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Can you keep a StreamReader from disposing the underlying stream?
(9 answers)
Closed 7 years ago.
I need to read a stream two times, from start to end.
But the following code throws an ObjectDisposedException: Cannot access a closed file exception.
string fileToReadPath = #"<path here>";
using (FileStream fs = new FileStream(fileToReadPath, FileMode.Open))
{
using (StreamReader reader = new StreamReader(fs))
{
string text = reader.ReadToEnd();
Console.WriteLine(text);
}
fs.Seek(0, SeekOrigin.Begin); // ObjectDisposedException thrown.
using (StreamReader reader = new StreamReader(fs))
{
string text = reader.ReadToEnd();
Console.WriteLine(text);
}
}
Why is it happening? What is really disposed? And why manipulating StreamReader affects the associated stream in this way? Isn't it logical to expect that a seekable stream can be read several times, including by several StreamReaders?
This happens because the StreamReader takes over 'ownership' of the stream. In other words, it makes itself responsible for closing the source stream. As soon as your program calls Dispose or Close (leaving the using statement scope in your case) then it will dispose the source stream as well. Calling fs.Dispose() in your case. So the file stream is dead after leaving the first using block. It is consistent behavior, all stream classes in .NET that wrap another stream behave this way.
There is one constructor for StreamReader that allows saying that it doesn't own the source stream. It is however not accessible from a .NET program, the constructor is internal.
In this particular case, you'd solve the problem by not using the using-statement for the StreamReader. That's however a fairly hairy implementation detail. There's surely a better solution available to you but the code is too synthetic to propose a real one.
The purpose of Dispose() is to clean up resources when you're finished with the stream. The reason the reader impacts the stream is because the reader is just filtering the stream, and so disposing the reader has no meaning except in the context of it chaining the call to the source stream as well.
To fix your code, just use one reader the entire time:
using (FileStream fs = new FileStream(fileToReadPath, FileMode.Open))
using (StreamReader reader = new StreamReader(fs))
{
string text = reader.ReadToEnd();
Console.WriteLine(text);
fs.Seek(0, SeekOrigin.Begin); // ObjectDisposedException not thrown now
text = reader.ReadToEnd();
Console.WriteLine(text);
}
Edited to address comments below:
In most situations, you do not need to access the underlying stream as you do in your code (fs.Seek). In these cases, the fact that StreamReader chains its call to the underlying stream allows you to economize on the code by not using a usings statement for the stream at all. For example, the code would look like:
using (StreamReader reader = new StreamReader(new FileStream(fileToReadPath, FileMode.Open)))
{
...
}
Using defines a scope, outside of which an object will be disposed, thus the ObjectDisposedException. You can't access the StreamReader's contents outside of this block.
I agree with your question. The biggest issue with this intentional side-effect is when developers don't know about it and are blindly following the "best practice" of surrounding a StreamReader with a using. But it can cause some really hard to track down bugs when it is on a long-lived object's property, the best (worst?) example I've seen is
using (var sr = new StreamReader(HttpContext.Current.Request.InputStream))
{
body = sr.ReadToEnd();
}
The developer had no idea the InputStream is now hosed for any future place that expects it to be there.
Obviously, once you know the internals you know to avoid the using and just read and reset the position. But I thought a core principle of API design was to avoid side effects, especially not destroying data you are acting upon. Nothing inherent about a class that supposedly is a "reader" should clear the data it reads when done "using" it. Disposing of the reader should release any references to the Stream, not clear the stream itself. The only thing I can think is that the choice had to be made since the reader is altering other internal state of the Stream, like the position of the seek pointer, that they assumed if you are wrapping a using around it you are intentionally going to be done with everything. On the other hand, just like in your example, if you are creating a Stream, the stream itself will be in a using, but if you are reading a Stream that was created outside of your immediate method, it is presumptuous of the code to clear the data.
What I do and tell our developers to do on Stream instances that the reading code doesn't explicitly create is...
// save position before reading
long position = theStream.Position;
theStream.Seek(0, SeekOrigin.Begin);
// DO NOT put this StreamReader in a using, StreamReader.Dispose() clears the stream
StreamReader sr = new StreamReader(theStream);
string content = sr.ReadToEnd();
theStream.Seek(position, SeekOrigin.Begin);
(sorry I added this as an answer, wouldn't fit in a comment, I would love more discussion about this design decision of the framework)
Dispose() on parent will Dispose() all owned streams. Unfortunately, streams don't have Detach() method, so you have to create some workaround here.
I don't know why, but you can leave your StreamReader undisposed. That way your underlying stream won't be disposed, even when StreamReader got collected.
