When I run code analysis on the following chunk of code I get this message:
Object 'stream' can be disposed more than once in method 'upload.Page_Load(object, EventArgs)'. To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object.
using(var stream = File.Open(newFilename, FileMode.CreateNew))
using(var reader = new BinaryReader(file.InputStream))
using(var writer = new BinaryWriter(stream))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
}
I don't understand why it might be called twice, and how to fix it to eliminate the error. Any help?
I struggled with this problem and found the example here to be very helpful. I'll post the code for a quick view:
using (Stream stream = new FileStream("file.txt", FileMode.OpenOrCreate))
{
using (StreamWriter writer = new StreamWriter(stream))
{
// Use the writer object...
}
}
Replace the outer using statement with a try/finally making sure to BOTH null the stream after using it in StreamWriter AND check to make sure it is not null in the finally before disposing.
Stream stream = null;
try
{
stream = new FileStream("file.txt", FileMode.OpenOrCreate);
using (StreamWriter writer = new StreamWriter(stream))
{
stream = null;
// Use the writer object...
}
}
finally
{
if(stream != null)
stream.Dispose();
}
Doing this cleared up my errors.
To illustrate, let's edit your code
using(var stream = File.Open(newFilename, FileMode.CreateNew))
{
using(var reader = new BinaryReader(file.InputStream))
{
using(var writer = new BinaryWriter(stream))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
} // here we dispose of writer, which disposes of stream
} // here we dispose of reader
} // here we dispose a stream, which was already disposed of by writer
To avoid this, just create the writer directly
using(var reader = new BinaryReader(file.InputStream))
{
using(var writer = new BinaryWriter( File.Open(newFilename, FileMode.CreateNew)))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
} // here we dispose of writer, which disposes of its inner stream
} // here we dispose of reader
edit: to take into account what Eric Lippert is saying, there could indeed be a moment when the stream is only released by the finalizer if BinaryWriter throws an exception. According to the BinaryWriter code, that could occur in three cases
If (output Is Nothing) Then
Throw New ArgumentNullException("output")
End If
If (encoding Is Nothing) Then
Throw New ArgumentNullException("encoding")
End If
If Not output.CanWrite Then
Throw New ArgumentException(Environment.GetResourceString("Argument_StreamNotWritable"))
End If
if you didn't specify an output, ie if stream is null. That shouldn't be a problem since a null stream means no resources to dispose of :)
if you didn't specify an encoding. since we don't use the constructor form where the encoding is specified, there should be no problem here either (i didn't look into the encoding contructor too much, but an invalid codepage can throw)
if you don't pass a writable stream. That should be caught quite quickly during development...
Anyway, good point, hence the edit :)
The BinaryReader/BinaryWriter will dispose the underlying stream for you when it disposes. You don't need to do it explicitly.
To fix it you can remove the using around the Stream itself.
A proper implementation of Dispose is explicitly required not to care if it's been called more than once on the same object. While multiple calls to Dispose are sometimes indicative of logic problems or code which could be better written, the only way I would improve the original posted code would be to convince Microsoft to add an option to BinaryReader and BinaryWriter instructing them not to dispose their passed-in stream (and then use that option). Otherwise, the code required to ensure the file gets closed even if the reader or writer throws in its constructor would be sufficiently ugly that simply letting the file get disposed more than once would seem cleaner.
Your writer will dispose your stream, always.
Suppress CA2202 whenever you are sure that the object in question handles multiple Dispose calls correctly and that your control flow is impeccably readable. BCL objects generally implement Dispose correctly. Streams are famous for that.
But don't necessarily trust third party or your own streams if you don't have unit tests probing that scenario yet. An API which returns a Stream may be returning a fragile subclass.
Related
I have the following code. In here I am using the StreamReader constructor with leaveOpen: true and in order to do that I need to give the previous parameters which I manage to get their default values. This is cumbersome. Since I use stream with using do I gain anything for using the StreamReader with using? Does answer change if it is a StreamWriter instead?
using (Strem stream = ...)
{
...
using (StreamReader sr = new StreamReader(stream, Encoding.UTF8, true, 1024, true))
{
...
}
...
}
What if any do I lose if use the following code instead?
using (Strem stream = ...)
{
...
StreamReader sr = new StreamReader(stream);
...
...
}
You do need to close a StreamWriter (generally via the using block), or else data in its buffer could be lost.
