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.
Related
With this code:
using (var stream = new MemoryStream())
{
thumbnail.Save(stream); // you get the idea
stream.Position = 0; // <- is this needed?
WriteStreamToDisk(stream);
}
If I have a method writing to a memory stream, and then I want to write that stream to disk, do I need to set the position to 0?
Or, do streams have different read / write pointers?
A stream has only a single position which is used for both reading and writing. So, assuming that...
Thumbnail.Save(O); doesn't rewind the stream after it's done writing to the stream, and
WriteStreamToDisk(O); doesn't rewind the stream before it starts reading from the stream,
then yes, you will need to rewind the stream yourself.
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
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 zip a memory stream into another memory stream so I can upload to a rest API. image is the initial memory stream containing a tif image.
WebRequest request = CreateWebRequest(...);
request.ContentType = "application/zip";
MemoryStream zip = new MemoryStream();
GZipStream zipper = new GZipStream(zip, CompressionMode.Compress);
image.CopyTo(zipper);
zipper.Flush();
request.ContentLength = zip.Length; // zip.Length is returning 0
Stream reqStream = request.GetRequestStream();
zip.CopyTo(reqStream);
request.GetResponse().Close();
zip.Close();
To my understand, anything I write to the GZipStream will be compressed and written to whatever stream was passed into it's constructor. When I copy the image stream into zipper, it appears nothing is actually copied (image is 200+ MB). This is my first experience with GZipStream so it's likely I'm missing something, any advice as to what would be greatly appreciated.
EDIT:
Something I should note that was a problem for me, in the above code, image's position was at the very end of the stream... Thus when I called image.CopyTo(zipper); nothing was copied due to the position.
[Edited: to remove incorrect info on GZipStream and it's constructor args, and updated with the real answer :) ]
After you've copied to the zipper, you need to shift the position of the MemoryStream back to zero, as the process of the zipper writing to the memory stream advances it's "cursor" as well as the stream being read:
WebRequest request = CreateWebRequest(...);
request.ContentType = "application/zip";
MemoryStream zip = new MemoryStream();
GZipStream zipper = new GZipStream(zip, CompressionMode.Compress);
image.CopyTo(zipper);
zipper.Flush();
zip.Position = 0; // reset the zip position as this will have advanced when written to.
...
One other thing to note is that the GZipStream is not seekable, so calling .Length will throw an exception.
I don't know anything about C# and its libraries, but I would try to use Close instead of (or after) Flush first.
(Java's GZipOutputStream has the same problem that it doesn't properly flush, until Java 7.)
See this example:
http://msdn.microsoft.com/en-us/library/system.io.compression.gzipstream.flush.aspx#Y300
You shouldn't be calling flush on the stream.
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.