Develop Reference

XmlWriter - disposable

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.

C# dispose unmanaged resource outside "using" and "try/finalize" block

I want a method to return an unmanaged resource and later in the program dispose of that resource. Does the following implementation do what I intend ?
class DocxManager
{
// DocX is the unmanaged resource
public Docx createDocX(string pathToFile)
{
return new DocX(pathToFile);
}
public void makeChangesAndSaveDocX(DocX wordDoc)
{
wordDoc.Save();
// IS THIS WAY THE UNMANAGED RESOURCE FREED CORRECTLY AND NO MEMORY LEAK ?
((IDisposable)wordDoc).Dispose();
}
}

First off all, you seem to be misunderstanding the concept of managed and unmanaged resources. wordDoc is not an unmanaged resource, its a managed resource which happens to either directly hold an unmanaged resource or acts as a wrapper around some other IDisposable object (you don't care wich of the two is true). Its important you are clear on this because otherwise you will not implement correctly the IDisposable pattern when you need to. Read this for a very instructive answer on the subject and a few chuckles courtesy of Eric Lippert.
Does the following implementation do what I intend ?
No, it does not and to make things worse, the contract of DocXManager is simply horrible (more on that later).
What hapens if wordDoc.Save() throws and exception because the file is being used by another process, or maybe you've run out of space in the hard drive, or you've lost connection, etc.?
If the thrown exception is not recoverable (it's not handled anywhere in your code or it is but you will terminate it ASAP) then it's not really an issue and the runtime will clean up everything behind you when the process is terminated. On the other hand, if the exception is handled (warn the user the file is in use, the directory is not available, etc.) and the process keeps running then you've just maybe leaked a resource.
How to avoid this? Use a try-finally block:
public void makeChangesAndSaveDocX(DocX wordDoc)
{
try
{
wordDoc.Save();
}
finally
{
((IDisposable)wordDoc).Dispose();
}
}
Now you are sure that Dispose() will be called in any recoverable scenario.
So, is this good enough? Well....not exactly. The problem here is that makeChangesAndSaveDocX's (that should be MakeChangesAndSaveDocX by the way) contract is unclear. Who is responsible of disposing wordDoc? MakeChangesAndSaveDocX or the caller? Why one or the other? How does the consumer know that he doesn't need to worry about wordDoc once he's called MakeChangesAndSaveDocX? Or how is he supposed to know he can't use wordDoc after calling the public method MakeChangesAndSaveDocX? Why does DocXManager assume that the consumer will not need to use wordDoc after calling MakeChangesAndSaveDocX? Yuck...this is a mess.
I'd recommend you reconsider your approach and do one of the following:
If the method signature MakeChangesAndSaveDocX(DocX wordDoc) makes sense (somebody else can own wordDoc), then do not dispose wordDoc in MakeChangesAndSaveDocX. Let the caller carry that burden, he should be responsible because the object belongs to him not to MakeChangesAndSaveDocX.
If on the other hand, it makes no sense that somebody else who is not DocXManager owns wordDoc then wordDoc should be part of the state of DocXManager and you should reconsider the implementation of DocXManager to something allong the following lines:
public class DocXManager: IDisposable
{
private readonly DocX docX;
private bool disposed;
public DocXManager(string filePath)
{
docX = new DocX(filePath);
}
public void MakeChangesAndSaveDocX()
{
if (disposed)
throw new ObjectDisposedException();
docX.Save();
}
public void FrobDocX(Frobber frobber)
{
if (disposed)
throw new ObjectDisposedException();
frobber.Frob(docX);
}
public void Dispose()
{
if (disposed)
return;
Dispose(true);
disposed = true;
GC.SupressFinalize(this);
}
public void Dispose(bool disposing)
{
if (disposing)
{
//we can sefely dispose managed resources here
((IDisposable)docX).Dispose();
}
//unsafe to clean up managed resources here, only clean up unmanaged resources if any.
}
~DocXManager()
{
Dispose(false);
}
}
Now you've got a clear contract; DocManagerX is responsible of disposing correctly of DocX and the consumer is responsible of disposing correctly of any instance of DocManagerX he might use. Once responsibilities are clear, its easier to reason about code correctness and who should do what.
You'd use the manager in the following way:
using (var manager = new DocXManager(path))
{
manager.FrobDoxX(frobber);
manager.MakeChangesAndSaveDocX();
} //manager is guaranteed to be disposed at this point (ignoring scenarios where finally blocks are not executed; StackOverflowException, OutOfMemoryException, etc.)