Because both StreamReader and StreamWriter default to closing the stream automatically, if you want to eliminate one using block from your code, it should be the Stream that you remove from using.
If you can't do that, for example you've borrowed the Stream from elsewhere that doesn't want you to close it, then you must use the leaveOpen parameter you're already aware of. The reason that you can't just omit the using statement for a StreamReader/StreamWriter in order to leave it open, is that the garbage collector will still trigger some cleanup (although not as much) since the object is unreachable... only this will now occur at an unrelated time, creating an unpredictable bug that's very hard to find.
It is indeed ugly that you can't specify leaveOpen without explicitly controlling the buffer size, etc. May I suggest a helper method along the lines of StreamReader CreateStreamReaderLeaveOpen(Stream)?
Because you have leaveOpen set to true in the constructor disposing of a StreamReader does nothing execpt call the the Dispose method of the TextReader class, which itself does nothing at all.
protected override void Dispose(bool disposing)
{
// Dispose of our resources if this StreamReader is closable.
// Note that Console.In should be left open.
try {
// Note that Stream.Close() can potentially throw here. So we need to
// ensure cleaning up internal resources, inside the finally block.
if (!LeaveOpen && disposing && (stream != null))
stream.Close();
}
finally {
if (!LeaveOpen && (stream != null)) {
stream = null;
encoding = null;
decoder = null;
byteBuffer = null;
charBuffer = null;
charPos = 0;
charLen = 0;
base.Dispose(disposing);
}
}
}
With a StreamWriter it does change a bit because it will not flush it's internal buffers to the underlying stream unless you dispose of the writer.
in my WPF application code i got the following Warnings:
CA2202 Do not dispose objects multiple times Object 'fs' can be
disposed more than once in method
'MainWindow.TestResults_Click(object, RoutedEventArgs)'. To avoid
generating a System.ObjectDisposedException you should not call
Dispose more than one time on an object. : Lines:
429 yesMonitor MainWindow.xaml.cs 429
for code:
FileStream fs = new FileStream(System.AppDomain.CurrentDomain.BaseDirectory + "TestResult.htm", FileMode.Create);
using (fs)
{
using (StreamWriter w = new StreamWriter(fs, Encoding.UTF8))
{
w.WriteLine(GetTestResultsHtml());
}
}
what should be the reason for these warning?
Nested using statements can cause violations of the CA2202 warning. If the IDisposable resource of the nested inner using statement contains the resource of the outer using statement, the Dispose method of the nested resource releases the contained resource. When this situation occurs, the Dispose method of the outer using statement attempts to dispose its resource for a second time.
In the following example, a Stream object that is created in an outer using statement is released at the end of the inner using statement in the Dispose method of the StreamWriter object that contains the stream object. At the end of the outer using statement, the stream object is released a second time. The second release is a violation of CA2202.
using (Stream stream = new FileStream("file.txt", FileMode.OpenOrCreate))
{
using (StreamWriter writer = new StreamWriter(stream))
{
// Use the writer object...
}
}
To resolve this issue, use a try/finally block instead of the outer using statement. In the finally block, make sure that the stream resource is not null.
Stream stream = null;
try
{
stream = new FileStream("file.txt", FileMode.OpenOrCreate);
using (StreamWriter writer = new StreamWriter(stream))
{
stream = null;
// Use the writer object...
}
}
finally
{
if(stream != null)
stream.Dispose();
}
Personally in this case I would use:
public StreamWriter(
string path,
bool append
)
Initializes a new instance of the StreamWriter class for the specified
file by using the default encoding and buffer size. If the file
exists, it can be either overwritten or appended to. If the file does
not exist, this constructor creates a new file.
But there is NO good solution, see CA2202, how to solve this case
I have the following code, which uses a stream to open and modify an Open XML document, and then save the new binary representation of that stream:
MemoryStream stream = null;
try
{
stream = new MemoryStream();
stream.Write(this.GetBinaryRepresentation(), 0, this.GetBinaryRepresentation().Length);
using (WordprocessingDocument document = WordprocessingDocument.Open(stream, true))
{
OfficeDocument.ModifyDocument(document);
this.SetBinaryRepresentation(stream.ToArray());
stream = null;
}
}
finally
{
if (stream != null)
{
stream.Dispose();
}
}
I had originally used two using blocks (one for the MemoryStream and the second for the WordprocessingDocument), but received warning CA2202: "Object 'stream' can be disposed more than once in method..." Per the MSDN article, I modified the code to above (converting the outer using to a try), but I am still receiving this warning.
