I have a method which return an instance of resource class, how can use the "using" statement to avoid resource leak?
public ResourceClass method()
{
return new ResourceClass ();
}
I hope my question is clear.
Thanks in advance
You can relegate the responsibility of disposing it to your caller by declaring your using block around the entire usage of the resource
public ResourceClass method()
{
return new ResourceClass();
}
then
using(var s = method())
{
//do something with s
}
This is as simple as just utilising the method method to create the instance in your using statement:
public void YourMethod()
{
using (ResourceClass Foo = method())
{
...
}
}
This will only work of course if ResourceClass implements IDisposable. Otherwise, your program won't compile.
We have the following construct in our codebase, used to ensure a particular resource is disposed of after use:
using (var disposableThing = DisposableThing.Begin())
{
// do something
disposableThing.Finish(); // must always be called
}
Here's an example of its usage:
List<int> ids;
using (var disposableThing = DisposableThing.Begin())
{
ids = disposableThing.GetSomeIds();
disposableThing.Finish();
}
DoSomethingElseWith(ids);
Since this pattern is so common, we wrote a method on DisposableThing to encapsulate it:
static void ExecuteWithFinish(Action<DisposableThing> action)
{
using (var disposableThing = Begin())
{
action(disposableThing);
disposableThing.Finish();
}
}
which allows us to rewrite the second sample as:
// #4
List<int> ids;
DisposableThing.ExecuteWithFinish(disposableThing =>
{
ids = disposableThing.GetSomeIds();
});
DoSomethingElseWith(ids); // compiler error "Use of unassigned local variable 'ids'"
But the compiler refuses to compile that code because it has no way to know that ids will always be assigned after ExecuteWithFinish has completed (or thrown an exception, which will prevent the execution of DoSomethingElseWith anyway).
I know I could add an overload of ExecuteWithFinish that returns values from a passed-in Func, which is ugly.
I know I could subclass DisposableThing and override its Dispose method to call Finish, which is a cleaner, neater, and faster way than constructing a delegate each time (this is probably what I'll end up doing).
But for my own edification and in the spirit of "what if", is it possible to inform or even trick the compiler into allowing the code in #4 as written?
edit: Yes, I know I could write List<int> ids = null; and circumvent this issue entirely, but (a) I'd prefer not to perform unnecessary assignments (b) I'd like to change the code as little as possible.
I would take a different approach here.
I'm going to make the assumption that for some reason you must have a Finish() method that must always be called before Dispose(), which must also always be called.
That may be a rash assumption, and it does rather beg the question: Why don't you put the functionality of Finish() into the Dispose()? However...
Firstly, create an interface to encapsulate a disposable thing with a Finish() method:
public interface IDisposableThingWithFinish : IDisposable
{
void Finish();
}
and change your DisposableThing class so that it implements IDisposableThingWithFinish.
Then you could write a disposable class that encapsulates calling Finish() and then Dispose() like so:
public sealed class DisposingFinisher : IDisposable
{
readonly IDisposableThingWithFinish _item;
public Disposing(IDisposableThingWithFinish item)
{
if (item == null)
throw new ArgumentNullException(nameof(item));
_item = item;
}
public void Dispose()
{
try
{
_item.Finish();
}
finally
{
_item.Dispose();
}
}
}
You would use Finisher like so:
using (var disposableThing = new DisposingFinisher(DisposableThing.Begin()))
{
// Do something.
}
A simple null-assignment will avoid the compiler warning as explained in the documentation of compiler error CS0165:
List<int> ids = null;
Can we use the using statement in a constructor to declare an instance of an object for later usage. For example.
public class TestClass {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
using (this._dataContext = DataClassesDataContext(connString));
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
}
You must implement IDisposable yourself and call Dispose on the data context from you Dispose method.
public class TestClass : IDisposable {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
_dataContext = new DataClassesDataContext(connString);
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
public void Dispose(){
_dataContext.Dispose();
}
}
It's not clear what you expect the using statement to do here. All it does is make sure that Dispose is called at the end of the block.
So basically you'd be creating a DataClassesDataContext (I assume you missed the new keyword...), storing a reference in a field, and then immediately disposing of it. That's not going to work well - you should get rid of the using statement, but quite possibly make your class implement IDisposable so that when the instance of TestClass is disposed, you dispose of the data context.
According to MSDN:
The using statement calls the Dispose method on the object in the
correct way, and (when you use it as shown earlier) it also causes the
object itself to go out of scope as soon as Dispose is called.