How to properly use the dispose method on a class

I'm working on a database and I need to write the database to file when the database class is destroyed along with every else when the form is closed
This is how I'm currently calling it:
class database: IDisposable
{
List<databaseEntry> dDatabase;
public database()
{
dDatabase = new List<databaseEntry>;
}
protected virtual void Dispose(bool disposing)
{
if (!this.disposed)
{
StreamWriter sw = new StreamWriter(path);
string toWrite;
foreach (databaseEntry dE in dDatabase)
{
toWrite = dE.rfid.ToString() + " " + dE.currentArea.ToString() + " " + dE.itemName;
sw.WriteLine(toWrite);
}
sw.Close();
disposed = true;
}
}//destructor for database (saves database to file)
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
The Dispose method isn't called when I close the windows form that this class is open in.
This is for an assignment and I'm not allowed to use SQL for this.

You do not have control over when the Dispose method is called by the garbage collector. If you want to dispose resources when the form is closed, you should manually call the Dispose method when the form is closed.
This is achieved by listening to the FormClosed event of the form.
So you would do something similar to this:
Create this method:
private void Form1_FormClosed(Object sender, FormClosedEventArgs e)
{
if(this.database != null)
this.database.Dispose();
}
Put this line in your form's constructor:
this.FormClosed += Form1_FormClosed;
Since you're implementing IDisposable, another solution would be to wrap your database object instantiation in a using statement - this would automatically call Dispose once the code in the using statement completes.
Syntax for your use case is as follows:
using(database db = new database())
{
//use your db object
} //db is disposed here

Edit:
Matthew Watson’s comment made me realize my answer is over simplified and could be confusing (thanks for the comment Matthew). Here’s (most of) the real story:
All objects have a constructor and a destructor. If these are not specified the default constructor and destructor are used.
Constructor:
ClassName()
Destructor:
~ClassName()
The Constructor is called when you use the new keyword. The Destructor can be called once there is no longer a reference to that specific instance of an object. Notice I said can be called, not is called. Meaning at this point the object is eligible for garbage collection and could be freed when the garage collector runs. I will not go into detail here but just know that the garage collector does not always collect all the unreachable objects for performance reasons. This is one of the key reasons you would want to the IDisposable interface. You can immediately clean up resources when you know they are no longer being used vs. waiting for the GC to clean up the resources. When working with streams or database connections it is a good idea to implement the IDisposable interface.
With that said here is the updated answer (replacing the previous "default Dispose()" references with the destructor reference).
Since this is an assignment I am not going to give you the answer per se, but I'll get you close. The destructor (~database) is normally called by the garage collector when an instance of a database object is no longer reachable. By implementing the IDisposable interface you have provided an alternative to allowing the garage collector to dispose of the object. You are telling the garage collector "I know when and how these resources need to be freed" vs. free these resources when there are no longer references to the objects. But that only applies if you call your Dispose method; more specifically the line:
Dispose(true);
GC.SuppressFinalize(this);
This means "Garage collector, do not call the destructor (~database()) method, I have already handled it".
What you should do in this method is close any streams (StreamWriter) you have open.
Usage: The IDisposable interface has a nice construct in C# the using statement. This is in effect a try catch block where the Dispose method is called in the finally block. StreamWriter also implements IDisposable so you should think about implementing the using statement in your database class. Example:
using(StreamWriter writer = new StreamWriter()){
//do stuff with writer here
}
This is the pattern you should use for your database class in your form. I will let you figure out how to code that up.

Suppressing premature finalization of stream in .NET

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.

Using the using-statement with an already instantiated object

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});
}