I know this might seem silly, but why does the following code only work if I Close() the file? If I don't close the file, the entire stream is not written.
Steps:
Run this code on form load.
Close form using mouse once it is displayed.
Program terminates.
Shouldn't the file object be flushed or closed automatically when it goes out of scope? I'm new to C#, but I'm used to adding calls to Close() in C++ destructors.
// Notes: complete output is about 87KB. Without Close(), it's missing about 2KB at the end.
// Convert to png and then convert that into a base64 encoded string.
string b64img = ImageToBase64(img, ImageFormat.Png);
// Save the base64 image to a text file for more testing and external validation.
StreamWriter outfile = new StreamWriter("../../file.txt");
outfile.Write(b64img);
// If we don't close the file, windows will not write it all to disk. No idea why
// that would be.
outfile.Close();
C# doesn't have automatic deterministic cleanup. You have to be sure to call the cleanup function if you want to control when it runs. The using block is the most common way of doing this.
If you don't put in the cleanup call yourself, then cleanup will happen when the garbage collector decides the memory is needed for something else, which could be a very long time later.
using (StreamWriter outfile = new StreamWriter("../../file.txt")) {
outfile.Write(b64img);
} // everything is ok, the using block calls Dispose which closes the file
EDIT: As Harvey points out, while the cleanup will be attempted when the object gets collected, this isn't any guarantee of success. To avoid issues with circular references, the runtime makes no attempt to finalize objects in the "right" order, so the FileStream can actually already be dead by the time the StreamWriter finalizer runs and tries to flush buffered output.
If you deal in objects that need cleanup, do it explicitly, either with using (for locally-scoped usage) or by calling IDisposable.Dispose (for long-lived objects such as referents of class members).
Because Write() is buffered and the buffer is explicitly flushed by Close().
Streams are objects that "manage" or "handle" non-garbage collected resources. They (Streams) therefore implement the IDisposable interface that, when used with 'using' will make sure the non-garbage collected resources are clean up. try this:
using ( StreamWriter outfile = new StreamWriter("../../file.txt") )
{
outfile.Write(b64img);
}
Without the #Close, you can not be sure when the underlying file handle will be properly closed. Sometimes, this can be at app shutdown.
Because you are using a streamwriter and it doesn't flush the buffer until you Close() the writer. You can specify that you want the writer to flush everytime you call write by setting the AutoFlush property of the streamwriter to true.
Check out the docs. http://msdn.microsoft.com/en-us/library/system.io.streamwriter.aspx
If you want to write to a file without "closing", I would use:
System.IO.File
Operating system cache write to block devices to enable the OS to have better performance. You force a write by flushing the buffer after a write of setting the streamwriter to autoflush.
Because the C# designers were cloning Java and not C++ despite the name.
In my opinion they really missed the boat. C++ style destruction on scope exit would have been so much better.
It wouldn't even have to release the memory to be better, just automatically run the finalizer or the IDisposable method.
Related
I have a readonly FileStream which is a method local variable:
public void SomeMethod()
{
var fileStream = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite));
... //some stream operations
}
Should I call Dispose (explicitly or by "using") at the end of the method? What will it change?
It doesn't matter what it does, IDisposable is implemented by FileStream, and so you need to call Dispose implicitly or explicitly when you've finished using it. It is part of the contract of using the class in your code.
I think it's best to wrap that in a using statement. You also need exception handling if you really want that code to be robust. It will work as it is now, it's just bad practice.
It is a must to dispose any instance implementing IDisposable and a good practice to dispose it trough using statement.
Don't think that analyzing every case in particular would help.
If you fail to dispose, the FileStream won't be closed until the GC kicks in (non-deterministically).
And until this happens, you will be holding an open file handle, preventing some types of access to the file (e.g. writing, deletion).
Note that specifying FileShare.ReadWrite won't necessarily help - if another writer attempts to open the file with FileShare.None (e.g. by calling File.OpenWrite), he won't be able to do so until you close the file.
So, yes, do close the file, with a using statement.
If an object can be disposed, you should dispose it as early as you don't need it anymore. From FileStream Class topic:
If a process terminates with part of a file locked or closes a file
that has outstanding locks, the behavior is undefined.
As everyone suggested: dispose every IDisposable, preferably with using.
