If MyClass implements IDisposable interface, will the both given snippets disposes MyClass object correctly? and which method is preferable over the other?
Code 1
public MyModel MethodOne()
{
using (MyClass myClass = new MyClass())
{
var myModel = new MyModel();
myModel.MyModelData = myClass.GetData();
return myModel;
}
}
Code 2
public MyModel MethodTwo()
{
var myData = new MyData();
using (MyClass myClass = new MyClass())
{
myData = myClass.GetData();
}
var myModel = new MyModel();
myModel.MyModelData = myData;
return myModel;
}
It is the same: the object will be disposed in both way.
the using statement is translated by the compiler as a try block which calls Dispose in a finally block.
finally is guaranteed to execute after the try block has finished execution, regardless of its execution path.
Dispose is guaranteed to be called, no matter what.
Reference MSDN
As a style and best practice IMHO it is better to dispose an object as soon as it is not needed anymore because you will free resources earlier and because if Dispose throw an exception you will have your method return anything. and it will be easier to debug.
Update
As KrisVandermotten pointed out there are some cases where the finally block will not execute.
Environment.FailFast
the Uncatchable execeptions
Power Failure
using is translated to try with finally block. Your code
using (MyClass myClass= new MyClass())
{
myData = myClass.GetData();
}
Is the same as
MyClass myClass= new MyClass();
try
{
myData = myClass.GetData();
}
finally
{
if (myClass != null)
((IDisposable)myClass).Dispose();
}
As for which method is preferable, both solutions will work fine. The second one disposes of the object earlier and for that reason is preferable, in many cases. There's no need to hold on to objects, especially if they use external resources.
There are exceptions to that, though. You may want to or need to hold on to an object for longer to achieve your objective, e.g. when using a transaction scope. Also, sometimes the objects derived from the disposable object (as in your case) may become invalid if the originating object is disposed.
In both cases, you can rely on the fact that, if an object of MyClass was constructed and the method completes, the object is also disposed.
In the latter case, the object is constructed later than in the first case, and it is disposed earlier than in the first case, but in both cases it will be disposed.
Which is better? Assuming that a disposable object somehow holds on to something that needs to be freed, and that this holding on to it is expensive, all else being equal, the better approach would be the second.
So why the two caveats above?
"if an object of MyClass was constructed"
In the second case, it is possible that the call to new MyData() throws an exception, and the MyClass object construction is not even attempted. In the first case, constructing this object is the first thing that happens. It may fail, but it will be tried. It may fail in the second case as well of course. And if it fails, there is nothing to be disposed.
"and the method completes"
Nothing will be disposed if you lose power before the dispose can be executed, and there are a few other pathological cases where execution of a finally block is not guaranteed.
IMHO the first version is more concise and therefore better, since the extra time taken before disposing is likely to be negligible. It typically doesn't take long to construct a couple of objects and set a property.
Only if there were a call to "myModel.DoSomethingThaTakesALongTime()" would I consider using the second version.
Related
What is an alternate/better way I can safely dispose of the variables purge and factory in the following code?
public void Run( string XmlFragment ) {
XmlNode xmlNode = null;
try
{
xmlNode = Common.ConstructXmlNodeFromString(XmlFragment, "Params");
var list = DataList();
foreach (var item in list)
{
var factory = new PurgerFactory(item);
IPurger purge = factory.Purger;
purge.Purge();
purge = null;
factory = null;
}
Common.PurgeEmail(SuccessEmail());
}
catch (Exception ex)
{
string errorMessage = $"Purge Error: {ex.Message}. {Environment.NewLine} Stack Trace: {ex.StackTrace}";
Common.PurgeEmail(FailEmail(errorMessage));
}
}
As I think you know, C# has a garbage collector. So for normal objects that don't access an unmanaged resource, just letting the garbage collector clean them is fine.
If you want to deterministically close a managed resource the primary paradigm is inheriting from IDisposable and a using statement. This will call the Dispose function upon exiting the code block of the using statement.
If you otherwise want to clean stuff up, but you don't care when it happens you can use ~(MyType). This is called when the GC does a GC cycle, this is called the Finalizer. I personally haven't encountered a use-case for this vs IDisposable. But if you just want to make sure say a file, or otherwise is deleted when this email object is Garbage Collected, then it might be a good use case for you.