I'm unsure of how I can structure this method to ensure that Dispose is called exactly once on the stream. I would prefer not to simply suppress this warning since the MSDN article states that you shouldn't rely on Dispose being safely callable multiple times.
Disposing of an object multiple times should always be safe. From the documentation for Dispose:
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times.
That being said, a using statement is definitely the way to go here. The only reason you'd receive that method was if you were explicitly disposing of the object, which would not be required, as the using statement should always dispose the object exactly once.
The reason that the example from the MSDN article did not work for you is that they set the stream to null as soon as they enter the using block, whereas you use the stream inside your using block and set the stream to null after. If an exception is thrown before your stream = null statement, stream would be disposed of as the using block is exited, and then again in your finally block.
Unfortunately, since you need to access your stream after document has updated it, I don't see a clean way to use their example of setting stream = null within your using statement to avoid the multiple Dispose() calls. An alternative would be to you could declare both stream and document outside of the try block, and then clean both of them up inside your finally, like so:
MemoryStream stream = null;
WordprocessingDocument document = null;
try
{
stream = new MemoryStream();
stream.Write(this.GetBinaryRepresentation(), 0, this.GetBinaryRepresentation().Length);
document = WordprocessingDocument.Open(stream, true));
OfficeDocument.ModifyDocument(document);
this.SetBinaryRepresentation(stream.ToArray());
}
finally
{
if( document != null)
{
document.Dispose();
}
// Catch the case where an error occurred before document was defined.
else
{
stream.Dispose();
}
}
The stream may still be disposed twice if an exception is thrown in the using block before stream is set to null. Try this:
MemoryStream stream = null;
MemoryStream streamToDispose = null;
try
{
streamToDispose = stream = new MemoryStream();
stream.Write(this.GetBinaryRepresentation(), 0, this.GetBinaryRepresentation().Length);
using (WordprocessingDocument document = WordprocessingDocument.Open(stream, true))
{
streamToDispose = null;
OfficeDocument.ModifyDocument(document);
this.SetBinaryRepresentation(stream.ToArray());
}
}
finally
{
if (streamToDispose != null)
{
streamToDispose.Dispose();
}
}
The using statement disposes the object - so essentially you are calling dispose twice
When your code leaves the using block around the WordProcessingDocument, it will call dispose.
using (WordprocessingDocument document = WordprocessingDocument.Open(stream, true))
Since the WordProcessingDocument takes an instance of stream in its constructor, it will call dispose on that stream instance when WordProcessingDocument.Dispose is called. You then enter the finally block where you call stream.Dispose() - you have now called Dispose() on the stream instance twice.
I am trying to write an object to an Xml string and take that string and save it to a DB. But first I need to get the string...
private static readonly Encoding LocalEncoding = Encoding.UTF8;
public static string SaveToString<T> (T settings)
{
Stream stream = null;
TextWriter writer = null;
string settingsString = null;
try
{
stream = new MemoryStream();
var serializer = new XmlSerializer(typeof(T));
writer = new StreamWriter(stream, LocalEncoding);
serializer.Serialize(writer, settings);
var buffer = new byte[stream.Length];
stream.Read(buffer, 0, (int)stream.Length);
settingsString = LocalEncoding.GetString(buffer);
}
catch (Exception ex)
{
// If the action cancels we don't want to throw, just return null.
}
finally
{
if (stream != null)
stream.Close();
if (writer != null)
writer.Close();
}
return settingsString;
}
This seems to work, the stream gets filled with bytes. But when I come to read it back into the buffer and then into the string... the buffer is filled with '0'! Not sure what I doing wrong here guys.
If you'd checked the results of stream.Read, you'd have seen that it hadn't read anything - because you haven't rewound the stream. (You could do this with stream.Position = 0;.) However, it's easier to just call ToArray:
settingsString = LocalEncoding.GetString(stream.ToArray());
(You'll need to change the type of stream from Stream to MemoryStream, but that's okay as it's in the same method where you create it.)
Alternatively - and even more simply - just use StringWriter instead of StreamWriter. You'll need to create a subclass if you want to use UTF-8 instead of UTF-16, but that's pretty easy. See this answer for an example.
