I am using some XmlReader and XmlWriter object to do some needed work on strings inside some try...catch blocks.
I know that using the notation using (XmlReader NewReader = XmlReader.Create(...)) is the prefered syntax, but I don't really like that so I am also appending finally blocks and executing NewReader.Close(); and NewWriter.Close();.
However, code analysis is complaining that these objects aren't being disposed, thus forcing me to somehow call Dispose() method.
The problem is that in these classes the Dispose() method is implemented explicitly, so I have to use ((IDisposable)(NewReader)).Dispose(); and ((IDisposable)(NewWriter)).Dispose();.
Are there any drawbacks to this technique?
There are good reasons for not using using:
When the lifetime of the object might need to survive longer than the current block
and there are poor reasons for avoiding using:
"I don't really like that"
Does the good reason apply to your code?
Please also note that a simple extension method would make the syntax nice and clean again.
The C# using statement will always call Dispose for you. It roughtly translates to the following:
XmlReader NewReader = XmlReader.Create(...);
try
{
// do stuff on NewReader
}
finally
{
((IDisposable)NewReader).Dispose();
}
So wrapping it in finally youself doesn't add any value. Although Close and Dispose are often equivalent, it is not always the case. Because of this FxCop is right, you should always call Dispose and when you do this (or let the using statement do this), there is no reason to call Close manually.
The using statement is really the preferred solution. It's idiomatic in C#. These classes implement IDisposable explicitly because they already provide a method that has closing semantics: Close. My bet is that Dispose calls Close, or vice-versa. But you shouldn't count on that, and should always call Dispose anyway.
In the end, all these are equivalent:
Use using, which is preferred;
Call Close on the finally block, and suppress the static analysis warnings or;
Call Dispose on the finally: ((IDisposable)NewReader).Dispose();
Related
This question already has answers here:
What is the C# Using block and why should I use it? [duplicate]
(9 answers)
Closed 9 years ago.
I was wondering if it necessary to call Close() in the following situation (if not necessary, why?)
using (var file = System.IO.File.OpenText(myFilePath))
{
...
}
I presume it is necessary in the following situation
StreamWriter file2 = new System.IO.StreamWriter(myFilePath);
newFileContents.ForEach(file2.WriteLine);
file2.Close();
Is it correct?
Edit:
I thought my question is close() - specific, might have been the difference between reading and writing....
With the using construct you get IDisposable.Dispose called automatically at the end of the code block which will close the file. If you don't use the using statement you have to call Close yourself.
With using you also automatically get built-in try/finally exception handling which will behave more gracefully if something goes wrong before you exit your using block. That's another reason using using is a good idea instead of rolling your own.
In your case the using construct is shorthand for:
StreamWriter file2 = new System.IO.StreamWriter(myFilePath);
try
{
newFileContents.ForEach(file2.WriteLine);
}
finally
{
if (file2!= null)
((IDisposable)file2).Dispose();
}
If you decompile StreamWriter's implementation of Dispose you will see this call (among others):
this.stream.Close();
in the finally block (If you need absolute proof ;-)
OpenText returns a StreamReader, which inherits TextReader, which inherits the IDisposable interface, which specifies a Dispose() method.
When the using statement goes out of scope, it calls the Dispose() method on the StreamReader's implementation of Dispose(), which in turn closes the stream (i.e. the underlying file) The whole thing is wrapped in a try/finally block under the covers, which guarantees that Dispose() will always be called.
the using structure calls the object's Dispose method when it reaches the end of the script block. This method closes the file.
Any time you work with a file, you need to make sure to call Dispose or Close in order to let Windows know that the file is free for editing.
Correct.
If wrapped in using clause, a file.Close() is not required
If not wrapped, you are responsible for Close() and Dispose()
I was wondering if it necessary to call Close() in the following situation (if not necessary, why?)
No because the using statement disposes of the object - and thus removes unmanaged resources.
I presume it is necessary in the following situation
Yes, here it would be required so that the unmanaged resources were cleaned up.
It's not necessary because Dispose() will close the underlying file. However since that's an implementation detail it's good practice to explicitly call Close() when you're done with the stream.
What is better, the using directive, or the dispose directive when finished with an object?
using(FileStream fileStream = new FileStream(
"logs/myapp.log",
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite))
{
using(StreamReader streamReader = new StreamReader(fileStream))
{
this.textBoxLogs.Text = streamReader.ReadToEnd();
}
}
On the other hand, when I'm dealing with System.Net.Mail, I'm told I need to Dispose() of the object to release any stray locks.
Is there any consistent guidance? How do I tell what is more appropriate in a given situation for a given object?