The using statement is basically syntactic sugar for try/finally.
try
{
_dataContext = new DataClassesDataContext(connString);
}
finally
{
if (_dataContext != null)
((IDisposable)dataContext).Dispose();
}
Looking at it in this way it should become obvious that datacontext is no longer in scope and therefore can't be used by other methods as you desire. To solve the problem, you should make the class implement IDisposeable.
using (this._dataContext = DataClassesDataContext(connString));
is the same as
try
{
this._dataContext = DataClassesDataContext(connString);
}
catch
{
}
finally
{
if(this._dataContext!=null)
((IDisposable)this._dataContext).Dispose();
}
So you'll get _dataContext disposed in you constructor and it will be no longer available. You should implement IDisposable interface and you'll be able to employ using statement where you want like this:
using (TestClass test = new TestClass("conn"))
{
//your code
}
If I am using the using keyword, do I still have to implement IDisposable?
You can't have one without the other.
When you write :
using(MyClass myObj = new MyClass())
{
myObj.SomeMthod(...);
}
Compiler will generate something like this :
MyClass myObj = null;
try
{
myObj = new MyClass();
myObj.SomeMthod(...);
}
finally
{
if(myObj != null)
{
((IDisposable)myObj).Dispose();
}
}
So as you can see while having using keyword it is assumed/required that IDisposable is implemented.
If you use the using statement the enclosed type must already implement IDisposable otherwise the compiler will issue an error. So consider IDisposable implementation to be a prerequisite of using.
If you want to use the using statement on your custom class, then you must implement IDisposable for it. However this is kind of backward to do because there's no sense to do so for the sake of it. Only if you have something to dispose of like an unmanaged resource should you implement it.
// To implement it in C#:
class MyClass : IDisposable {
// other members in you class
public void Dispose() {
// in its simplest form, but see MSDN documentation linked above
}
}
This enables you to:
using (MyClass mc = new MyClass()) {
// do some stuff with the instance...
mc.DoThis(); //all fake method calls for example
mc.DoThat();
} // Here the .Dispose method will be automatically called.
Effectively that's the same as writing:
MyClass mc = new MyClass();
try {
// do some stuff with the instance...
mc.DoThis(); //all fake method calls for example
mc.DoThat();
}
finally { // always runs
mc.Dispose(); // Manual call.
}
You are confusing things. You can only use the "using" keyword on something that implements IDisposable.
Edit: If you use the using keyword, you don't have to explicity invoke Dispose, it will be called automatically at the end of the using block. Others have already posted examples of how the using statement is translated into a try - finally statement, with Dispose invoked within the finally block.
Yes, the using keyword is syntactic sugar for this type of pattern...(from msdn)
Font font1 = new Font("Arial", 10.0f);
try
{
byte charset = font1.GdiCharSet;
}
finally
{
if (font1 != null)
((IDisposable)font1).Dispose();
}
Edit: a useful example.
When you find you are doing things in a finally section consistantly, for example resetting a Cursor back to its default after having set it to a wait cursor, this is a candidate for this pattern...
public class Busy : IDisposable
{
private Cursor _oldCursor;
private Busy()
{
_oldCursor = Cursor.Current;
}
public static Busy WaitCursor
{
get
{
Cursor.Current = Cursors.WaitCursor;
return new Busy();
}
}
#region IDisposable Members
public void Dispose()
{
Cursor.Current = _oldCursor;
}
#endregion
}
Called like...
using(Busy.WaitCursor)
{
// some operation that needs a wait cursor.
}
Using will only dispose of disposable objects. So wraping a using block around an object that does not implement IDisposable is rather useless will in fact cause a compiler error.
http://msdn.microsoft.com/en-us/library/yh598w02.aspx
As a rule, when you use an IDisposable
object, you should declare and
instantiate it in a using statement.
The using statement calls the Dispose
method on the object in the correct
way, and it also causes the object
itself to go out of scope as soon as
Dispose is called. Within the using
block, the object is read-only and
cannot be modified or reassigned.
The using statement ensures that
Dispose is called even if an exception
occurs while you are calling methods
on the object. You can achieve the
same result by putting the object
inside a try block and then calling
Dispose in a finally block; in fact,
this is how the using statement is
translated by the compiler.
