Given a method
public static string[] Foo(System.IO.Stream stream)
{
XmlTextWriter xmlWriter = new XmlTextWriter(stream, System.Text.Encoding.ASCII);
xmlWriter.WriteStartDocument();
xmlWriter.WriteStartElement("Element");
xmlWriter.WriteEndElement();
xmlWriter.WriteEndDocument();
xmlWriter.Flush();
return new string[3]{"1", "2", "3"};
}
and a calling code
using(MemoryStream memStream = new MemoryStream())
{
string[] playerIDs = PlayerCommand.Foo(stream);
// do some stuff with stream and playerIDs
}
When the control passes out of the using directive, is there a problem that xmlWriter was not explicitly closed in Foo (which would make the stream useless)?
Also, is passing a Stream between methods unsavory, is there a better way to pass the results of XMLWriting back to the calling code (which will zip it up and file it away)?
Thanks in advance,
Greg
No problem here. A StreamWriter's Dispose() method basically first calls Flush() and then disposes the underlying stream, so as long as you have the Flush() covered yourself (which you do above) it's OK to leave it hanging as long as the stream's alive.
The other answers suggest disposing the Writer itself, but that's not really an option here, because it will dispose the underlying stream as well.
That said, if you're just putting an XML document in the stream and then using it later, I'd rather pass the XML document itself around instead of keeping track of the stream. But perhaps your code isn't as simple as all that.
Related
This question already has answers here:
Cannot write to file after reading
(5 answers)
Closed 3 years ago.
This question stems from this other thread: How to lock a file with C#?
Basically let's say you want to lock a JSON file, read it, then write to it afterward, and finally unlock it. You can lock the file using the answers from the other question.
However I'm having trouble where this is allowing me to read the file, but not write to it afterward without first unlocking the file. That is, the recommended method, which seems to be fairly well-respected, is locking the same thread out of its own resource.
Example:
using (var fs = new FileStream(GetJsonPath(), FileMode.Open, FileAccess.ReadWrite,
FileShare.None))
{
SomeDtoType dto;
using (StreamReader reader = new StreamReader(fs))
{
dto = ((SomeDtoType)(new JsonSerializer()).Deserialize(reader,
typeof(SomeDtoType)));
}
// Make changes to the DTO.....
using (StreamWriter writer = new StreamWriter(fs))
{
new JsonSerializer().Serialize(writer, dto);
}
}
The using line that creates the StreamWriter throws the following exception:
Stream was not writable.
Now one thing that comes to mind is the value of FileShare.None. The problem here is that that particular enum is evidently setting lock permissions for more than just external processes.
How can you lock external threads/processes out of changing/deleting the file, yet allow your own to make these two subsequent read/write accesses?
EDIT:
Evidently moving everything into the using block for the StreamReader, then setting fs.Position to 0 between the read and the write kind of fixes the issue. The fs.Position part is fine, but having to move the write logic into the using block for the StreamReader, just so they can both use the same FileStream lock, seems a tad odd...
StreamReader closes the stream if you don't use ctor overload and instruct it don't do that:
using (StreamReader reader = new StreamReader(fs, Encoding.UTF8, true, 4096, leaveOpen:true))
There is no finalizer in StreamReader, you can move it out using block and keep as undisposed, however I'd recommend explicitly control the lifetime and behavior.
Another issue is that you'll append to the file. If you want to override a content you need to reset it before you write to it:
fs.SetLength(0);
I came across the following example code in the MSDN documentation, demonstrating the use of the System.IO.StreamReader class to read UTF-8 text from a System.IO.FileStream object. The two nested using statements struck me as redundant - surely calling Dispose() on one of the objects does the trick, and properly releases the file handle? (Source: http://msdn.microsoft.com/en-us/library/yhfzs7at.aspx)
using (FileStream fs = new FileStream(path, FileMode.Open))
{
using (StreamReader sr = new StreamReader(fs))
{
while (sr.Peek() >= 0)
{
Console.WriteLine(sr.ReadLine());
}
}
}
Would it not be simpler, and equally correct, to rewrite that code in the following way?
using (FileStream fs = new FileStream(path, FileMode.Open))
{
StreamReader sr = new StreamReader(fs);
while (sr.Peek() >= 0)
{
Console.WriteLine(sr.ReadLine());
}
}
According to the documentation, The StreamReader object calls Dispose() on the provided Stream object when StreamReader.Dispose is called. This means using the StreamReader guarantees disposal of the underlying Stream. It does not hold vice versa: disposing only the Stream does not suffice - the StreamReader may be allocating other native resources. So the second sample is not correct.
(using only the StreamReader does not cover the case that the StreamReader constructor can throw. To cover this case, both using's would be needed. Since it only throws for non-readable or null streams, this may not be relevant though.)
In general, you should always dispose on every disposable object. It is part of the IDisposable contract that it does not hurt to dispose an object multiple times, and the overhead of doing so is low.
The second sample is simply wrong on principle.
It won't leak anything but that relies on the knowledge that a StreamReader has no resources of its own and does not actually needs Disposing even though it is IDisposable.
A single using(){} around the StreamReader would have been more or less correct here, based on the documented (and criticized) feature that the Reader will close its Stream.
The best practice here is to use 2 using statments. Note that they are very cheap, and you simply want code that's consistent.
I ran into something interesting when using a StreamWriter with a FileStream to append text to an existing file in .NET 4.5 (haven't tried any older frameworks). I tried two ways, one worked and one didn't. I'm wondering what the difference between the two is.
