposable pattern so that users can write
// sensible subclasses without needing to inspect all their base
// classes, and without worrying about version brittleness, from a
// base class switching to the Dispose pattern. We're moving
// Stream to the Dispose(bool) pattern - that's where all subclasses
// should put their cleanup starting in V2.
public virtual void Close()
{
Dispose(true);
GC.SuppressFinalize(this);
}
public void Dispose()
{
Close();
}
Stream.Close is implemented by a call to Stream.Dispose or vice versa - so the methods are equivalent. Stream.Close exists just because closing a stream sounds more natural than disposing a stream.
Besides you should try to avoid explicit calls to this methods and use the using statement instead in order to get correct exception handling for free.