Related
A helper method to turn a string into a zipped up text file:
public static System.Net.Mail.Attachment CreateZipAttachmentFromString(string content, string filename)
{
using (MemoryStream memoryStream = new MemoryStream())
{
using (ZipArchive zipArchive = new ZipArchive(memoryStream, ZipArchiveMode.Update))
{
ZipArchiveEntry zipArchiveEntry = zipArchive.CreateEntry(filename);
using (StreamWriter streamWriter = new StreamWriter(zipArchiveEntry.Open()))
{
streamWriter.Write(content);
}
}
MemoryStream memoryStream2 = new MemoryStream(memoryStream.ToArray(), false);
return new Attachment(memoryStream2, filename + ".zip", MediaTypeNames.Application.Zip);
}
}
I was really hoping to avoid turning the first memory stream into an array, making another memory stream on it to read it, and passing that to attachment. My logic was, why copy X megabytes to another place in memory to establish another stream pointing to the copy, when it's essentially just what we started out with.. It's the multi-megabyte equivalent of redundancy like if(myBool == true)
So I figured instead I would Seek back to the start of the first memory stream, and then attachment could just read that.. Or I would establish another memorystream pointing to the buffer of the first, and with the offset and length parameters set so it would know what to read
Neither of these approaches work out because it seems that ZipArchive only pushes data into the memory stream (in my case maybe) when control falls out of the using block and the ziparchive is disposed. Disposing it also disposes the MemoryStream and nearly everything (other than ToArray() and GetBuffer()) throw ObjectDisposedException.
Ultimately I can't seek it or get its length after the ZipArchive pumps data into it and before it pumps it in, the offset is usually zero and the length is definitely zero so the values are useless
Is there a nice optimal way, short of configuring my own over-large buffer (which then makes it non expandable by MemoryStream), to avoid having to burn up around 2x the memory bytes of the archive size with this method?
Most well designed streams and stream-users in .NET have an additional boolean parameter that can be used to instruct them to leave the "base stream" (terrible name) open when disposing.
This is ZipArchive's constructor:
public ZipArchive(
Stream stream,
ZipArchiveMode mode,
bool leaveOpen
)
There is no need for a second MemoryStream. You need to do two things:
Ensure, that the MemoryStream is not disposed before the last usage point. This is harmless. Disposing a MemoryStream does nothing helpful and for compatibility reasons can never do anything in the future. The .NET Framework has a very high compatibility bar. They often don't even dare to rename fields.
Seek to offset zero.
So remove the using around the MemoryStream and use the ctor for ZipArchive that allows you to leave the stream open.
Since the Attachment you are returning makes use of the MemoryStream you can't dispose it before exiting the method. Again, this is harmless. The only negative point is that the code becomes less obvious.
There's an entirely different approach: You can write your own Stream class that creates the bytes on demand. That way there is no need to buffer the string and ZIP bytes at all. This is much more work, of course. And it does not detract from the fact that the whole string must sit in memory at once, so it's still not a O(1) space solution.
public static System.Net.Mail.Attachment CreateZipAttachmentFromString(string content, string filename)
{
MemoryStream memoryStream = new MemoryStream();
using (ZipArchive zipArchive = new ZipArchive(memoryStream, ZipArchiveMode.Update, true))
{
ZipArchiveEntry zipArchiveEntry = zipArchive.CreateEntry(filename);
using (StreamWriter streamWriter = new StreamWriter(zipArchiveEntry.Open()))
{
streamWriter.Write(content);
}
}
memoryStream.Position = 0;
return new Attachment(memoryStream, filename + ".zip", MediaTypeNames.Application.Zip);
}
Maybe it is a trival question, but it's bothering me. And don't shout laud if it is a duplicate - I tried to search, but there are so many questions regarding using that it was hard for me to find the answer.
I've a code like this:
using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
using (StreamWriter writeFile = new StreamWriter(new IsolatedStorageFileStream("example.txt", FileMode.Create, ISF)))
writeFile.WriteLine("Example");
And my questions are: What happens to my created IsolatedStorageFileStream, when StreamWriter is disposed, while leaving using? Will it be also disposed?
Is there any difference in comparison to this code:
using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
using (IsolatedStorageFileStream stream = ISF.CreateFile("example.txt"))
using (StreamWriter writeFile = new StreamWriter(stream))
writeFile.WriteLine("Example");
Thanks in advance.
You have a constructor for StreamWriter (NET Framework 4.5 only) that allows specifying the leaveOpen boolean that defines whether your instance takes ownership of the underlying stream or not.
If not specified (as in your example, or for previous versions of the framework), by default it's false, so closing (or disposing) the instance closes the underlying stream.
Unless you set the leaveOpen parameter to true, the StreamWriter
object calls Dispose() on the provided Stream object when
StreamWriter.Dispose is called.
So there is no difference between both pieces of code you provided.
Once it leaves the using block, Dispose is called.
using Statement (C# Reference)
The using statement calls the Dispose method on the object in the
correct way, and (when you use it as shown earlier) it also causes the
object itself to go out of scope as soon as Dispose is called. Within
the using block, the object is read-only and cannot be modified or
reassigned.
The using statement ensures that Dispose is called even if an
exception occurs while you are calling methods on the object.
Your stream remains open even though the stream writer is disposed of. You could, for example, open another stream writer and continue writing to the stream.
Use {} always! It makes the intention of your code a lot better.