You must implement IDisposable to use using. If you try to use using() on a type that does not implement IDisposable you get the following compile time error:
error CS1674: 'SomeType': type used in a using statement must be implicitly convertible to 'System.IDisposable'
The using keyword already implements, so if you use the using keyword, you do not have to invoke IDisposable
I've got a bunch of properties which I am going to use read/write locks on. I can implement them either with a try finally or a using clause.
In the try finally I would acquire the lock before the try, and release in the finally. In the using clause, I would create a class which acquires the lock in its constructor, and releases in its Dispose method.
I'm using read/write locks in a lot of places, so I've been looking for ways that might be more concise than try finally. I'm interested in hearing some ideas on why one way may not be recommended, or why one might be better than another.
Method 1 (try finally):
static ReaderWriterLock rwlMyLock_m = new ReaderWriterLock();
private DateTime dtMyDateTime_m
public DateTime MyDateTime
{
get
{
rwlMyLock_m .AcquireReaderLock(0);
try
{
return dtMyDateTime_m
}
finally
{
rwlMyLock_m .ReleaseReaderLock();
}
}
set
{
rwlMyLock_m .AcquireWriterLock(0);
try
{
dtMyDateTime_m = value;
}
finally
{
rwlMyLock_m .ReleaseWriterLock();
}
}
}
Method 2:
static ReaderWriterLock rwlMyLock_m = new ReaderWriterLock();
private DateTime dtMyDateTime_m
public DateTime MyDateTime
{
get
{
using (new ReadLock(rwlMyLock_m))
{
return dtMyDateTime_m;
}
}
set
{
using (new WriteLock(rwlMyLock_m))
{
dtMyDateTime_m = value;
}
}
}
public class ReadLock : IDisposable
{
private ReaderWriterLock rwl;
public ReadLock(ReaderWriterLock rwl)
{
this.rwl = rwl;
rwl.AcquireReaderLock(0);
}
public void Dispose()
{
rwl.ReleaseReaderLock();
}
}
public class WriteLock : IDisposable
{
private ReaderWriterLock rwl;
public WriteLock(ReaderWriterLock rwl)
{
this.rwl = rwl;
rwl.AcquireWriterLock(0);
}
public void Dispose()
{
rwl.ReleaseWriterLock();
}
}
From MSDN, using Statement (C# Reference)
The using statement ensures that Dispose is called even if an exception occurs while you are calling methods on the object. You can achieve the same result by putting the object inside a try block and then calling Dispose in a finally block; in fact, this is how the using statement is translated by the compiler. The code example earlier expands to the following code at compile time (note the extra curly braces to create the limited scope for the object):
{
Font font1 = new Font("Arial", 10.0f);
try
{
byte charset = font1.GdiCharSet;
}
finally
{
if (font1 != null)
((IDisposable)font1).Dispose();
}
}
So basically, it is the same code but with a nice automatic null-checks and an extra scope for your variable. The documentation also states that it "ensures the correct use of IDisposable object" so you might as well gets even better framework support for any obscure cases in the future.
So go with option 2.
Having the variable inside a scope that ends immediately after it's no longer needed is also a plus.
I definitely prefer the second method. It is more concise at the point of usage, and less error prone.
In the first case someone editing the code has to be careful not to insert anything between the Acquire(Read|Write)Lock call and the try.
(Using a read/write lock on individual properties like this is usually overkill though. They are best applied at a much higher level. A simple lock will often suffice here since the possibility of contention is presumably very small given the time the lock is held for, and acquiring a read/write lock is a more expensive operation than a simple lock).
Consider the possibility that both solutions are bad because they mask exceptions.
A try without a catch should obviously be a bad idea; see MSDN for why the using statement is likewise dangerous.
Note also Microsoft now recommends ReaderWriterLockSlim instead of ReaderWriterLock.
Finally, note that the Microsoft examples use two try-catch blocks to avoid these issues, e.g.
try
{
try
{
//Reader-writer lock stuff
}
finally
{
//Release lock
}
}
catch(Exception ex)
{
//Do something with exception
}
A simple, consistent, clean solution is a good goal, but assuming you can't just use lock(this){return mydateetc;}, you might reconsider the approach; with more info I'm sure Stack Overflow can help ;-)
I personally use the C# "using" statement as often as possible, but there are a few specific things that I do along with it to avoid the potential issues mentioned. To illustrate:
void doSomething()
{
using (CustomResource aResource = new CustomResource())
{
using (CustomThingy aThingy = new CustomThingy(aResource))
{
doSomething(aThingy);
}
}
}
void doSomething(CustomThingy theThingy)
{
try
{
// play with theThingy, which might result in exceptions
}
catch (SomeException aException)
{
// resolve aException somehow
}
}
Note that I separate the "using" statement into one method and the use of the object(s) into another method with a "try"/"catch" block. I may nest several "using" statements like this for related objects (I sometimes go three or four deep in my production code).
