How to I dispose the File.OpenRead() correctly. I am currently using the below code?
using (BinaryReader br = new BinaryReader(File.OpenRead(path)))
{
myByte = br.ReadByte();
}
I get the following in Visual Studio when analyzing the code:
Warning 1 CA2000 :
Microsoft.Reliability : In method
'Program.Main(string[])', object
'File.OpenRead(path)' is not disposed
along all exception paths. Call
System.IDisposable.Dispose on object
'File.OpenRead(path)' before all
references to it are out of scope.
At first glance, this looks like a false positive, because disposing the BinaryReader will also dispose the FileStream returned by File.OpenRead:
From: http://msdn.microsoft.com/en-us/library/azy2k2bx.aspx
When the disposing parameter is true, this method releases all resources held by any managed objects that this BinaryReader references. This method invokes the Dispose method of each referenced object.
However, there is one corner case, where the FileStream is really not disposed: When the constructor of BinaryReader throws an exception!
Solution:
The correct way to write your code would be like this:
using (var fs = File.OpenRead(path))
{
BinaryReader br = new BinaryReader(fs);
myByte = br.ReadByte();
}
Background:
BinaryReader only holds a reference to the FileStream and therefore doesn't need to be disposed.
Code Analysis shares this opinion.
BTW: When using this solution for a writable stream, it is important to flush the writer before the stream is disposed:
using (var fileStream = new FileStream(...))
{
var writer = new StreamWriter(fileStream);
writer.WriteLine(...);
writer.Flush(); // <-- Important
}
If you forget this, your stream might not contain everything that has been written using the StreamWriter.
Hows about:
using (Filestream fs = File.OpenRead(Path))
{
using (BinaryReader br = new BinaryReader(fs))
{
myByte = br.ReadByte();
}
}
File.OpenRead returns a FileStream, which is also IDisposible. You can put it in an outer using block if you like, or declare it and dispose it outside your current using.
Both the FileStream that's created by File.OpenRead and the BinaryReader that you create on that FileStream need to be disposed of, so you need an explicit reference to each:
using(FileStream fs = File.OpenRead(path))
using(BinaryReader br = new BinaryReader(fs))
{
myByte = br.ReadByte();
}
Related
The new Visual Studio 2012 is complaining about a common code combination I have always used. I know it seems like overkill but I have done the following in my code 'just to be sure'.
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
using (var sr = new StreamReader(fs))
{
// Code here
}
}
Visual studio is 'warning' me that I am disposing of fs more than once. So my question is this, would the proper way to write this be:
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
var sr = new StreamReader(fs);
// do stuff here
}
Or should I do it this way (or some other variant not mentioned).
var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (var sr = new StreamReader(fs))
{
// Code here
}
I searched several questions in StackOverflow but did not find something that addressed the best practice for this combination directly.
Thank you!
The following is how Microsoft recommends doing it. It is long and bulky, but safe:
FileStream fs = null;
try
{
fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (TextReader tr= new StreamReader(fs))
{
fs = null;
// Code here
}
}
finally
{
if (fs != null)
fs.Dispose();
}
This method will always ensure that everything is disposed that should be despite what exceptions may be thrown. For example, if the StreamReader constructor throws an exception, the FileStream would still be properly disposed.
Visual studio is 'warning' me that I am disposing of fs more than once.
You are, but that is fine. The documentation for IDisposable.Dispose reads:
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.
Based on that, the warning is bogus, and my choice would be to leave the code as it is, and suppress the warning.
As Dan's answer only appears to work with StreamWriter, I believe this might be the most acceptable answer.
(Dan's answer will still give the disposed twice warning with StreamReader - as Daniel Hilgarth and exacerbatedexpert mentions, StreamReader disposes the filestream)
using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
string line;
while ((line = tr.ReadLine()) != null)
{
// Do work here
}
}
This is very similar to Daniel Hilgarth's answer, modified to call dispose via the Using statement on StreamReader as it is now clear StreamReader will call dispose on FileStream (According to all the other posts, documentation referenced)
Update:
I found this post. For what it is worth.
Does disposing streamreader close the stream?
Yes, the correct way would be to use your first alternative:
using (FileStream fs = new FileStream(filePath, FileMode.Open,
FileAccess.Read, FileShare.ReadWrite))
{
TextReader tr = new StreamReader(fs);
// do stuff here
}
The reason is the following:
Disposing the StreamReader only disposes the FileStream so that's actually the only thing you need to dispose.
Your second option (just the inner "using") is no solution as it would leave the FileStream undisposed if there was an exception inside the constructor of the StreamReader.