The using statement (not directive) involves an implicit call to Dispose(), in a finally block. So there is no contradiction here. Can you link to that discussion?
The official definition of
using (x) { ... }
is
try ... finally if (x != null) x.Dispose(); }
What is better?
From a notational perspective, the using() { } block. Technically they are the same.
It's the same thing. Usage is simple, if you create the object and use it in only one method then use using. If you need to keep it alive beyond the method call then you have to use Dispose().
The runtime callable wrappers for COM objects don't have a Dispose() method.
There's no reason that I can think of to manually call Dispose(), other than in another implementation of Dispose() (for example in a class you've created that implements IDisposable) when you can wrap an object in a using block. The using block puts the creation and disposal of the object in a try/catch/finally block to pretty much gaurantee that the object will be disposed of correctly.
The compiler is more reliable than me. Or you. =)
MSDN documents the using statement and calls out where you can obtain the C# language specification where you can review section 8.13 "The using statement" (at least in the v4.0 reference it's 8.13) that gives a comprehensive explanation of the using statement and how to use it. The fifth paragraph gives the following:
A using statement is translated into
three parts: acquisition, usage, and
disposal. Usage of the resource is
implicitly enclosed in a try statement
that includes a finally clause. This
finally clause disposes of the
resource. If a null resource is
acquired, then no call to Dispose is
made, and no exception is thrown.
using(foo)
{
foo.DoStuff();
}
is just syntactic sugar for this:
try
{
foo.DoStuff();
}
finally
{
if(foo != null)
foo.Dispose();
}
So I'm not sure where the debate comes from. using blocks do call dispose. Most people prefer using blocks when possible as they are cleaner and clearer as to what is going on.
As long as the lifetime of the object is within a block of code, use using, if your object needs to be long lived, for example to be disposed after an asynchronous call you need to manually call Dispose.
A using block is way better than you of remembering the call to Dispose in all possible and impossible ways execution can leave a block of code.
using calls Dispose upon exit. using is better because it assures calling dispose.
using blocks automatically call Dispose() when the end of the block is reached.
There is one really important reason to use the "using statement" anywhere you can.
If the code that wrapped via using statement threw an exception, you could be sure that the "using object" would be disposed.
Typically I declare streams inside of a using statement to ensure that the stream is properly disposed when I am done with it, and so that I don't mistakenly call it when I'm outside the using block.
Some examples here:
MSDN using Statement Reference
How does one use a using statement with an array of streams?
Would it be equivalent to declare the array outside of a try/catch/finally block and call each stream's dispose method in the finally block?
Lastly, how does one test that the streams have been properly disposed?
I would create a new object that holds the Streams in it. Something like this (not totally fleshed out):
class StreamHolder : IDisposable
{
List<Stream> Streams {get;}
public void Dispose()
{
Streams.ForEach(x=>x.Dispose()):
}
}
This way you can put the container object in the using statment, it will will handle the stream disposal for you. Your other option is the handle it in the Finally block, but if I were going to do this in more than one place, I would like to encapsulate it, so I don't accidentally forget to dispose of all the streams when I am done.
You have to do what using does without using using.
This means looping through the array and calling Dispose on each instance ( after casting explicitly to IDisposable, if needed). Do that loop in the finally block.
Lastly, you don't test for them being properly disposed. Once you call Dispose, you should just null them out, since you can't use them anymore.
The using () statement is a pattern for disposal intended to re-inforce good habits, but it is not required. There are plenty of situations where you need to store objects outside the lifetime of a scoped function.
While I think you should probably architect in a way where you can use the using() statement, the essential thing is that in some encopassing finally block, you dispose each of the streams.
To answer the last question, it does not appear that the Stream class has any method or property to determine if it has been closed/disposed, however if the IDisposable pattern is implemented correctly, you can call Close/Dispose again if you need to be sure.
This question already has answers here:
Closed 13 years ago.
Possible Duplicates:
Which is better, and when: using statement or calling Dispose() on an IDisposable in C#?
When should I use “using” blocks in C#?
using a using if statement?
Properly, how will I use a using statement? I have a tutorial open and i do not understand it. And i can see more than 1 different ways to implement. Which is correct or favored way?
The using statement is for any object which implements IDisposable.
using (var my_object = new IDisposableObject ())
{
//do my_object code here.
} //once the program reaches here, it calls my_object.Dispose();
Generally, this is used for objects with connections that manually need to be handled (closed) when the program is finished with them. For example, open connections to files and to the database.
The using statement will call Dispose even if there is an error in the code so it is akin to calling the Dispose method in a finally block of the try catch statement.
Example/Tutorial
It's a shorter syntax to make sure that dispose is called:
using (File f = File.Open("..."))
