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});
}
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);
}
}
Narrowed question:
I need to implement reusable functionality that initializes a temporary file, runs some custom logic provided by the user, and then deletes the file. I can do this either through a static utility method that takes the user's logic as a delegate, or through a class that implements IDisposable to perform cleanup during Dispose. The two approaches would be used as follows:
// Delegate-parameterized method approach:
TempFile.Using(filePath =>
{
// use temporary file here
});
// IDisposable implementation approach:
using (var tempFile = new TempFile())
{
// use temporary file here
}
Which are the advantages and drawbacks to each approach? Which one should I use?
Original question:
I am developing some general-purpose functionalities that require a fixed “initialization – custom logic – clean-up” sequence. The clean-up must be performed as part of the same construct used for executing the operation; I don't want to confer the responsibility for calling a Cleanup or Close method to the user.
For example, I might want to provide functionality that automatically creates and deletes a temporary file. The simplest way to implement this is through a method that takes an Action<T> delegate:
public static void UsingTempFile(Action<string> action)
{
// initialization
string tempFilePath = Path.GetTempFileName();
try
{
action(tempFilePath);
}
finally
{
// clean-up
File.Delete(tempFilePath);
}
}
This could be used like so:
UsingTempFile(filePath =>
{
File.WriteAllText(filePath, "Hello world");
string text = File.ReadAllText(filePath);
});
However, this approach often requires me to implement four method overloads to support returned results and anonymous functions:
public static void UsingTempFile(Action<string> action) { /* ... */ }
public static TResult UsingTempFile<TResult>(Func<string, TResult> func) { /* ... */ }
public static async Task UsingTempFile(Func<string, Task> asyncAction) { /* ... */ }
public static async Task<TResult> UsingTempFile<TResult>(Func<string, Task<TResult>> asyncFunc) { /* ... */ }
The former three can implemented as simple wrappers that call the last overload. However, in a public API, they still need to be documented and unit-tested, substantially cluttering my codebase.
An alternative approach is to design an instantiatable class that implements IDisposable to represent the operation. This class would perform the initialization in its constructor, and the clean-up in its Dispose method. It could then be consumed like so:
using (var tempFile = new TempFile())
{
File.WriteAllText(tempFile.FilePath, "Hello world");
string text = File.ReadAllText(tempFile.FilePath);
}
The advantage of this approach is that the C# compiler automatically handles all my four cases ‒ the user can specify return and await keywords in the using block.
However, I often need to be able to throw exceptions from my clean-up logic – like IOException being thrown from File.Delete if the temporary file is still in use in the above example. The MSDN documentation for the Dispose Pattern states:
X AVOID throwing an exception from within Dispose(bool) except under critical situations where the containing process has been corrupted (leaks, inconsistent shared state, etc.).
Users expect that a call to Dispose will not raise an exception.
If Dispose could raise an exception, further finally-block cleanup logic will not execute. To work around this, the user would need to wrap every call to Dispose (within the finally block!) in a try block, which leads to very complex cleanup handlers.
A related question makes a stronger case about the downsides to throwing exceptions from finally blocks:
A first exception is thrown
A finally block is executed as a result of the first exception
The finally block calls a Dispose() method
The Dispose() method throws a second exception
[…] You lose information because .NET unceremoneously replaces the first exception with the second one. A catch block somewhere up the call stack will therefore never see the first exception. However, one is usually more interested in the first exception because that normally gives better clues as to why things started to go wrong.
This argument has merit – if the user's custom logic throws an exception, I would not want it to be hidden (and lost) by any exceptions thrown in the clean-up. There are solutions to this, such as Marc Gravell's wrapper for IDisposable objects (which swallows all Dispose exceptions), but this imparts more responsibility on the user. On the other hand, the .NET Framework Class Library itself seems to disregard this rule ‒ FileStream can throw exceptions from its Dispose method if it fails to flush the buffer's contents to disk.
Which of the two approaches (delegate-parameterized methods vs IDisposable) would be recommendable for these general-purpose implementations? Are there any implications apart from the ones mentioned above?
Update: This is an example of how delegate-parameterized methods can prevent main-logic exceptions from being swallowed:
public static void UsingTempFile(Action<string> action)
{
// initialization
string tempFilePath = Path.GetTempFileName();
bool isSuccess = false;
try
{
// main logic
action(tempFilePath);
isSuccess = true;
}
finally
{
try
{
// clean-up
File.Delete(tempFilePath);
}
catch
{
// only propagate exceptions from clean-up if there were
// no unhandled exceptions from the main logic
if (isSuccess)
throw;
}
}
}
I want to encapsulate the process of unzipping a zip file, making the files available for use, and then automatically cleaning them up when they are no longer needed. I did this with a class that implements the IDisposable interface, so that I can instantiate it with "using" and the files will be cleaned up when going out of scope, eliminating the need to specifically delete the files. The class, TempUnzip, can therefore be used thus:
static void AccessZipFileContents(string zipFilePath)
{
using (var temp = new TempUnzip(zipFilePath)
{
var tempPath = temp.TempPath;
if (tempPath != null)
{
// read/use the files in tempPath
}
} // files automatically get deleted when it goes out of scope! Woohoo!