It's because the way you used StreamReader disposes the stream when it is disposed. So, if you dispose the stream too, it's being disposed twice. Some consider this a flaw in StreamReader--but it's there none-the-less. In VS 2012 (.NET 4.5) there is an option in StreamReader to not dispose of the stream, with a new constructor: http://msdn.microsoft.com/en-us/library/gg712952
Two solutions:
A) You trust Reflector or Documentation and you know *Reader and *Writer will close the underlying *Stream. But warning: it won't work in case of a thrown Exception. So it is not the recommended way:
using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
// Code here
}
B) You ignore the warning as documentation states The object must not throw an exception if its Dispose method is called multiple times. It's the recommended way, as it's both a good practice to always use using, and safe in case of a thrown Exception:
[SuppressMessage("Microsoft.Usage", "CA2202:Do not dispose objects multiple times")]
internal void myMethod()
{
[...]
using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (TextReader tr = new StreamReader(fs))
{
// Code here
}
}
Given all the nonsense this (perfectly legitimate!) question generated, this would be my preference:
FileStream fs = null;
TextReader tr= null;
try
{
fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
tr= new StreamReader(fs);
// Code here
}
finally
{
if (tr != null)
tr.Dispose();
if (fs != null)
fs.Dispose();
}
The links below illustrate perfectly legal syntax. IMO, this "using" syntax is far preferable to nested "using". But I admit - it does not solve the original question:
http://blogs.msdn.com/b/ericgu/archive/2004/08/05/209267.aspx
.NET - Replacing nested using statements with single using statement
IMHO...
On this code:
public static string Base64FromFileName(string fileName)
{
try
{
FileInfo fInfo = new FileInfo(fileName);
long numBytes = fInfo.Length;
FileStream fStream = new FileStream(fileName, FileMode.Open, FileAccess.Read);
BinaryReader br = new BinaryReader(fStream);
byte[] bdata = br.ReadBytes((int)numBytes);
br.Close();
fStream.Close();
return Convert.ToBase64String(bdata);
}
catch(Exception e)
{
throw e;
}
}
...I get, courtesy of Visual Studio's Code Analysis tool, the warning, "Do not dispose objects multiple times...To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object" on the "fStream.Close();" line.
Why? Is fStream disposed in the line above, where the BinaryReader is closed?
Wouldn't I be better off refactoring it like this anyway:
. . .
using (FileStream fStream = new FileStream(fileName, FileMode.Open, FileAccess.Read))
{
using (BinaryReader br = new BinaryReader(fStream))
{
byte[] bdata = br.ReadBytes((int)numBytes);
} //br.Close();
} //fStream.Close();
. . .
?
BinaryReader.Close also closes the underlying stream, so this would indeed cause the stream to be disposed of twice. But that's not a real problem, disposing twice doesn't hurt.
You could write this much better as
using (var fs = new FileStream(fileName, FileMode.Open, FileAccess.Read))
using (var br = new BinaryReader(fs, new UTF8Encoding(), true))
{
return Convert.ToBase64String(br.ReadBytes((int)numBytes));
}
This is the bomb-proof version:
Anything that is successfully constructed is guaranteed to be disposed
You won't dispose of the stream twice because the boolean leaveOpen argument on the BinaryReader constructor ensures that disposing (closing) it won't also close the stream
Code Analysis is right; Code Analysis is wrong.
Yes, you're closing the FileStream twice. This is harmless. So is disposing it twice. Multiple disposal happens. It is the responsibility of the developer of a disposable component to handle multiple disposal properly and without throwing exceptions1.
However, while calling Dispose() on a disposed FileStream is a no-op by convention, the same isn't true of your code, which calls Close() on a disposed stream. Don't do that.
Your suggested fix with the nested using is fine.
1 The contract for IDisposable.Dispose requires:
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. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.
The formal term for this behavior is idempotence.
Is there any chance that fileStream object will likely be destroyed before its call to the Close method as below?
FileStream fileStream = new FileStream(xxx);
StreamReader txtReader = new StreamReader(fileStream);
curLog = txtReader.ReadToEnd();
txtReader.Close();
fileStream.Close();
Is there any chance that fileStream object will likely be destroyed
before its call to the Close method as below?
No.
But you should never write code like that. You should always wrap IDisposable resources in using statements to ensure that they will be disposed even if an exception is thrown and that you won't be leaking handles.
using (FileStream fileStream = new FileStream(xxx))
using (StreamReader txtReader = new StreamReader(fileStream))
{
curLog = txtReader.ReadToEnd();
}
But for the purpose of this specific example you could simply use the ReadAllText method.
string curLog = File.ReadAllText(xxx);
No, there isn't any chance that it is closed before that. And i would recommend using it like this
FileStream fileStream = new FileStream(xxx);
using (StreamReader txtReader = new StreamReader(fileStream))
{
curLog = txtReader.ReadToEnd();
}
Should I be using the using keyword or a dispose method with the following code (since I am opening a stream):
class Program
{
static void Main(string[] args)
{
var x = Deserialize<Dog>(new FileStream(#"C:\Documents and Settings\name\Desktop\demo.xml", FileMode.Open));
}
static T Deserialize<T>(Stream s)
{
XmlSerializer ser = new XmlSerializer(typeof(T));
return (T)ser.Deserialize(s);
}
}
If not, can you please explain why not (does a new FileStream automatically dispose/close the stream)?
You should be using using:
using(var stream = new FileStream(#"C:\path\demo.xml", FileMode.Open))
{
var x = Deserialize<Dog>(stream);
// more code ...