Now for files there could be special case when you really want to block everyone else from accessing/modifying the file. In this case you'd still dispose file at some point, but this "some point" can be significantly later in the code/application lifetime.
I am redirecting the output of a process into a streamreader which I read later. My problem is I am using multiple threads which SHOULD have separate instances of this stream. When I go to read this stream in, the threading fudges and starts executing oddly. Is there such a thing as making a thread-safe stream? EDIT: I put locks on the ReadToEnd on the streamreader, and the line where I did: reader = proc.StandardOutput;
There's a SyncrhonizedStream built into the framework, they just don't expose the class for you to look at/subclass etc, but you can turn any stream into a SynchronizedStream using
var syncStream = Stream.Synchronized(inStream);
You should pass the syncStream object around to each thread that needs it, and make sure you never try to access inStream elsewhere in code.
The SynchronizedStream just implements a monitor on all read/write operation to ensure that a thread has mutually exclusive access to the stream.
Edit:
Appears they also implements a SynchronizedReader/SynchronizedWriter in the framework too.
var reader = TextReader.Synchronized(process.StandardOutput);
A 'thread-safe' stream doesn't really mean anything. If the stream is somehow shared you must define on what level synchronization/sharing can take place. This in terms of the data packets (messages or records) and their allowed/required ordering.
So, after discovering that the Bitmap class expects the original stream to stay open for the life of the image or bitmap, I decided to find out if the Bitmap class actually closes the stream when it is disposed.
Looking at the source code, the Bitmap and Image classes create a GPStream instance to wrap the stream, but do not store a reference to either the GPStream or the Stream instance.
num = SafeNativeMethods.Gdip.GdipLoadImageFromStreamICM(new GPStream(stream), out zero);
Now, the GPStream class (internal), does not implement a Release or Dispose method - nothing that would allow GDI to close or dispose of the stream. And since the Image/Bitmap class doesn't keep a reference to the GPStream instance, it seems that there is absolutely no way for either GDI, Drawing.Bitmap, or Drawing.Stream to close the stream properly.
I could subclass Bitmap to fix this, but, oh wait, it's sealed.
Please tell me I'm wrong, and that MS didn't just make it impossible to write code that doesn't leak resources with their API.
Keep in mind (a), Bitmap has no managed reference to the stream, meaning GC will collect it while it is still in use, and (b) .NET APIs take Bitmap/Image references and aren't deterministic about when they're done with them.
Since you supply the stream in this example, I'd imagine you are responsible for disposing it.
It is a good practice to have the method that opens a stream, close it as well. That way it is easier to keep track of leaks. It would be quite strange to have an other object closing the stream that you opened.
Because bitmap can't guarantee in which order the destructor is called it will not close the stream because it may already have been closed with its own destructor during garbage collection. Jeffrey Richter's CLR via C# has a chapter on memory management that explains with much more clarity than I can.
An easy workaround to the problem is:
var image = new Bitmap(stream);
image.Tag = stream;
Now the stream is referenced by the image and won't be garbage collected before the image is. If your stream happens to be a MemoryStream, it doesn't need to be disposed (its Dispose is a no-op). If not, you can dispose it when you dispose the image, or just let the GC do it when it gets around to it.
This seems like a fairly straightforward question, but I couldn't find this particular use-case after some searching around.
Suppose I have a simple method that, say, determines if a file is opened by some process. I can do this (not 100% correctly, but fairly well) with this:
public bool IsOpen(string fileName)
{
try
{
File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None);
}
catch
{
// if an exception is thrown, the file must be opened by some other process
return true;
}
}
(obviously this isn't the best or even correct way to determine this - File.Open throws a number of different exceptions, all with different meanings, but it works for this example)
Now the File.Open call returns a FileStream, and FileStream implements IDisposable. Normally we'd want to wrap the usage of any FileStream instantiations in a using block to make sure they're disposed of properly. But what happens in the case where we don't actually assign the return value to anything? Is it still necessary to dispose of the FileStream, like so:
try
{
using (File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None));
{ /* nop */ }
}
catch
{
return true;
}
Should I create a FileStream instance and dispose of that?
try
{
using (FileStream fs = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None));
}
...
Or are these totally unnecessary? Can we simply call File.Open and not assign it to anything (first code example), and let the GC dispose of it right away?