In my Dispose() methods for these custom IDisposable classes, I catch exceptions (but NOT errors) and log them (using Log4net). I have never encountered a situation where any of those exceptions could possibly affect my processing. The potential errors, as usual, are allowed to propagate up the call stack and typically terminate processing with an appropriate message (the error and stack trace) logged.
If I somehow encountered a situation where a significant exception could occur during Dispose(), I would redesign for that situation. Frankly, I doubt that will ever happen.
Meanwhile, the scope and cleanup advantages of "using" make it one of my most favorite C# features. By the way, I work in Java, C#, and Python as my primary languages, with lots of others thrown in here and there, and "using" is one of my most favorite language features all around because it is a practical, everyday workhorse.
I like the 3rd option
private object _myDateTimeLock = new object();
private DateTime _myDateTime;
public DateTime MyDateTime{
get{
lock(_myDateTimeLock){return _myDateTime;}
}
set{
lock(_myDateTimeLock){_myDateTime = value;}
}
}
Of your two options, the second option is the cleanest and easier to understand what's going on.
"Bunch of properties" and locking at the property getter and setter level looks wrong. Your locking is much too fine-grained. In most typical object usage, you'd want to make sure that you acquired a lock to access more than one property at the same time. Your specific case might be different but I kinda doubt it.
Anyway, acquiring the lock when you access the object instead of the property will significantly cut down on the amount of locking code you'll have to write.
DRY says: second solution. The first solution duplicates the logic of using a lock, whereas the second does not.
Try/Catch blocks are generally for exception handling, while using blocks are used to ensure that the object is disposed.
For the read/write lock a try/catch might be the most useful, but you could also use both, like so:
using (obj)
{
try { }
catch { }
}
so that you can implicitly call your IDisposable interface as well as make exception handling concise.
The following creates extension methods for the ReaderWriterLockSlim class that allow you to do the following:
var rwlock = new ReaderWriterLockSlim();
using (var l = rwlock.ReadLock())
{
// read data
}
using (var l = rwlock.WriteLock())
{
// write data
}
Here's the code:
static class ReaderWriterLockExtensions() {
/// <summary>
/// Allows you to enter and exit a read lock with a using statement
/// </summary>
/// <param name="readerWriterLockSlim">The lock</param>
/// <returns>A new object that will ExitReadLock on dispose</returns>
public static OnDispose ReadLock(this ReaderWriterLockSlim readerWriterLockSlim)
{
// Enter the read lock
readerWriterLockSlim.EnterReadLock();
// Setup the ExitReadLock to be called at the end of the using block
return new OnDispose(() => readerWriterLockSlim.ExitReadLock());
}
/// <summary>
/// Allows you to enter and exit a write lock with a using statement
/// </summary>
/// <param name="readerWriterLockSlim">The lock</param>
/// <returns>A new object that will ExitWriteLock on dispose</returns>
public static OnDispose WriteLock(this ReaderWriterLockSlim rwlock)
{
// Enter the write lock
rwlock.EnterWriteLock();
// Setup the ExitWriteLock to be called at the end of the using block
return new OnDispose(() => rwlock.ExitWriteLock());
}
}
/// <summary>
/// Calls the finished action on dispose. For use with a using statement.
/// </summary>
public class OnDispose : IDisposable
{
Action _finished;
public OnDispose(Action finished)
{
_finished = finished;
}
public void Dispose()
{
_finished();
}
}
I think method 2 would be better.
Simpler and more readable code in your properties.
Less error-prone since the locking code doesn't have to be re-written several times.
While I agree with many of the above comments, including the granularity of the lock and questionable exception handling, the question is one of approach. Let me give you one big reason why I prefer using over the try {} finally model... abstraction.
I have a model very similar to yours with one exception. I defined a base interface ILock and in it I provided one method called Acquire(). The Acquire() method returned the IDisposable object and as a result means that as long as the object I am dealing with is of type ILock that it can be used to do a locking scope. Why is this important?
