This question already has answers here:
What is causing 'CA2202: Do not dispose objects multiple times' in this code and how can I refactor?
(2 answers)
Closed 10 years ago.
I'm currently using two objects as follows:
using (var ms = new MemoryStream())
using (var bw = new BinaryWriter(ms))
{
// work with ms and bw, both referenced here
}
It works "fine" and is in fact an answer here too. However, when I run VS2012's code analysis tool, I get a warning like:
CA2202 Do not dispose objects multiple times
Object 'ms' can be disposed more than once in method '<my method name>'.
To avoid generating a System.ObjectDisposedException you should not
call Dispose more than one time on an object.
This leads me to believe there might an alternative way to handle this situation, but I don't know what it is.
Does anyone know what is the 'proper' way to use two objects within a single using block in a warning-free manner?
The BinaryWriter class is written such that it will dispose the underlying stream that you pass to it when you dispose the BinaryWriter unless you use the optional constructor parameter to tell it not to. Since the binary writer is disposing the underlying memory stream and you are also disposing of it in the using block it is being disposed of twice.
Now, this isn't really an issue for most classes, as they should be (and memory stream is) written to work just fine if disposed twice, as long as you don't use it after it's dispose (you're not) so you can safely ignore the warning. If you want the warning to go away you can just take the memory stream out of the using since it's already being disposed.
If you don't need the object reference to MemoryStream this would work fine
using (var bw = new BinaryWriter(new MemoryStream()))
{
var memStream = bw.BaseStream as MemoryStream;
}
Otherwise go for the multiple using. Especially in cases where you are using .NET framework objects, since they are implemented with correct dispose pattern.
A double using statement can cause trouble when the object that could have a second dispose call does not implement the dispose pattern correctly.
PS: do you get the same warning this way:
using (var memStream = new MemoryStream())
{
using (var bw = new BinaryWriter(memStream))
{
// work with ms and bw
}
}
You assumption that this is a single using block is incorrect.
The code in your example is syntactic sugar for a nested using block like this:
using (var ms = new MemoryStream())
{
using (var bw = new BinaryWriter(ms))
{
// work with ms and bw, both referenced here
}
}
There is no single way to right this down correctly and avoid the warning. In cases where the inner resource wraps the outer resource and they both go out of scope together, you need to consult the documentation: if the contract states that the inner resource's dispose methods invoked the dispose of the wrapped object, you simply do not need to put the outer resource in a using block.
That being said, there will probably be no mention of such details in most docs. You can check the behavior by yourself (by inheriting and observing side effects) but than you must be careful since in the first place it was not documented, this behavior may change in future releases.
So if you are very worried about it, you can leave the double using block (just in case), suppress the warning and put the whole block in a try{} catch (ObjectDisposedException e) {} to safe gourd yourself completely.
Sounds paranoid? depends on what library you are working with :-)
I think you shouldn't use "using( ... ){}" for:
using (var ms = new MemoryStream())
because the OS managment memory and "dispose" it!
Related
This question already has answers here:
What are the uses of "using" in C#?
(29 answers)
Closed 2 years ago.
I have been working with C# ASP.NET MVC5 for a while and I was wondering something.
Why are we utilizing the using() to instantiate a class sometimes?
For an example:
using (CyrptoStream cs = new CryptoStream(PARAMETERS)
{
//SOMETHING
}
or
using (Aes encryptor = Aes.Create()
{
}
How is that different to MyClass myClass = new MyClass()
Thank you.