}
Yes you should dispose the stream. If you were to use the File.ReadAllText() for example, this static method would open a stream and dispose it for you. I would suggest a Using, this is because it'll handle exceptions too. For example in this noddy example:
This version correctly disposes the FileStream:
using(FileStream fs = FileStream(path, FileMode.Open))
{
throw new Exception();
}
This example leaks the resources used by the FileStream, you could add try/catch blocks but then it's less readable.
FileStream fs = new FileStream(path, FileMode.Open);
throw new Exception();
fs.Dispose();
If you use using block , he execute in the end of treatment Dispose method.
You use using, because FileStream is non managed object, so Garbage collector don't have informatiosn abouts this object in order to clean, so the developper must clean ressource in order to help your GC.
Link : http://msdn.microsoft.com/fr-fr/library/yh598w02(v=vs.80).aspx
you should use Using with everything that implements IDisposable :)
As already mentioned, you should use using. But why? Well, as already mentioned you should use using for all objects that implements IDisposable.
In your case, FileStream inherits from the Stream object which is implementing IDisposable. Read more about FileStream here: msdn
The new Visual Studio 2012 is complaining about a common code combination I have always used. I know it seems like overkill but I have done the following in my code 'just to be sure'.
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
using (var sr = new StreamReader(fs))
{
// Code here
}
}
Visual studio is 'warning' me that I am disposing of fs more than once. So my question is this, would the proper way to write this be:
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
var sr = new StreamReader(fs);
// do stuff here
}
Or should I do it this way (or some other variant not mentioned).
var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (var sr = new StreamReader(fs))
{
// Code here
}
I searched several questions in StackOverflow but did not find something that addressed the best practice for this combination directly.
Thank you!
The following is how Microsoft recommends doing it. It is long and bulky, but safe:
FileStream fs = null;
try
{
fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (TextReader tr= new StreamReader(fs))
{
fs = null;
// Code here
}
}
finally
{
if (fs != null)
fs.Dispose();
}
This method will always ensure that everything is disposed that should be despite what exceptions may be thrown. For example, if the StreamReader constructor throws an exception, the FileStream would still be properly disposed.
Visual studio is 'warning' me that I am disposing of fs more than once.
You are, but that is fine. The documentation for IDisposable.Dispose reads:
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.
Based on that, the warning is bogus, and my choice would be to leave the code as it is, and suppress the warning.
As Dan's answer only appears to work with StreamWriter, I believe this might be the most acceptable answer.
(Dan's answer will still give the disposed twice warning with StreamReader - as Daniel Hilgarth and exacerbatedexpert mentions, StreamReader disposes the filestream)
using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
string line;
while ((line = tr.ReadLine()) != null)
{
// Do work here
}
}
This is very similar to Daniel Hilgarth's answer, modified to call dispose via the Using statement on StreamReader as it is now clear StreamReader will call dispose on FileStream (According to all the other posts, documentation referenced)
Update:
I found this post. For what it is worth.
Does disposing streamreader close the stream?
Yes, the correct way would be to use your first alternative:
using (FileStream fs = new FileStream(filePath, FileMode.Open,
FileAccess.Read, FileShare.ReadWrite))
{
TextReader tr = new StreamReader(fs);
// do stuff here
}
The reason is the following:
Disposing the StreamReader only disposes the FileStream so that's actually the only thing you need to dispose.
Your second option (just the inner "using") is no solution as it would leave the FileStream undisposed if there was an exception inside the constructor of the StreamReader.
It's because the way you used StreamReader disposes the stream when it is disposed. So, if you dispose the stream too, it's being disposed twice. Some consider this a flaw in StreamReader--but it's there none-the-less. In VS 2012 (.NET 4.5) there is an option in StreamReader to not dispose of the stream, with a new constructor: http://msdn.microsoft.com/en-us/library/gg712952
Two solutions:
A) You trust Reflector or Documentation and you know *Reader and *Writer will close the underlying *Stream. But warning: it won't work in case of a thrown Exception. So it is not the recommended way:
using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
// Code here
}
B) You ignore the warning as documentation states The object must not throw an exception if its Dispose method is called multiple times. It's the recommended way, as it's both a good practice to always use using, and safe in case of a thrown Exception:
[SuppressMessage("Microsoft.Usage", "CA2202:Do not dispose objects multiple times")]
internal void myMethod()
{
[...]
using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (TextReader tr = new StreamReader(fs))
{
// Code here
}
}
Given all the nonsense this (perfectly legitimate!) question generated, this would be my preference:
FileStream fs = null;
TextReader tr= null;
try
{
fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
tr= new StreamReader(fs);
// Code here
}
finally
{
if (tr != null)
tr.Dispose();
if (fs != null)
fs.Dispose();
}
The links below illustrate perfectly legal syntax. IMO, this "using" syntax is far preferable to nested "using". But I admit - it does not solve the original question:
http://blogs.msdn.com/b/ericgu/archive/2004/08/05/209267.aspx
.NET - Replacing nested using statements with single using statement
IMHO...