Yes, you should definitely dispose of the FileStream. Otherwise the stream will remain open and the file won't be usable until a finalizer happens to clean it up.
The important thing here is ownership: for File.Open, the caller is assumed to "own" the stream returned to it - and if you own something which implements IDisposable, it's your responsibility to dispose of it.
Compare this with the situation of Image.FromStream: in that case, you pass in a stream and the Image then assumes that it owns that stream. You mustn't close the stream yourself, in that case - you have to dispose of the image when you're done, and it will dispose of the stream.
Calling a static method which returns something disposable almost always assumes that the caller takes ownership of the resource. Ditto constructors (which are effectively static methods.)
The irony in this case is that if you don't dispose of the stream returned by File.Open, you'll have found that the file is usable - at the same time as making it unusable until some indeterminate time.
If a method returns an IDisposable, I personally would always put it in a using block. Even if I don't assign the return value to anything.
Even if you don't assign it to a variable, the disposable object is still created. Dispose is not going to be called automatically. The only difference will be that the returned object will become immediately eligible for garbage collection, because there are no (strong) references to it.
The garbage collector does not call Dispose automatically when it reclaims an object. However, most IDisposable types provide a finalizer (which will be called just before the GC reclaims an object) that invokes Dispose as a fallback strategy (safety net) — study the IDisposable pattern to see how this is done:
~SomeClass // <-- the finalizer method will usually call Dispose;
{ // but you have no control over when it will be called!
Dispose(false);
}
Remember that you don't know when the garbage collector will run (because it's non-deterministic). Therefore, you also don't know when the finalizer method will be called. And because of that -- if you haven't called Dispose explicitly (either yourself, or with a using block) -- you don't know when it will be called by the finalizer.
That's the advantage of calling Dispose explicitly: You can free resources -- or at least allow the GC to free managed resources -- as soon as you're done with them, instead of holding on to resources until the finalizer gets called sometime in the future.
Yes, you don't want to leave the FileStream opened. For one, you won't even be able to open the file yourself after that. Calling Close() is good enough, but using using is probably the preferred pattern.
There's a much bigger problem with your code however. It cannot possibly work reliably on Windows. A typical scenario:
The File.Open() call succeeds. You close it
Your thread gets pre-empted by the Windows scheduler
Another thread in another process gets a chance to run, it opens the file
Your thread regains the CPU and continues after the File.Open() call
You open the file, trusting that it will work since IsOpen() returned false.
Kaboom.
Never write code like this, failure is extremely hard to diagnose. Only ever open a file when you are ready to start reading or writing to it. And don't close it until you are done.
Extra bonus: it is now obvious that you want to use a using statement.
When you call any method that returns something, an instance of that something is being created. Just because you're not actually capturing it, doesn't make it any less "there". Therefore, in the case of an IDisposable object, the object is being created by the method you're calling in spite of the fact that you're doing nothing with it. So yes, you still need to dispose of it somehow. The first approach with the using statement seems like it should work.
I know there are a number of threads on here about how to use the using statement and calling the Dispose() method. I have read the majority of these threads.
If I call Dispose(), does it call Close()?
If I want to use an object (say SqlDataReader), but then use it again in another code block, should I not call Dispose()? Which also means to omit the using statement.
Also, to clarify, if a FileStream is wrapping a StreamWriter and I call dispose on the FileStream, this will call Flush(), Close() and Dispose() (depending on whether Dispose() calls Close()) on the StreamWriter, right? Likewise, if I call Close on the FileStream, this will only call Flush() and Close() on the FileStream.
Is checking IL a good way to answer these questions about what is happening under the hood?
"If I call Dispose(), does it call Close()?"
In theory, it should. The BCL classes all do this, but it is up to the library author to correctly handle this. If the library you are using is done correctly, Dispose() should also Close() [and Close() will Dispose() - the calls should be interchangable].
"If I want to use an object (say SqlDataReader), but then use it again in another code block, should I not call Dispose()? Which also means to omit the using statement."
Correct. If you use the using statement, it will always call Dispose(). This will close the data reader before your other block can use it.