using cleans up after itself when control leaves the scope block. Use it to ensure Dispose is called on the object that was created in the top line (next to using)
Multiple using blocks can be nested, though the neater form is to see them stacked in a line with only the bottom one having curly brackets
using(X x = new X())
using(Y y = new Y())
using(Z z = new Z())
{
//code here
}
However it comes to be that control leaves the //code here part (return, reaching the end, throwing exception etcp) we can guarantee that x.Dispose(), y.Dispose() and z.Dispose() will be called. Not everything has to be disposed, which is why we don't see using all over the place, but for some things ensuring Dispose is called is highly advisable in order to free up resources. The documentation for whatever library you're using will advise on whether disposing is recommended. Some prime examples in everyday c# use where disposal is recommended are database connections, graphics/gdi drawing, stream writing.. if you're unsure whether to dispose something or not, take a look at it in the object browser and see if it implements IDisposable; if it does then err on the side of caution and dispose it when you're done with it, but don't dispose of anything you didn't create (it's not your job to do so)
So I have a code and it goes like this,
using (MemoryStream mStream = new MemoryStream())
{
MemoryStream fileStream = new MemoryStream();
}
I assign memory streams to both of them, and both of them can be very large. I know the nested memory stream is only accessible inside the using clause, but I'm worried if this will cause memory leaks or this memory stream would still be in the memory even after the code is executed. Is it safe to use this on GB files?
PS: Sorry if this question is obvious or stupid, I'm a newbie on this matter.
When using any class that implements IDisposable you should call the Dispose method as soon as you are done with it to allow it to release resources that your program no longer requires.
The using keyword handles this for you by calling Dispose on the indicated object at the end of the statement block. Only the specified object is disposed however. Thus in your case the mStream object will be disposed, but the fileStream object will not. It will remain in memory, using allocated memory space and so on, until the garbage collector cleans it up.
While the garbage collector will take care of it eventually, it is good practice to dispose of every object that implements IDisposable as soon as you're done with it. The using statement helps, but there are going to be times when you want to keep using the object outside the scope of the creating method. For instance, if you want to return a MemoryStream from a method, the object still needs to be disposed at some point.
Learn which classes you use implement IDisposable, and get into the habit of disposing them. It'll save you grief later on.
To actually dispose of the second MemoryStream properly, you need to nest your usings:
using (MemoryStream mStream = new MemoryStream())
using (MemoryStream fileStream = new MemoryStream())
{
…
}
Either way, though, if you need more memory and there are no references left to the MemoryStream, it’ll be reclaimed. Fitting everything in memory at once, on the other hand, is just a matter of actual memory versus file size…
You should use:
using (MemoryStream mStream = new MemoryStream())
{
//Don't name this a fileStream if it's actually a MemoryStream?
using(MemoryStream fileStream = new MemoryStream())
{
}
}
I know the Dispose() method is called on the StreamReader object when you have the following code:
//Sample 1
using (StreamReader sr1 = new StreamReader(#"C:\Data.txt"))
{
string s1 = sr1.ReadToEnd();
//Do something with s1...
}
But if you write the code like this (Sample 2) will the Dispose() method get called too?
//Sample 2
StreamReader sr2 = new StreamReader(#"C:\Data.txt");
using (sr2)
{
string s2 = sr2.ReadToEnd();
//Do something with s2...
}
Yes, Dispose() would be called in both examples. They are functionally equivalent except that in the second example the disposed StreamReader would still be in scope. Therefore the first method is preferred, as using a disposed object is usually a Bad Idea.
However as others have pointed out, it is sometimes OK to use a disposed object. In such cases you might want to use your second example. But you have to know what you're doing and I would avoid it if at all possible.
Yes, absolutely. The details are in section 8.13. There isn't a concise statement of your exact question, but:
A using statement of the form
using (expression) statement
has the same three possible expansions, but in this case ResourceType is implicitly the compile-time type of the expression, and the resource variable is inaccessible in, and invisible to, the embedded statement.
The "three possible expansions" referred to cover the more common case of declaring a variable at the same time. Basically the important thing is that it behaves the same way, aside from the variable's scope. Dispose will still be called - otherwise there'd be no point in putting in a using statement at all :)
Note that the two aren't quite equivalent in terms of what's valid within the block, because a variable declared by the using statement is readonly. So this is valid:
// Warning but no error
MemoryStream ms = new MemoryStream();
using (ms)
{
ms = null;
}
but this isn't:
using (MemoryStream ms = new MemoryStream())
{
// error CS1656: Cannot assign to 'ms' because it is a 'using variable'
ms = null;
}
Note that even in the case where it's valid, it's the original value of ms which is used for disposal. The compiler makes this clear:
warning CS0728: Possibly incorrect assignment to local 'ms' which is the argument to a using or lock statement. The Dispose call or unlocking will happen on the original value of the local.