I'm concerned by the way you're just catching Exception and assuming that it means something harmless, by the way - without even logging anything. Note that using statements are generally cleaner than writing explicit finally blocks.
string result = System.Text.Encoding.UTF8.GetString(fs.ToArray());
string result = Encoding.UTF8.GetString((stream as MemoryStream).ToArray());
In case of a very large stream length there is the hazard of memory leak due to Large Object Heap. i.e. The byte buffer created by stream.ToArray creates a copy of memory stream in Heap memory leading to duplication of reserved memory. I would suggest to use a StreamReader, a TextWriter and read the stream in chunks of char buffers.
In netstandard2.0 System.IO.StreamReader has a method ReadBlock
you can use this method in order to read the instance of a Stream (a MemoryStream instance as well since Stream is the super of MemoryStream):
private static string ReadStreamInChunks(Stream stream, int chunkLength)
{
stream.Seek(0, SeekOrigin.Begin);
string result;
using(var textWriter = new StringWriter())
using (var reader = new StreamReader(stream))
{
var readChunk = new char[chunkLength];
int readChunkLength;
//do while: is useful for the last iteration in case readChunkLength < chunkLength
do
{
readChunkLength = reader.ReadBlock(readChunk, 0, chunkLength);
textWriter.Write(readChunk,0,readChunkLength);
} while (readChunkLength > 0);
result = textWriter.ToString();
}
return result;
}
NB. The hazard of memory leak is not fully eradicated, due to the usage of MemoryStream, that can lead to memory leak for large memory stream instance (memoryStreamInstance.Size >85000 bytes). You can use Recyclable Memory stream, in order to avoid LOH. This is the relevant library
Can I close a file stream without calling Flush (in C#)? I understood that Close and Dispose calls the Flush method first.
MSDN is not 100% clear, but Jon Skeet is saying "Flush", so do it before close/dispose. It won't hurt, right?
From FileStream.Close Method:
Any data previously written to the buffer is copied to the file before
the file stream is closed, so it is not necessary to call Flush before
invoking Close. Following a call to Close, any operations on the file
stream might raise exceptions. After Close has been called once, it
does nothing if called again.
Dispose is not as clear:
This method disposes the stream, by writing any changes to the backing
store and closing the stream to release resources.
Remark: the commentators might be right, it's not 100% clear from the Flush:
Override Flush on streams that implement a buffer. Use this method to
move any information from an underlying buffer to its destination,
clear the buffer, or both. Depending upon the state of the object, you
might have to modify the current position within the stream (for
example, if the underlying stream supports seeking). For additional
information see CanSeek.
When using the StreamWriter or BinaryWriter class, do not flush the
base Stream object. Instead, use the class's Flush or Close method,
which makes sure that the data is flushed to the underlying stream
first and then written to the file.
TESTS:
var textBytes = Encoding.ASCII.GetBytes("Test123");
using (var fileTest = System.IO.File.Open(#"c:\temp\fileNoCloseNoFlush.txt", FileMode.CreateNew))
{
fileTest.Write(textBytes,0,textBytes.Length);
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileCloseNoFlush.txt", FileMode.CreateNew))
{
fileTest.Write(textBytes, 0, textBytes.Length);
fileTest.Close();
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileFlushNoClose.txt", FileMode.CreateNew))
{
fileTest.Write(textBytes, 0, textBytes.Length);
fileTest.Flush();
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileCloseAndFlush.txt", FileMode.CreateNew))
{
fileTest.Write(textBytes, 0, textBytes.Length);
fileTest.Flush();
fileTest.Close();
}
What can I say ... all files got the text - maybe this is just too little data?
Test2
var rnd = new Random();
var size = 1024*1024*10;
var randomBytes = new byte[size];
rnd.NextBytes(randomBytes);
using (var fileTest = System.IO.File.Open(#"c:\temp\fileNoCloseNoFlush.bin", FileMode.CreateNew))
{
fileTest.Write(randomBytes, 0, randomBytes.Length);
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileCloseNoFlush.bin", FileMode.CreateNew))
{
fileTest.Write(randomBytes, 0, randomBytes.Length);
fileTest.Close();
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileFlushNoClose.bin", FileMode.CreateNew))
{
fileTest.Write(randomBytes, 0, randomBytes.Length);
fileTest.Flush();
}
using (var fileTest = System.IO.File.Open(#"c:\temp\fileCloseAndFlush.bin", FileMode.CreateNew))
{
fileTest.Write(randomBytes, 0, randomBytes.Length);
fileTest.Flush();
fileTest.Close();
}
And again - every file got its bytes ... to me it looks like it's doing what I read from MSDN: it doesn't matter if you call Flush or Close before dispose ... any thoughts on that?