}
Here is the implementation of the TempUnzip class:
using System.IO;
using System.IO.Compression;
public class TempUnzip : IDisposable
{
public TempUnzip(string zipFilePath)
{
try
{
var tempFolderName = Path.GetRandomFileName();
var tempFolder = Path.GetTempPath();
var tempPath = Path.Combine(tempFolder, tempFolderName);
Directory.CreateDirectory(tempPath);
ZipFile.ExtractToDirectory(zipFilePath, tempPath);
TempPath = tempPath;
}
catch (Exception) { TempPath = null; }
}
public readonly string TempPath;
public void Dispose()
{
try
{
if (TempPath != null)
Directory.Delete(TempPath);
}
catch (Exception) { }
}
}
Is this a valid use of IDisposable?
If so, do I need to implement the full standard IDisposable pattern?
If not, is there a better way to encapsulate the creation and destruction of files in such a way that they're tied to the lifetime of an object, or should I avoid this altogether?
Is this a valid use of IDisposable?
From the documentation:
Provides a mechanism for releasing unmanaged resources.
Files on local disk are certainly unmanaged resources. Thus, this use fits with the stated purpose of IDisposable.
If so, do I need to implement the full standard IDisposable pattern?
You can. The usual cautions about finalizers need to be considered, but you already linked to those. It certainly won't hurt.
If not, is there a better way to encapsulate the creation and destruction of files in such a way that they're tied to the lifetime of an object, or should I avoid this altogether?
I also like a functional approach for this kind of problem. That would make your example look something like this:
static void AccessZipFileContents(string zipFilePath)
{
ZipManager.RunWithZipExtracted(zipFilePath, (string tempPath) =>
{
if (tempPath != null)
{
// read/use the files in tempPath
}
} // files automatically get deleted when it goes out of scope! Woohoo!
}
//from ZipManager.cs...
static void RunWithZipExtracted(string zipLocation, Action<string> toRun)
{
var tempPath = CalculateTempPath();
try
{
ExtractZip(zipLocation, tempPath);
toRun(tempPath);
}
finally
{
DeleteFolder(tempPath);
}
} //private methods left as exercise for the reader
A pattern like that avoids the problem of "what if they don't call Dispose?" entirely.
This is a situation where having a finalizer is probably a good idea, but the MS pattern is predicated on the idea of public objects having finalizers, which is almost always a bad idea. Instead, resources that will require finalization-based cleanup should be encapsulated in privately-held objects whose references are never exposed to the outside world. Because the inner objects are private, there's no need for them to use the IDisposable pattern--instead they may be designed in whatever fashion best fits the requirement.
Since trying to close a file handle more than once may have catastrophic consequences, and it's possible (though rare) for a finalizer to execute while other code is using an object (or even performing Dispose upon it!), writing a robust class can be difficult. A nasty issue that arises is that file accesses can block but finalizer actions shouldn't. One could work around that by creating a thread whose purpose was to wait until the file is supposed to be closed and deleted, and which would then close and delete the file. Even if the attempt to delete the file gets blocked, other finalizer actions could continue apace. Unfortunately, eagerly creating a thread for the purpose of allowing safe finalizer-based cleanup is apt to be a waste of resources, but thread creation seems like an excessively-heavyweight task to be performing within a finalizer. I don't know what the best solution is.
In this instance I would say that it is a good example of IDisposable, if Dispose isn't called straight away that you've finished using it then it's not the end of the world and quickly calling this for the same zip file wouldn't cause an exception as you're using unique temp folders each time; which again you're not leaving file pointers left open on the files, etc. The only issue I could see is that if disk space was really tight you might want to expose the delete folder as a method to allow the option of that being called directly and then Dispose is just used to clean up after for all cases.
Note: in this example you probably want to call Directory.Delete(TempPath, true); because the method you're calling will throw an IOException if the folder isn't empty (something your catch will hide) - see https://msdn.microsoft.com/en-us/library/62t64db3%28v=vs.110%29.aspx
public class SimpleLogger
{
static readonly string logFile = ConfigurationManager.AppSettings["LogFile"];
static StreamWriter GetStream()
{
return File.Exists(logFile) ?
File.AppendText(logFile) : File.CreateText(logFile);
}
public static void Write(string msg)
{
using (var sw = GetStream())
{
sw.Write(msg);
}
}
}
The above code fails in use as it doesn't appear to be closing/disposing of the stream correctly. Subsequent writes give a 'file in use' IOException.
If the class is modified to use non-static methods, it appears to work correctly.
I don't understand why there would be any behavioural difference?
The disposal is fine; GetStream delivers an open writer; Write closes/disposes it - sorted. if I had to guess, though, the issue is concurrent use - i.e. multiple threads (in particular in a web application) accessing the file at the same time. If that is the case, options:
make Write (and any other access to the file) synchronized, so only one caller can possibly try to have the file open at once
use a pre-canned logging framework that will already handle this scenario (common approaches here include synchronization, but also: buffering the data locally and then pushing the data down periodically - avoids opening the file over and over and over and over)
In particular; your only static state is the file path itself. There will therefore be no significant difference between using this as a static versus instance method.
As a side-note, File.AppendAllText may be useful here, but does not avoid the issue of concurrency.
I don't think changing from a static to an instance would fix the problem since they're all ultimately contending over a static resource (the file). This answer might help you. Perhaps if you left both methods static and declared a static synchronisation object for calling threads to lock with (since the resource is static itself) would help?, e.g.:
private static object _objectLock = new object();
for synchronising access to the file from multiple threads, hence:
public static void Write(string msg)
{
lock(_objectLock)
{
using (var sw = GetStream())
{
sw.Write(msg);
}
}
}
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.