Note that this form doesn't generate a warning:
// No warning
MemoryStream ms;
using (ms = new MemoryStream())
{
ms = null;
}
On the other hand, aside from that, they really will behave in the same way.
EDIT: As bzlm notes, the fact that the variable remains in scope after the using statement means it's usually not a good idea. However, objects which have been disposed aren't always unusable. For example:
MemoryStream ms = new MemoryStream();
using (ms)
{
// Do stuff
}
byte[] data = ms.ToArray();
That will work just fine - the MemoryStream keeps the data even when it's disposed. It still feels somewhat wrong to me though.
Both codes are pretty much the same. Dispose will be called once control leaves the using block. If the exception occurs before then it won't be called in either case. But apart from asynchronous exceptions this won't happen in your code.
Is C#'s using statement abort-safe?
Is a similar discussion focusing on the interaction with thread abortion.
using takes an object that implements IDisposable. How and where that object is created is not taken into account, when the compiler generates the code to call Dispose at the end for the using block.
Good question. I would say yes, because the using code block is little more than syntax sugar for the following:
try
{
var myObj = <parameter from using>
<using block code>
}
finally
{
myObj.Dispose();
}
Notice that, when substituted for your using block, the variable you end up disposing has another handle that is visible outside the code block (sr2). This reference will prevent the instance from being garbage-collected after the using statement, but since it's been disposed, unless it's smart enough to recover from a mid-scope Dispose(), it's not going to be of much use.
MSDN says not entirely equal regarding exceptions during initialization. Also consider the following scoping scenario:
//Sample 1
using (StreamReader sr1 = new StreamReader(#"C:\Data.txt"))
{
string s1 = sr1.ReadToEnd();
//Do something with s1...
}
sr1.ReadToEnd() // sr1 not in scope.
Does not compile, but
//Sample 2
StreamReader sr2 = new StreamReader(#"C:\Data.txt");
using (sr2)
{
string s2 = sr2.ReadToEnd();
//Do something with s2...
}
sr2.ReadToEnd() // possible to write, but accessing a disposed object.
compiles but access a disposed object.
This question already has answers here:
Closed 13 years ago.
Possible Duplicates:
Using the using statment in c#
What is the C# Using block and why should I use it?
Just wondering what this means? I've seen lots of tutorials online that have the syntax:
using (SqlCeCommand cmd2 = new SqlCeCommand("SELECT city FROM cities JOIN states ON states.id=cities.state WHERE states.state='" + read.GetString(0) + "'", con))
{
SqlCeDataReader readCities = cmd2.ExecuteReader();
while (readCities.Read())
{
parent.Nodes.Add(readCities.GetString(0));
}
}
Why is it used? I tried searching Google, but it comes up with the 'using' keyword which is used for including dll's and other files.
The using statement
using(var disposableObject = new object_that_implements_IDisposable()) { ... }
is syntactic sugar for code similar to following:
var disposableObject = new object_that_implements_IDisposable()
try
{
...
}
finally
{
if(disposableObject != null)
{
((IDisposable)your_object).Dispose();
}
}
This is only applicable for classes that implement IDisposable. It is helpful for cleaning up code where you have objects that take, for example, system resources (file handles, database connections, sockets, etc.) that need to be cleaned up after you are done to free the resource for the rest of the system.
In theory, you could leave out the .Dispose() call, but then you would have to wait for the Garbage Collector to free the kept resources. The GC is awesome at knowing when to free objects to reclaim their memory usage, but it has no idea that it needs to free objects to have them give up other system resources. Thus, these critical resources might not be given up until after the GC decides it needs the memory used by the owner. Thus, you definitely want to dispose of your objects when you are done with them (if they are disposable)!