Your code doesn't indicate whether purge or factory implement IDisposable. If they do, you would call
purge.Dispose();
factory.Dispose();
or use using which ensures that Dispose is called even if the method throws an exception:
using(var factory = new PurgerFactory(item))
{
// do stuff
} // factory is disposed.
What we don't need to do is set variables to null. An object gets garbage collected sometime after there are no longer any references to it. It's true that setting factory = null removes that reference to the object, but the same thing happens anyway when the variable goes out of scope. So it's redundant.
There's really no need to try to force that to happen sooner. It's not like the object will get garbage collected the instant there are no more references to it, so we don't worry so much about that.
In this case, the variables are both declared within the foreach loop, so the references go out of scope immediately after each iteration of the loop. In other words, they already go out of scope right at the point when you're setting them to null.
That's part of what's awesome about .NET. Imagine if we had to set every reference variable to null. We'd need try/catch everywhere to make sure it happened. It would be as if every single object was disposable. What a nightmare. Instead it's handled for us in most normal scenarios so that we don't have to think about it or clutter our code with it.
I have this code:
Method1(Method2());
However, Method2 returns an object that needs to be disposed. Here is how I can handle this:
using (var x = Method2())
{
await Method1(x);
}
Method1 and Method2 belong to framework and it is not up to me to change them.
I have number of such cases. For example, Method2 creates HTTP request and Methid1 sends it.
Again, both methods belong to library that I cannot change.
I know if I do not dispose object, Garbage Collector will eventually do this. May be not soon.
I am wandering, if may be in case when there is no any variable that references the object (as it will be after Method1 returns), I can count on Garbage Collector to dispose the object immediately, and thus it is ok to use the original short option.
Use the using statement, just like you have shown, yes it is the elegant way
using (var something = Method2())
{
Method1(something);
}
Or if you will
using (var something = Method2())
Method1(something);
Anything else would be unusual and confusing (ie disposing it in your method1)... As pointed out by the comments, unless this was some sort of Command/Query Service, or other Dependency that could be injected with a disposable scope
Also, calling wait on anything is suspicious these days
Until recently it never really bothered me as to how to best declare and dispose of a local variable but I thought I'd ask once and for all and get some feedback as it's starting to bug me more and more these days.
When creating a function/method that creates a local object, which method is best to create and dispose of the object.
For simplicity sake, assume that the method of the object being called will never generate an exception i.e. ConvertThisToString
private string myFirstFunction()
{
MyDataType myObject = null;
try
{
myObject = new MyDataType();
return myOjbect.ConvertThisToString();
}
finally
{
myObject = null;
}
}
or
private string mySecondFunction()
{
MyDataType myObject = new MyDataType();
return myOjbect.ConvertThisToString();
}
Are both functions ok and is it just about coding preferences or is there one method that's better than the other? Why?
My opinion is that one always requires the try/catch in order to nullify the object, which might be an overkill of try/catch for nullifying's sake, while the other method doesn't call any explicit way to destroy the object, which might be to reliant on .NET GC to release it from memory.
Should I be using the "using" statement instead?
Well, this is probably an incorrect statement. Is a local object immediately destroyed and disposed of when leaving a function or will it be cleared at a later stage by the GC management or other.
Thanks for feedback.
Thierry
UPDATED:
Removed the catch block as it caused confusion in my question. Should haven't been there in the first place since I did say, no error would ever occur.
That's very wrong.
Don't swallow exceptions.
Assigning a variable to null at the end of its scope will not help the GC at all.
If your object actually has expensive resources, it should implement IDisposable (correctly!), and you should dispose it using a using statement (but only when you're finished with it!)
You don't need to assign to null. When the object leaves scope, it will automatically be eligible for GC. There is no need to do anything special.
Basically, just write the second, simple version.
Should I be using the "using" statement instead?
If your object is wrapping resources (not memory allocated via new ..., but native resources) and implements IDisposable, then yes, you should use the using statement to guarantee those are cleaned up.
Is a local object immediately destroyed and disposed of when leaving a function or will it be cleared at a later stage by the GC management or other.
It will become eligible to be collected. At some point in the future, the GC will clean it up, but the time when this happens in indeterminant.
The best approach here is the Using statement.
something like this:
private string myFirstFunction()
{
using(MyDataType myObject = new MyDataType())
{
return myObject.ConvertThisToString();
}
}
this will dispose the object after execution.