We deal with many different locking mechanisms and behaviors. Your lock object may have a specific timeout that employs. Your lock implementation may be a monitor lock, reader lock, writer lock or spin lock. However, from the perspective of the caller all of that is irrelevant, what they care about is that the contract to lock the resource is honored and that the lock does it in a manner consistent with it's implementation.
interface ILock {
IDisposable Acquire();
}
class MonitorLock : ILock {
IDisposable Acquire() { ... acquire the lock for real ... }
}
I like your model, but I'd consider hiding the lock mechanics from the caller. FWIW, I've measured the overhead of the using technique versus the try-finally and the overhead of allocating the disposable object will have between a 2-3% performance overhead.
I'm surprised no one has suggested encapsulating the try-finally in anonymous functions. Just like the technique of instantiating and disposing of classes with the using statement, this keeps the locking in one place. I prefer this myself only because I'd rather read the word "finally" than the word "Dispose" when I'm thinking about releasing a lock.
class StackOTest
{
private delegate DateTime ReadLockMethod();
private delegate void WriteLockMethod();
static ReaderWriterLock rwlMyLock_m = new ReaderWriterLock();
private DateTime dtMyDateTime_m;
public DateTime MyDateTime
{
get
{
return ReadLockedMethod(
rwlMyLock_m,
delegate () { return dtMyDateTime_m; }
);
}
set
{
WriteLockedMethod(
rwlMyLock_m,
delegate () { dtMyDateTime_m = value; }
);
}
}
private static DateTime ReadLockedMethod(
ReaderWriterLock rwl,
ReadLockMethod method
)
{
rwl.AcquireReaderLock(0);
try
{
return method();
}
finally
{
rwl.ReleaseReaderLock();
}
}
private static void WriteLockedMethod(
ReaderWriterLock rwl,
WriteLockMethod method
)
{
rwl.AcquireWriterLock(0);
try
{
method();
}
finally
{
rwl.ReleaseWriterLock();
}
}
}
SoftwareJedi, I don't have an account, so I can't edit my answers.
In any case, the previous version wasn't really good for general purpose use since the read lock always required a return value. This fixes that:
class StackOTest
{
static ReaderWriterLock rwlMyLock_m = new ReaderWriterLock();
private DateTime dtMyDateTime_m;
public DateTime MyDateTime
{
get
{
DateTime retval = default(DateTime);
ReadLockedMethod(
delegate () { retval = dtMyDateTime_m; }
);
return retval;
}
set
{
WriteLockedMethod(
delegate () { dtMyDateTime_m = value; }
);
}
}
private void ReadLockedMethod(Action method)
{
rwlMyLock_m.AcquireReaderLock(0);
try
{
method();
}
finally
{
rwlMyLock_m.ReleaseReaderLock();
}
}
private void WriteLockedMethod(Action method)
{
rwlMyLock_m.AcquireWriterLock(0);
try
{
method();
}
finally
{
rwlMyLock_m.ReleaseWriterLock();
}
}
}
Actually in your first example, to make the solutions comparable, you would also implement IDisposable there as well. Then you'd call Dispose() from the finally block instead of releasing the lock directly.
Then you'd be "apples to apples" implementation (and MSIL)-wise (MSIL will be the same for both solutions). It's still probably a good idea to use using because of the added scoping and because the Framework will ensure proper usage of IDisposable (the latter being less beneficial if you're implementing IDisposable yourself).
Silly me. There's a way to make that even simpler by making the locked methods part of each instance (instead of static like in my previous post). Now I really prefer this because there's no need to pass `rwlMyLock_m' off to some other class or method.
class StackOTest
{
private delegate DateTime ReadLockMethod();
private delegate void WriteLockMethod();
static ReaderWriterLock rwlMyLock_m = new ReaderWriterLock();
private DateTime dtMyDateTime_m;
public DateTime MyDateTime
{
get
{
return ReadLockedMethod(
delegate () { return dtMyDateTime_m; }
);
}
set
{
WriteLockedMethod(
delegate () { dtMyDateTime_m = value; }
);
}
}
private DateTime ReadLockedMethod(ReadLockMethod method)
{
rwlMyLock_m.AcquireReaderLock(0);
try
{
return method();
}
finally
{
rwlMyLock_m.ReleaseReaderLock();
}
}
private void WriteLockedMethod(WriteLockMethod method)
{
rwlMyLock_m.AcquireWriterLock(0);
try
{
method();
}
finally
{
rwlMyLock_m.ReleaseWriterLock();
}
}
}