I'm writing an application that creates a "Catalog" of files, which can be attributed with other meta data files such as attachments and thumbnails.
I'm trying to abstract the interface to a catalog to the point where a consumer of a catalog does not need to know about the underlying file system used to store the files. So I've created an interface called IFileSystemAdaptor which is shown below.
public interface IFileSystemAdaptor:IDisposable
{
void WriteFileData(string fileName, Stream data);
Stream ReadFileData(string filename);
void DeleteFileData(string filename);
void ClearAllData();
void WriteMetaFileData(string filename, string path, Stream data);
Stream ReadMetaFileData(string filename, string path);
void DeleteMetaFileData(string filename, string path);
void ClearMetaFilesData(string filename);
}
Essentially my IFileSystemAdaptor interface exposes a flat list of files, that can also be associated with additional meta data files.
As you can see I'm using references to generic Stream objects to abstract the interface to a file's data. This way one implementation of a Catalog could return files from a hard disk, while another could return the data from a web server.
Now I'm trying to figure out how to keep my program from leaving streams open. Is there a rule of thumb for what members should close streams? Should the consumer of a stream close it, or should the member that original created the stream be responsible for closing it.
My Rules:
Should the consumer of a stream close it
If I return a stream from a method, the consumer is responsible. I'm giving it to you, It's your responsilibity.
If I accept a stream as a parameter in a method, I don't close it. When exiting the method, I don't know if the calling method still needs it. It's your stream, I'm just borrowing it, and I don't want to mess you up.
If I create a stream and pass it to another method, my method closes it (or tries to) when I am done with it. I don't know what you are going to do with it, but it's my stream, so I am responsible for it.
My spontaneous thought in this case is that the consumer should hold the responsibility for closing the streams. An IFileSystemAdaptor can't know when the consumer is done using the stream, so it also can't decide when to close it.
In effect the last object to use the stream should be responsible for closing it, and that is generally the caller.
Enjoy!
Related
In my program, I write a file, then call an external program that reads that file. Do I need Flush(true) to make sure that the data is written entirely to disk, or is Flush() sufficient?
class ExampleClass : IDisposable {
private FileStream stream = File.Open("command-list.txt", FileMode.Append, FileAccess.Write, FileShare.Read);
public void Execute(string command) {
var buffer = Encoding.UTF8.GetBytes(command);
stream.WriteByte(10);
stream.Write(buffer, 0, buffer.Length);
stream.Flush(true);
Process.Start("processor", "command-list.txt");
}
public void Dispose() {
stream.Close();
}
}
Calling Dispose, or any kind of Flush, will make it so that other software will see the data that has been written, but it won't guarantee that the data will actually make it all the way to the disk. If e.g. the data is being written to a USB drive and the cable gets unplugged before the data actually gets written, the program will only find out about the problem if uses a Flush(true) or other similar means to ensure that the data has been written before it it's finished. Whether a program should use Flush(true) depends on the application. If the hard drive is not removable and could only fail in cases where failure to write the file would be the least of one's problems, then Flush(true) is not necessary. If a user might yank a USB drive as soon as the program thinks it's done, then Flush(true) may be a good idea.
Neither; you should simply dispose the stream in a using statement.
Or, better yet, replace the whole thing with File.AppendAllText().
I have a stream that contains text, now I want to edit some text (replace some values) in that stream.
What is the most efficient way to do this, so without breaking the stream?
I want to use this in a custom pipeline component for BizTalk.
public IBaseMessage Execute(IPipelineContext pContext, IBaseMessage pInMsg)
{
string msg = "";
using (VirtualStream virtualStream = new VirtualStream(pInMsg.BodyPart.GetOriginalDataStream()))
{
using(StreamReader sr = new StreamReader(VirtualStream))
{
msg = sr.ReadToEnd();
}
// modify string here
msg = msg.replace("\r\n","");
while (msg.Contains(" <"))
msg = msg.Replace(" <", "<");
VirtualStream outStream = new VirtualStream();
StreamWriter sw = new StreamWriter(outStream, Encoding.Default);
sw.Write(msg);
sw.Flush();
outStream.Seek(0, SeekOrigin.Begin);
pInMsg.BodyPart.Data = outStream;
pContext.ResourceTracker.AddResource(outStream);
}
return pInMsg;
}
This is the code, but as you can see I am breaking the stream when I do sr.ReadToEnd().
Is there a beter way to do this?
The fact that you are using Stream classes in your pipeline component does not make it a Streaming pipeline component per-se as you have intuitively wondered.
The most appropriate way is to split the responsabilities into two components:
First, you create a customer System.IO.Stream class - that is a class that wraps the original incoming stream and exposes a streaming interface. In this class, you would effectively process the bytes as they are read by the calling code. This class should have no dependency over BizTalk and you should be able to create a sample unit-test program for this class outside BizTalk.
For the first case, I recommend you browse to one of several articles with source code samples.
Second, the pipeline component itself, whose sole responsability is to substitute the incoming stream with an instance of your custom stream. This is an idiomatic pattern that you'll find in good pipeline components. In particular, during the execution of the Execute method, you should not read the original incoming stream. The reading will happen - automagically - by itself when the Messaging Agent takes over.
The following snippet should be the canonical source code for an Executemethod, barring additional code for error handling, of course:
IBaseMessage IComponent.Execute(IPipelineContext pContext, IBaseMessage pInMsg)
{
// assign a new CustomStream to the incoming message
System.IO.Stream stream = pInMsg.BodyPart.GetOriginalDataStream();
System.IO.Stream customStream = new CustomStream(stream);
// return the message for downstream pipeline components (further down in the pipeline)
pInMsg.BodyPart.Data = customStream;
pContext.ResourceTracker.AddResource(customStream);
return pInMsg;
}
See? No reading whatsoever in the preceding method. The entire processing should happen during the (repated) calls of the Read method in your custom Stream class.
