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).
Related
I'm working with the Azure Relay Service at the moment and faced a problem handling the stream. I need to use this service in a synchronous way as it is required by the software which will use this implementation.
I'm opening a stream to the service and reading the data from it with StreamReader(), works fine. But now I must leave the StreamReader without closing the underlying stream as I have to send an answer back to the sender.
The problem is, that I can't leave the StreamReader() without closing the underlying stream and its not posible to reopen the stream to send an answer back.
Any ideas how to solve this problem?
Thanks for your help.
There is an overload of the StreamReader constructor which accepts a bool leaveOpen parameter. Passing true prevents the StreamReader from closing the stream when the StreamReader is disposed.
leaveOpen
Type: System.Boolean
true to leave the stream open after the StreamReader object is disposed; otherwise, false.
Example, using the default UTF8 encoding and 1024-byte buffer that you get with the simpler StreamReader constructors:
using (var reader = new StreamReader(stream, Encoding.UTF8, true, 1024, true))
{
// use reader
} // stream will NOT be closed here
I'm working on an assignment for a professor that is strict about LOC. For this reason I'd like to do the following:
(new StreamWriter(saveFileDialog.FileName)).Write(textBox.Text);
instead of
StreamWriter sw = new StreamWriter(saveFileDialog.FileName);
sw.Write(textBox.Text);
sw.Close();
In the first example I don't close the stream. Is this ok? Will it cause any security or memory problems?
You may not get any output, or incomplete output. Closing the writer also flushes it. Rather than manually calling Close at all, I'd use a using statement... but if you're just trying to write text to a file, use a one-shot File.WriteAllText call:
File.WriteAllText(saveFileDialog.FileName, textBox.Text);
Maybe your tutor is looking for:
File.WriteAllText(saveFileDialog.FileName, textbox.Text);
It's reasonable to prefer concise code, but not at the expense of readability or correctness.
Simplest solution without fout.Close() should be:
using (StreamWriter fout = new StreamWriter(saveFileDialog.FileName))
{
fout.Write(textBox.Text);
}
If you don't close it, you can't guarantee that it'll write out the last piece of data written to it. This is because it uses a buffer and the buffer is flushed when you close the stream.
Second, it will lock the file as open preventing another process from using it.
The safest way to use a filestream is with a using statement.
Short answer, the resources allocated for that operation will not be freed not to mention that it could pottentially lock that file.
Consider
using( var fout = new StreamWriter(saveFileDialog.FileName){ fout.write(textBox.Text); }
Any how GC will close it for you. But the thing is until the GC closes that stream you are unnecessary putting on hold to the resources
You can try with using blok in order to clean your no managed object
using (var streamWriter = new StreamWriter(saveFileDialog.FileName))
{
streamWriter.Write(textBox.Text);
}
It would be a memory hazard.
I would always use StreamWriter in a 'using' statement
using(StreamWriter fout = new StreamWriter(saveFileDialog.FileName)
{
fout.Write(textBox.Text);
}
Are streams disposed in NancyFx when using StreamResponse()?
Using a streamWriter to write to the stream, I can't dispose of it without closing the stream I want to send out. And the way I see it, I can't dispose of the stream manually either since I effectively left the method at the return.
This code works:
// Open a stream and write to it with streamReader
Stream memStream = new MemoryStream();
StreamWriter memWriter = new StreamWriter(memStream, Encoding.UTF8);
memWriter.Write(someStringText);
memWriter.Flush();
memStream.Position = 0;
// Add stream to the respose which should get downloaded
var response = new StreamResponse(() => memStream, MimeTypes.GetMimeType(contentType));
return response.AsAttachment(someFileName);
Do I need to worry about the stream lingering on until the garbage collector comes along or do it get disposed correctly by the StreamResponse?
According to StreamResponse.cs, StreamResponse.Dispose() disposes of the stream returned by the Func<Stream>, and Nancy handles disposing the Response at the end of the NancyContext.
As for disposing of StreamWriter without affecting the underlying stream, see this answer.
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.
This question already has answers here:
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.