Your code looks like:
using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
{
using (StreamWriter writeFile = new StreamWriter(new IsolatedStorageFileStream("example.txt", FileMode.Create, ISF)))
{
writeFile.WriteLine("Example");
}
}
You can then see that the StreamWriter is executed in the context of the ISF. If I understand the ISF correctly, the ISF should not be closed when the Streamwriter closes the file. And you could open another File in the ISF Block.
I have a program that copies video files, while they are being played.
More often than not, the video player (3rd party) is unable to access the file for around half a second (playback freezes) whenever I Close() the FileStream in my program.
From what I can tell, it won't matter if I use Dispose() instead of Close().
The file is copied to the new location as expected, so the playback freeze is the issue I need to solve, but frankly I do not know the specifics on how the 3rd party software reads the file.
I just can't wrap my head around why the problem appears exactly when I close the reading stream.
I would like to avoid having to postpone the copy process until the video player is done playing.
Any suggestions will be appreciated.
FileStream SourceFile = new FileStream(sourcePath, FileMode.Open, FileAccess.Read);
while(bytesLeftToCopy > 0)
{
int _BlockSize = 1024 * 1024;
byte[] _BlockBuffer = new byte[_BlockSize];
SourceFileBytes = SourceFile.Read(_BlockBuffer, 0, _BlockSize);
//you get the idea
}
SourceFile.Close();
Have you tried wrap your filestream into using construction? It automatically disposes the object after execution.
using (FileStream SourceFile = new FileStream(sourcePath, FileMode.Open, FileAccess.Read))
{
//read
}
Sorry to have bothered you guys, but it turns out that the problem was in the writing part of the program.
The guy who wrote this didn't flush the writing stream, which meant alot of data was cached when the read loop came to an end. Which in turn meant alot of system resources were used at Close() time.
I switched to the constructor (for the write stream) that accepts the FileOptions.WriteThough argument. This seems to have solved the problem.
Thanks for the suggestions you provided... I'll try to be more thorough when I create my next questions.
How do I manage closing StreamReader and StreamWriter which are using the same underlying stream?
var stream = /*...*/;
var reader = new StreamReader(stream);
var writer = new StreamWRiter(stream);
I know that I could simply ignore closing the reader/writer and just close the underlying stream. However, that seems a bit of a hack, since it is based on the assumption that the reader/writer doesn't have anything to dispose (which might not be the case in the future).
I know this have been solved in .NET 4.5 with an extra constructor argument, but until .NET 4.5 is released, how do I solve it in a proper way?
Make sure you Flush() the writer first.
And then just Dispose or Close the 2 decorators and the stream (nested usings are OK).
I'm trying to download a file from the web and save it locally, but I get an exception:
C# The process cannot access the file
'blah' because it is being used by
another process.
This is my code:
File.Create("data.csv"); // create the file
request = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url));
request.Timeout = 30000;
response = (HttpWebResponse)request.GetResponse();
using (Stream file = File.OpenWrite("data.csv"), // <-- Exception here
input = response.GetResponseStream())
{
// Save the file using Jon Skeet's CopyStream method
CopyStream(input, file);
}
I've seen numerous other questions with the same exception, but none of them seem to apply here. Any help?
Update:
Thanks for the answers! Removing the File.Create(...) fixed it!
One comment on the documentation of OpenWrite: it is a little misleading, the brief description says:
Opens an existing file for writing.
The detailed description says:
If the file exists, it is opened for
writing at the beginning. The existing
file is not truncated.
Update 2.0:
It looks like the discrepancy is between IntelliSense/F1 and the online documentation. I thought it should be the same since I allow F1 to connect to the web when it's displaying documentation.
File.Create returns a FileStream - which you're not closing. That means you won't be able to open another stream writing to the same file until the finalizer has closed the existing stream.
Just get rid of the call to File.Create - File.OpenWrite will create it anyway. Alternatively, keep the FileStream around to write to:
using (Stream file = File.Create("data.csv"))
{
request = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url));
request.Timeout = 30000;
using (var response = (HttpWebResponse)request.GetResponse())
using (Stream input = response.GetResponseStream())
{
// Save the file using Jon Skeet's CopyStream method
CopyStream(input, file);
}
}
Note that I'm also disposing of the WebResponse here, which you should do to make sure the connection is freed to the connection pool.
looks like File.Create returns an open FileStream object
http://msdn.microsoft.com/en-us/library/aa328775(v=VS.71).aspx
try
using (FileStream fs = File.Create("data.csv"))
and leave off the first File.Create
The problem is that the File.Create method actually opens the file and returns a FileStream object. It won't close the file until the object is garbage collected (which will happen at an indeterminate time). The FileStream object still gets created, regardless of whether or not you use it. Try this:
using (Stream file = File.Create("data.csv"),
input = response.GetResponseStream())
{
// Save the file using Jon Skeet's CopyStream method
CopyStream(input, file);
}
First. File.Create will return a stream that you should use for accessing the file.
Second, if that doesn't resolve the issue, if you google who lock you will find a program that let's you determine what process is accessing a file at the same time.