{
}
is the same as
File f;
try
{
f = File.Open("...");
}
finally
{
f.Dispose();
}
There are 2 fundamental ways you can use the using statement. As extracted from using Directive (C#) from MSDN.
Create an alias for a namespace (a using alias).
Permit the use of types in a namespace, such that, you do not have to qualify the use of a type in that namespace (a using directive).
Just to expand on Kevin's answer, a using statement effectively wraps your object instantiation in a try/finally block calling the object Dispose() method in the finally section i.e.
using(myObject m = new myObjecyt())
{
// Code here
}
is the same as
myObject m = new myObjecyt()
try
{
// Code here
}
finally
{
m.Dispose();
}
this can be verified by checking the MSIL.
The "Using" keyword helps do a certain thing to be done safely and clearly in .net. This is propertly disposing of certain objects. You may have learned how in .Net we have garbage collection, which means for many object we don't have to care about them when we are done using them. Other object however need to have a method called on them called Dispose. The best practice is that whenever an object has a Dispose method, then we should call that method when we're done with that object.
(They typically handle unmanaged resources. This means that it's using memory or other computer parts that are outside the control of the .NET runtime. So, when garbage collection reaches the discarded .Net object, it is unable to propertly let go of these resources. This can cause memory leaks and all kinds of other problems. A good example is an ADO.NET Connection object. Repeatedly not Disposing of your connection objects can cause DB problems.)
The Using keyword is also tied to this "Dispose" method. It's more accurate to say that when an object has a Dispose method, we either (A.) call .Dispose when we're done with it, or (B.) put our code that uses that object within a Using block. The Using block does several things for you:
When the code moves out of the block, the important Dispose method is automatically called for you.
More importantly, if there is an error in the code block, the Dispose method will still be called. This is why the using block is really helpful. Otherwise you have to put in a lot of error handling code.
The key is that for many of these object that have a Dispose method, error handling is particularly imortant. For a lot our code we don't need error handling; the consequences of an error happening are not really a problem. But for these IDisposable objects errors are often a problem, maybe a big problem. So, .Net provides a syntax for busy developers to add the most basic error handling and move on. Always start off with a Using block at least; maybe later you'll move to fancier error handling, but at least you've got this basic safety.
Here's a good explanation of the Using keyword.
This question already has answers here:
What are the uses of "using" in C#?
(29 answers)
Closed 8 years ago.
Are there particular instances where I should (or shouldn't?) be using "using" blocks:
using(SomeType t = new SomeType()){
...
}
Some objects need some action to be taken when you have finished with them. Usually this is because the object uses some kind of resource that needs to be disposed of. For example, if you have a file object of class File, and this object opens a file from the file system, the file in the file system will need to be closed again.
If you just left the file object, and forgot to call file.Close() it wouldn't be cleaned up until the Garbage Collector (GC) ran and worked out nothing was still using the file object. When the Garbage Collector runs should be left to the Common Language Runtime (CLR) to decide. If the GC doesn't run for quite a while after you have finished with the file, the file could remain open potentially for a long time. This can pose a big problem if there are many file objects, or if something wants to open a file, but can't because the file object you left is still hanging around.
To solve this problem, C# has the IDisposable interface. This has one method called Dispose. Classes that require some cleanup implement this Dispose method. This gives you a standard way for cleaning up any objects that use resources. There are a lot of classes that need to have Dispose called. The problem with this is that code gets covered with calls to Dispose, and they are tricky to follow because the place where you new'ed the object and call Dispose to clean it up are different. So, you had to look around the code a lot and be very careful to check there were calls to Dispose in the right place.
To solve this problem C# introduced the 'using' keyword. You can put a 'using' keyword around where you new an object, and this ensures Dispose will be called on it for you. It guarantees that Dispose will be called whatever happens... even if there is an exception thrown within the body of the using statement.
So, you should use 'using' when you want to be sure an object that allocates resources will be cleaned up.
using can only be used for objects that are declared on the stack, i.e. in a function. It doesn't work for objects that are declared as members of a class. For them, you have to call Dispose yourself. You may have to implement Dispose in your class so that in can call Dispose on any member objects it has that require it.
Common objects that need using called on them are: Files, Database connections, Graphics objects such as Pen and Brush.
Sometimes it is also used when you want two operations to happen together. For example if you want to write a log statement when a block of code is entered and when it exits you could write a log class that you could use like this:
using( Log log = new Log("Doing stuff") )
{
// Stuff
}
The constructor for the log class could be made to write out the message, and the Dispose method could also write it out. Implement the finalizer (~Log) to assert if the Dispose method doesn't get called to ensure the 'using' is remembered around the 'new Log'.
When the SomeType class implements IDisposable.