You don't have to call Flush() on Close()/Dispose(), FileStream will do it for you as you can see from its source code:
http://referencesource.microsoft.com/#mscorlib/system/io/filestream.cs,e23a38af5d11ddd3
[System.Security.SecuritySafeCritical] // auto-generated
protected override void Dispose(bool disposing)
{
// Nothing will be done differently based on whether we are
// disposing vs. finalizing. This is taking advantage of the
// weak ordering between normal finalizable objects & critical
// finalizable objects, which I included in the SafeHandle
// design for FileStream, which would often "just work" when
// finalized.
try {
if (_handle != null && !_handle.IsClosed) {
// Flush data to disk iff we were writing. After
// thinking about this, we also don't need to flush
// our read position, regardless of whether the handle
// was exposed to the user. They probably would NOT
// want us to do this.
if (_writePos > 0) {
FlushWrite(!disposing); // <- Note this
}
}
}
finally {
if (_handle != null && !_handle.IsClosed)
_handle.Dispose();
_canRead = false;
_canWrite = false;
_canSeek = false;
// Don't set the buffer to null, to avoid a NullReferenceException
// when users have a race condition in their code (ie, they call
// Close when calling another method on Stream like Read).
//_buffer = null;
base.Dispose(disposing);
}
}
I've been tracking a newly introduced bug that seems to indicate .NET 4 does not reliably flush changes to disk when the stream is disposed (unlike .NET 2.0 and 3.5, which always did so reliably).
The .NET 4 FileStream class has been heavily modified in .NET 4, and while the Flush*() methods have been rewritten, similar attention seems to have been forgotten for .Dispose().
This is resulting in incomplete files.
Since you've stated that you understood that close & dispose called the flush method if it was not called explicitly by user code, I believe that (by close without flush) you actually want to have a possibility to discard changes made to a FileStream, if necessary.
If that is correct, using a FileStream alone won't help. You will need to load this file into a MemoryStream (or an array, depending on how you modify its contents), and then decide whether you want to save changes or not after you're done.
A problem with this is file size, obviously. FileStream uses limited size write buffers to speed up operations, but once they are depleted, changes need to be flushed. Due to .NET memory limits, you can only expect to load smaller files in memory, if you need to hold them entirely.
An easier alternative would be to make a disk copy of your file, and work on it using a plain FileStream. When finished, if you need to discard changes, simply delete the temporary file, otherwise replace the original with a modified copy.
Wrap the FileStream in a BufferedStream and close the filestream before the buffered stream.
var fs = new FileStream(...);
var bs = new BufferedStream(fs, buffersize);
bs.Write(datatosend, 0, length);
fs.Close();
try {
bs.Close();
}
catch (IOException) {
}
Using Flush() is worthy inside big Loops.
when you have to read and write a big File inside one Loop. In other case the buffer or the computer is big enough, and doesn´t matter to close() without making one Flush() before.
Example: YOU HAVE TO READ A BIG FILE (in one format) AND WRITE IT IN .txt
StreamWriter sw = .... // using StreamWriter
// you read the File ...
// and now you want to write each line for this big File using WriteLine ();
for ( .....) // this is a big Loop because the File is big and has many Lines
{
sw.WriteLine ( *whatever i read* ); //we write here somrewhere ex. one .txt anywhere
sw.Flush(); // each time the sw.flush() is called, the sw.WriteLine is executed
}
sw.Close();
Here it is very important to use Flush(); beacause otherwise each writeLine is save in the buffer and does not write it until the buffer is frull or until the program reaches sw.close();
I hope this helps a little to understand the function of Flush
I think it is safe to use simple using statement, which closes the stream after the call to GetBytes();
public static byte[] GetBytes(string fileName)
{
byte[] buffer = new byte[4096];
using (FileStream fs = new FileStream(fileName))
using (MemoryStream ms = new MemoryStream())
{
fs.BlockCopy(ms, buffer, 4096); // extension method for the Stream class
fs.Close();
return ms.ToByteArray();
}
}