As to why you'd use using over try/finally, it is purely a coder's preference. I prefer using because you can cascade them:
using(var a = new class())
using(var b = new class())
using(var c = new class())
using(var d = new class())
{
...
}
You'd need quite a few more lines of code to do that with try/finally.
using has additional advantages as well. For example, whereas calling x.Dispose directly might throw a NullReferenceException if x is null, using(x) will not.
See also:
Link
Using the using statement in C#
What is the C# Using block and why should I use it?
http://msdn.microsoft.com/en-us/library/yh598w02%28VS.80%29.aspx
The using just instructs the compiler to write code that will call the Dispose method on the variable you're using. Only types that implement 'IDisposable' can be used with using statements.
In your example, cmd2 will be disposed when the code in the {} finishes.
"Defines a scope, outside of which an object or objects will be disposed"
See using statement
using is applied to objects that implement IDisposable. It ensures that, when leaving the using block (whether normally, or via an exception, or whatever), the disposable object's Dispose method is called.
Using statement is defines a scope, that outside of it the object or objects will be disposed.
Using is a cool way of cleaning up resources, it is equivalent to try{}catch{}finally{dispose}. Effective c# has an item on this and I bet you willget 10+ similar answers.
http://www.amazon.com/Effective-Specific-Ways-Improve-Your/dp/0321245660
See http://msdn.microsoft.com/en-us/library/yh598w02(VS.80).aspx
The using statement encapsulates variables inside of it in a scope. When the execution exits the using block, all objects inside of it are disposed from memory. You might see it with DB connections so that when the using block exits the resources allocated to that connection are cleaned up and closed.
The using statement defines a scope in which to use an object which implements the IDisposable interface. The object will be cleaned up once the block has been exited. See:
http://msdn.microsoft.com/en-us/library/yh598w02.aspx
What is the difference between the following two snippets of code:
using (Object o = new Object())
{
// Do something
}
and
{
Object o = new Object();
// Do something
}
I have started using using a lot more but I am curious as to what the actually benefits are as compared to scoping objects.
Edit: Useful tidbits I took from this:
Jon Skeet:
Note that this does not force garbage collection in any way, shape or form. Garbage collection and prompt resource clean-up are somewhat orthogonal.
Will Eddins comment:
Unless your class implements the IDisposable interface, and has a Dispose() function, you don't use using.
The first snippet calls Dispose at the end of the block - you can only do it with types which implement IDisposable, and it basically calls Dispose in a finally block, so you can use it with types which need resources cleaning up, e.g.
using (TextReader reader = File.OpenText("test.txt"))
{
// Use reader to read the file
}
// reader will be disposed, so file handle released
Note that this does not force garbage collection in any way, shape or form. Garbage collection and prompt resource clean-up are somewhat orthogonal.
Basically, you should use a using statement for pretty much anything which implements IDisposable and which your code block is going to take responsibility for (in terms of cleanup).
At the end of using the object gets disposed (the object you put inside the parenthesis has to implement IDisposable). The object gets disposed also in exception cases. And you do not have to wait for the GC to do it at some time (you control it).
EDIT: The disadvantage of the scoping are:
you do not control the disposition of the object
even if you would call dispose at the end of your scope, it would not be exception safe
Just to literally show the difference...
using (FileStream fileStream = new FileStream("log.txt", FileMode.OpenCreate))
{
//stuff with file stream
}
is the same as...
{
FileStream fileStream = new FileStream("log.txt", FileMode.OpenCreate);
try
{
//stuff with filestream
}
finally
{
if (fileStream != null)
((IDisposable)fileStream).Dispose();
}
}
where as
{
FileStream fileStream = new FileStream("log.txt", FileMode.OpenCreate);
fileStream.Dispose();
}
is as it is.
See the documentation regarding IDisposable and determinable resource deallocation.
Simply put, at the end of the using{} block, you can reliably dispose of allocated resources (e.g. close file handles, database connections etc.)
using just requires an implementation of the IDisposable interface, and calls the Dispose method at the end of the scope.
For plenty of raging arguments about properly disposing objects, there's plenty of other threads.