Use using whenever the type implements IDisposable, unless you're going to wrap it in a try/catch block anyway, then you might as well (depending on what look you prefer) use a finally block.
I see plenty of other answers indicated when you should have a using statement. I want to address when specifically should not have a using statement:
If you need to use your object outside of the scope of the current function, don't have a using block. Good example are a factory method that returns a database connection or a method that needs to return a datareader. In either of those cases if you create your object with a using statement it would be disposed before the method returned, and therefore not usable outside the method.
Now, you still want to be sure that those objects are disposed, so you still might want a using statement somewhere. Just don't include it in the method where the object is actually created. Instead, you can wrap the function call itself in a using statement.
When SomeType implements IDisposable.
That is a clue to you the developer that SomeType uses unmanaged resources that need to be cleaned up.
Example:
using(SqlConnection MyConnection = new SqlConnection("Connection string"))
{
MyConnection.Open();
//...
// 1. SQLConnection is a type that implements IDisposable
// 2. So you can use MyConnection in a using statement
// 3. When using block finishes, it calls Dispose method of
// SqlConnection class
// 4. In this case, it will probably close the connection to
// the database and dispose MyConnection object
}
You can create your own objects that implements IDisposable:
public class MyOwnObjectThatImplementsIDisposable : IDisposable
{
//... some code
public void Dispose()
{
// Put here the code you want to be executed when the
// using statement finish.
}
}
So you could use an object of MyOwnObjectThanImplementsIDisposable type in a using statement:
using(MyOwnObjectThatImplementsIDisposable MyObject = new MyOwnObjectThatImplementsIDisposable)
{
// When the statement finishes, it calls the
// code you´ve writed in Dispose method
// of MyOwnObjectThatImplementsIDisposable class
}
Hope this helps
In this context the using statement is handy for types that implement IDisposable. When the code block exits the scope of the using statement, Dispose() is called implicitly. It's a good habit when working with objects you want to dispose immediately after use.
One specific instance in which you should be careful using a using block is with a WCF Service Client.
As noted in this MSDN article, wrapping a WCF client (which does implement IDisposable) in a using block could mask any errors which result in the client being left in a faulted state (like a timeout or communication problem). Long story short, when Dispose() is called, the client's Close() method fires, but throws and error because it's in a faulted state. The original exception is then masked by the second exception. Not good.
There are various workarounds out there, including one in the MSDN article itself. Others can be found at IServiceOriented and blog.davidbarret.net.
I prefer the last method, myself.
If you want a summary rule. Anytime an object using IDisposable where you would not have a catch, use using. Using, essentially, is this pattern:
try
{
//instantiate and use object
}
finally
{
//dispose object
}
If you do not need a catch, using can save you typing, which is a good thing.
The primary rule is:
* Use USING statement when objects implements IDisposable interface.
This interface provides the Dispose method, which should release the object's resources. If this method is not invoked then the object will stay in memory as long, as CLR wants to perform garbage collection. If the programmer use the USING statement then on the end the object will be disposed, and all resources will be free.
It is very important that all resources that are no longer in use be free as soon as possible.
For more information about it just visit this link: microsoft
Maybe it is worth mentioning that underlying reason for adding “using” lo C# languge is following: some resources can be scarce enough that it doesn’t make sense to wait for GC to call IDisposable. For example, DB connections. If you use try/catch/finally you won’t end up with a dangling connection, but connection will be left hanging until GC doesn’t kick in and this can take a while (if you do not close it explicitly). IF you use "using" (excuse the pun) you will release the connection immediately even if you forgot to close it and even if some exception occured inside the using block.
Another reason, as previous post mentions, is that programmers do not always use finally to clean up. If not using finally in the case of exception you end up with leaking resources…
One situation is when you want to do something at the beginning of a code block, and then undo it at the end of the block, unconditionally (even if there is a throw).
The ctor for the disposable class that you build (and call within the using) would perform the action, and then the Dispose method would undo that action. This is typically how I use it.
Other people has mentioned about "IDisposable" already.
But one of the caveats when using "using" statement is that,
any exceptions thrown within "using" will not be caught
even thought "SomeType" will be disposed regardless.
So in the following snippet,
using (SomeType t = new SomeType()){
throw new Exception("thrown within using");
}
throw new Exception("thrown within using"); should not be disregarded.
I would also add that use a using() statement if something implements IDispose and also if that something you want to dispose of holds on to NON-MANAGED resources like database connections and file handles.
If it's a normal object with say a List<T>, where T is like a Customer object that holds names and address, then you don't need to. The garbage collector is smart enough to manage this for you. But the garbage collector WILL NOT return connections to the connection pool or close file handles.