As I wrote in my response to the following question, I strongly encourage you to check out the whole series of posts Nic Barden has done about Developing Streaming Pipeline Components.
For the simple case of non-seekable read-only stream, you can create a wrapper stream that does the replaces on-the-fly as needed in the Stream.Read (and possibly the Stream.ReadByte) method. However these work with raw bytes, so you may have to account for the stream encoding too.
I think the way would be to keep a done and a processing buffer, then you when you get new content written to your stream you keep it in the pending buffer until you know there is nothing more to replace.
After replacing or deciding there's nothing to replace move that data from the pending to the done buffer.
The read method should only read from the done buffer, the write should only write to the processing buffer and flush moves all the pending to done.
Not shure about the flushing part, i am thinking myself how to do this in the best way as i need to write a generic string replacer stream.
[edit] sorry, replied a 2 years old post...[/edit]
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.
Using method: System.IO.File.Create()
After the file gets created, it still remains used by a process, and I can't delete it.
Any idea how I can better create the file, should be a 0byte file, and then somehow close and dispose?
JL,
You should wrap your call to .Create in a using statement so that the FileStream that .Create returns will be closed properly. IE:
using (File.Create("path")){...}
The Create method not only creates the file, it opens it and return a FileStream object that you can use to write to the file.
You have to close the file after yourself, otherwise it will not be closed before the garbage collector cleans up the FileStream object.
The easiest way is to simply close the file using the reference that the Create method returns:
File.Create(fileName).Close();
nikmd23 has the short answer, the long answer is: the FileStream that File.Create(...) is returning is not being deterministically disposed of, therefore it's file handle is not closed when you're trying to delete it.
As nikmd23 put it, wrapping your File.Create(...) call will with a using statement will make sure the stream is closed and disposed of:
using (FileStream fs = File.Create(path)) {
// do anything with the stream if need-be...
}
File.Delete(path); //after it's been disposed of.
The using(...) block is really just compiler-sugar for:
FileStream fs = File.Create(path);
try {
// do anything with the stream if need-be...
}
finally {
fs.Dispose();
}
File.Delete(path)
You should use nikmd23's answer in almost all cases. If you can't, because you need to pass the FileStream somewhere else, make sure to call the FileStream.Close method eventually. Preferably you would have the class that 'owns' the FileStream implement IDisposable itself, and close the stream in its Dispose method.
For more information on implementing IDisposable, refer to the MSDN documentation. Easier reading, and more up to date, is Joe Duffy's post on the subject.
using(FileStream f = File.Create(file_path))
{
// ... do something with file
f.Close();
}
The "f.Close();" line closing file immediately. If not close manually, Disposing may not close it.
See System.IO.File.Create(String) Method paramter and return value description
Parameters
path
Type: System.String
The path and name of the file to create.
Return Value
Type: System.IO.FileStream
A FileStream that provides read/write access to the file specified in path.
The FileStream return value is there for IO access to the created file. If you are not interested in writing (or reading) the newly created file, close the stream. That is what the using block is ensuring.
I've been playing around with what I thought was a simple idea. I want to be able to read in a file from somewhere (website, filesystem, ftp), perform some operations on it (compress, encrypt, etc.) and then save it somewhere (somewhere may be a filesystem, ftp, or whatever). It's a basic pipeline design. What I would like to do is to read in the file and put it onto a MemoryStream, then perform the operations on the data in the MemoryStream, and then save that data in the MemoryStream somewhere. I was thinking I could use the same Stream to do this but run into a couple of problems:
Everytime I use a StreamWriter or StreamReader I need to close it and that closes the stream so that I cannot use it anymore. That seems like there must be some way to get around that.
Some of these files may be big and so I may run out of memory if I try to read the whole thing in at once.
I was hoping to be able to spin up each of the steps as separate threads and have the compression step begin as soon as there is data on the stream, and then as soon as the compression has some compressed data available on the stream I could start saving it (for example). Is anything like this easily possible with the C# Streams? ANyone have thoughts as to how to accomplish this best?
Thanks,
Mike
Using a helper method to drive the streaming:
static public void StreamCopy(Stream source, Stream target)
{
byte[] buffer = new byte[8 * 1024];
int size;
do
{
size = source.Read(buffer, 0, 8 * 1024);
target.Write(buffer, 0, size);
} while (size > 0);
}
You can easily combine whatever you need:
using (FileStream iFile = new FileStream(...))
using (FileStream oFile = new FileStream(...))
using (DeflateStream oZip = new DeflateStream(outFile, CompressionMode.Compress))
StreamCopy(iFile, oZip);
Depending on what you are actually trying to do, you'd chain the streams differently. This also uses relatively little memory, because only the data being operated upon is in memory.
StreamReader/StreamWriter shouldn't have been designed to close their underlying stream -- that's a horrible misfeature in the BCL. But they do, they won't be changed (because of backward compatibility), so we're stuck with this disaster of an API.
But there are some well-established workarounds, if you want to use StreamReader/Writer but keep the Stream open afterward.
For a StreamReader: don't Dispose the StreamReader. It's that simple. It's harmless to just let a StreamReader go without ever calling Dispose. The only effect is that your Stream won't get prematurely closed, which is actually a plus.
For a StreamWriter: there may be buffered data, so you can't get away with just letting it go. You have to call Flush, to make sure that buffered data gets written out to the Stream. Then you can just let the StreamWriter go. (Basically, you put a Flush where you normally would have put a Dispose.)
Unless you're reading in streams bigger than your hard drive, I don't think you'll run out of memory:
http://blogs.msdn.com/ericlippert/archive/2009/06/08/out-of-memory-does-not-refer-to-physical-memory.aspx