Both methods contained the following code at the top
if (!File.Exists(filepath))
using (File.Create(filepath));
I have the creation in a using statement because I've found through personal experience that it's the best way to ensure that the application fully closes the file.
Non-Working Method:
using (FileStream f = new FileStream(filepath, FileMode.Append,FileAccess.Write))
(new StreamWriter(f)).WriteLine("somestring");
With this method nothing ends up being appended to the file.
Working Method:
using (FileStream f = new FileStream(filepath, FileMode.Append,FileAccess.Write))
using (StreamWriter s = new StreamWriter(f))
s.WriteLine("somestring");
I've done a bit of Googling, without quite knowing what to search for, and haven't found anything informative. So, why is it that the anonymous StreamWriter fails where the (non-anonymous? named?) StreamWriter works?
It sounds like you did not flush the stream.
http://msdn.microsoft.com/en-us/library/system.io.stream.flush.aspx
It looks like StreamWriter writes to a buffer before writing to the final destination, in this case, the file. You may also be able to set the AutoFlush property and not have to explicitly flush it.
http://msdn.microsoft.com/en-us/library/system.io.streamwriter.autoflush.aspx
To answer your question, when you use the "using" block, it calls dispose on the StreamWriter, which must in turn call Flush.
The difference between the two code snippets is the use of using. The using statement disposes the object at the end of the block.
A StreamWriter buffers data before writing it to the underlying stream. Disposing the StreamWriter flushes the buffer. If you don't flush the buffer, nothing gets written.
From MSDN:
You must call Close to ensure that all data is correctly written out to the underlying stream.
See also: When should I use “using” blocks in C#?
I am currently working on a C# project that uses a FileStream to open a file and passes the information to a StreamReader for it to be processed.
What I want to be able to do is I have a method which passes the stream to another method, that does the processing and once that method finishes, the original method calls another method passing it the same stream. However, when the first stream process completes when the second method is called it displays an exception saying that the thread is not readable.
When I look at debugging the stream, when I look at the details about the stream it says that it cannot be read, seek or write, and the length parameter of the stream says threw an exception of type System.ObjectDisposedException.
The question is, how can I keep the stream readable after the first method has completed so that the same stream can be processed in the second stream.
Thanks for any help you can provide.
if your streamReader is part of "using" statement, it disposes the file stream in the end of the statement block.
using (StreamReader reader = new StreamReader(fileStream))
{
..
}
Simple solution is not to dispose the reader explicitly, leaving it to GC
[More thoughts]
If most of the methods are accessing file stream through TextReader interface, you can pass reader thus avoiding the problem with the ownership.
If I understood you correctly, your stream is getting closed too fast. Based on what you say, it might be because you are Closing or Disposing StreamReader which, according to documentation, will close underlying stream.
Try not closing StreamReader (just ignore it, after it's not needed).
For example, if your code looks like this:
void P()
{
var stream = new FileStream();
P1(stream);
P2(stream);
}
void P1(FileStream stream)
{
using (var reader = new StreamReader(stream))
{
......
} //Here you would have disposed StreamReader and close FileStream
}
void P2(FileStream stream) // Stream is already closed
{
}
You have closed your stream in 1st method. You will have the same problem if you call:
reader.Dispose();
reader.Close();
stream.Dispose();
stream.Close();
using (stream);
So make sure aren't doing any of those things.
Btw: in C#5 I have heard, that Readers/Writers will be parametrized, if you want then to close underlying stream when they are closed (just like CryptoStream have right now)
I have a class that essentially wraps a Stream for reading/writing, but that stream is expected to be managed by the consumer of that class. For ease of use, I use StreamReader and StreamWriter classes to perform I/O operations on the stream. Normally I'd wrap the reader and writer in using blocks, but I want to avoid closing the reader and writer because doing so also closes the underlying stream and I have to keep it open.
Is it safe in terms of memory/resource management to not close a StreamReader/StreamWriter if I expect the underlying Stream to be managed by the caller? Will the reader and writer be garbage collected when the stream is explicitly closed elsewhere?
public class Wrapper
{
private Stream _underlyingStream;
public Wrapper(Stream underlyingStream)
{
_underlyingStream = underlyingStream;
}
public string GetValue()
{
_underlyingStream.Seek(0, SeekOrigin.Begin);
var reader = new StreamReader(_underlyingStream);
return reader.ReadToEnd(); // we're done, but the stream is not ours to close
}
}
If nobody closes the streams then ultimately the finalizer will be called which should call dispose and close them upon GC. But that's quite a crap-shoot resource-wise because it leaves whatever possibly-expensive resources allocated until GC. It could get worse the longer your object lives, especially if it survives collections to be promoted to gen 1 or even 2.
It sure would be nice if you could present something to your caller that isolates this. Perhaps you can cache something from the stream so you can close it while still serving the content to your caller?
EDIT after your edit: Now that I see your caller PASSES you a stream to operate on, my answer has to be different! It's very clear that your caller should be managing the stream's lifetime. I had the impression at first that your class created a stream and hoped the caller managed it.
The easiest way to solve this is to wrap the stream in your own class that derives from System.IO.Stream
Example:
http://csharptest.net/browse/src/Library/IO/NonClosingStream.cs
It is definetelly not ok.
read this from msdn
The close calls the Dispose method passing a true value.
Flushing the stream will not flush its underlying encoder unless you explicitly call Close.
Try to encapsulate all IO in a class.