"Also, to clarify, if a FileStream is wrapping a StreamWriter and I call dispose on the FileStream, this will call Flush(), Close() and Dispose() (depending on whether Dispose() calls Close()) on the StreamWriter, right? Likewise, if I call Close on the FileStream, this will only call Flush() and Close() on the FileStream."
If you are wrapping a FileStream around a StreamWriter, I highly recommend treating them consistently. Use a single using statement with both members, so they are both disposed of at the end of the block. This is the safest, most clean approach.
"Is checking IL a good way to answer these questions about what is happening under the hood?"
It is a way - although a more difficult way. Read up on MSDN about using and streams, and the documentation will explain it in simpler terms than trying to parse the IL. The IL will tell you EXACTLY what happens, though, if you are curious.
If I call Dispose(), does it call Close()?
Close() and Dispose() do the same if implemented properly; it is just a naming thing. It sounds more plain to close a file than to dispose it. See Implementing Finalize and Dispose to Clean Up Unmanaged Resources esspecialy 'Customizing a Dispose Method Name'.
If I want to use an object (say SqlDataReader), but then use it again in another code#
block, should I not call Dispose()? Which also means to omit the using statement.
Yes, because the object gets disposed on exiting the using block.
Also, to clarify, if a FileStream is wrapping a StreamWriter and I call dispose on the > FileStream, this will call Flush(), Close() and Dispose() (depending on whether Dispos()
calls Close()) on the StreamWriter, right? Likewise, if I call Close on the FileStream, > this will only call Flush() and Close() on the FileStream.
It is the other way; a StreamWriter is based on an underlying stream an closing the StreamWriter closes the underlying stream that may be a FileStream; see the MSDN for reference. Hence a single using statement for the StreamWriter is sufficent.
If I call Dispose(), does it call Close()?
Calling Dispose should take any required actions to dispose of the resource, which should be similar, if not identical to, calling Close. This, however, is an implementation detail and not necessarily guaranteed (though we can expect that the BCL follows this guideline).
If I want to use an object (say SqlDataReader), but then use it again in another code block, should I not call Dispose()? Which also means to omit the using statement.
If you want to use the object again, you definitely should not dispose it. However, you should typically use two separate connections if you're going to the database two separate times. It's generally not a good idea to keep an IDataReader around an longer than is needed to grab your needed data.
Also, to clarify, if a FileStream is wrapping a StreamWriter and I call dispose on the FileStream, this will call Flush(), Close() and Dispose() (depending on whether Dispose() calls Close()) on the StreamWriter, right? Likewise, if I call Close on the FileStream, this will only call Flush() and Close() on the FileStream.
Disposing an object that wraps another disposable object should call Dispose on the interior object. Calling Close on a FileStream will call its Dispose method under the good, so it will also act on both streams.
Is checking IL a good way to answer these questions about what is happening under the hood?
Checking IL will definitely answer most of these questions definitively. As #Rich says, you can also just try debugging your own Dispose implementations. There is also, of course, MSDN documentation to start with before you try to figure it out yourself, and Reflector if you don't want to muck around in IL.
If I call Dispose(), does it call Close()?
Not necessarily. I sometimes use Reflector to check what actually happens in Close and Dispose.
If I want to use (...) it again in another code block, should I not call Dispose()?
Correct. Call Dispose when you're done. But that doesn't mean you always want to keep your object alive for a long time - you can sometimes benefit from creating multiple instances (multiple using constructs) -- e.g. You might wan to close a connection as soon as possible, but then create a new one again when you need it.
As you said, there are lot of resources on that, but I will include the MSDN link for some guidelines: Implementing Finalize and Dispose to Clean Up Unmanaged Resources.
An easier way to debug this than going through the IL code would be to derive from your IDisposable, override the necessary methods doing nothing but calling base.[Method Name](), and set a breakpoint in each one. Then if you wrap your derived class in a using block, you'll see the lifecycle of these calls.
No, IDisposable does not require Close(), but the object implementing IDispose may be nice enough to include it in the Dispose() method.
You should dispose it as soon as you have the piece of data you are getting from the DB. Don't leave a reader open any longer than you need. If you are doing any real work with the data, use a dataAdapter/dataset instead of reader.
No Idea. Check the Generated IL
I try to move the using clause higher up, since I prefer to use that syntax. Then call the other blocks using that resource from inside that using block.