As with almost everything, it all depends on what object you are using. If the object you are creating implements IDisposable then you would be best served to place in a using (typically). Outside of that most objects will get cleaned up by the garbage collector. IF you are the producer of a class that accesses COM objects then as the producer you should have provided a away for a proper cleanup, e.g. implement the IDisposable interface and handle the Dispose() correctly. As others have commented swallowing exceptions or even try/catching EVERY method doesn't seem like a reasonable or good idea. If a call you are making has the potential of throwing an exception and you have unmanaged or leaky objects then you should handle via a try/finally or a using (again, if appropriate).
You don't need to use a try/catch/finally to set a variable to null. The .NET GC will clear up any unreferenced classes when a method ends in it's own time.
If you are doing something really intensive (more so than your sample shows) then you can set the variable reference to null as a pointer to the GC that this is no longer referenced. This will only make a difference to your program if you are doing something which ends up marking the variable reference for Gen2 collection and then you do more stuff which prevents the GC collecting your variable (because the scope has not been left - in this case the method).
This is a bit of an extreme case as the .NET GC is designed to remove this aspect of programming from your daily concern, and regardless, it should get cleared up when the scope ends - it might just a bit longer.
You use 'using' if the referenced object implements IDisposable, and you do this typically to release unmanaged resources - though there may be other reasons why a class implements this interface.
Your first method is total overkill... it should just be as described in MySecondFunction(), and just don't swallow exceptions (I'm referring to the empty catch block) like that - because it leads to buggy, unmaintanable code! :)
using(YourType yourObject = new YourType())
{
//Treatment on you object
//Exception occurs here
}
when we write this way, the garbage collector automatically disposes the object, but is exceptions occurs inside this will the garbage collector still dispose the object or I have to write something for it, I know this is a lame q but m really confused, thnx
please explain the treatment in different .net frameworks to make things crystal clear.
Yes, the using block will still call Dispose on an exception. See MSDN documentation.
The using statement ensures that Dispose is called even if an exception occurs while you are calling methods on the object.
Also another useful article Avoiding Problems with the Using Statement.
From MSDN Documentation
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.
using (var object = new Object())
{
object.DoSomething();
}
equal with:
var object = new Object();
try
{
object.DoSomething();
}
finally
{
object.Dispose();
}
I have this class:
class Foo : IDisposable
{
SomeBigResource resource;
void UsingResource()
{
using(Bar bar = new Bar(SomeBigResource)
bar.doStuff();
}
void Dispose()
{
resource.Dispose();
}
}
void Function()
{
using (Foo foo = new Foo(new SomeBigResource))
foo.UsingResource();
}
The Bar object has exactly the same Dispose() function.
Will my SomeBigResource be released or is the GC smart enough to release it later on when the second using is done?
If both the Dispose methods in Foo and Bar calls Dispose on the SomeBigResource object, the method will be called twice. If the method is implemented correctly it will release the resources the first time and do nothing the second time.
What you have is a confusion of responsibility, where both objects take responsibility for calling Dispose on the SomeBigResource object. This is something that you want to avoid, as one object can't know if the other object still needs the resource, so you want to put the responsibility in one place only.
You should either make the Foo object responsible for the life cycle of the resource, or handle it outside of the objects completely. The latter makes more sense, as that's where you create the SomeBigResource instance:
using (SomeBigResource resource = new SomeBigResource()) {
using (Foo foo = new Foo(resource)) {
foo.UsingResource();
}
}
Your resource will be disposed twice.
The GC is completely unaware of IDisposables and the using statement.
Each using statement translates into a try / finally block with a Dispose() call in the finally block.
This is a normal method call which will always execute.
Therefore, the inner using statement in UsingResource() will dispose the resource, and then the outer using statement in Function() will dispose it again.
All IDisposable implementations should be idempotent (calling it a second time should do no harm).
Therefore, (assuming that SomeBigResource implements IDisposable correctly), your code should work fine.
Note, though, that with your class, an empty using block will throw a NullReferenceException, which is very wrong.
You should add a null check in your Dispose method.
This depends on the implementation of Bar.Dispose(). Assuming it has a similar implementation as Foo, then, yes, the release will occur early.
Take a look at the Dispose pattern guidance and this SO question. There are more corner cases than you may think of immediately.
I assume this line should read:
using(Bar bar = new Bar(resource))
If that's the case, then when bar is disposed, it should dispose of resource.
So, yes. When bar is disposed in UsingResource, it will dispose resource, so that if you attempt to use it later, it should throw an ObjectDisposedException. (Although in your simple example, that shouldn't happen.)