Myself and a colleague are having a difference of opinion on when an object can be garbage collected in .NET. Take the following code:
Stream stream=getStream();
using(var request=new Request(stream))
{
Stream copy=request.Stream;
// From here on can "request" be garbage collected?
DoStuff1();
DoStuff2(copy);
}
My colleague claims that when a release build is run using the server garbage collector that it is valid for the request
object to be garbage collected after the call to request.Stream
. He asserts that this only happens with the server garbage collector, and never with the workstation garbage collector.
The reason this is that the Request
class had a finalizer that was closing the Stream
given to the request. As a result, when DoStuff2
went to use the stream it got an "object disposed" exception. Since the finalizer can only be run by the garbage collector my colleague says that a garbage collection must have taken place before the end of the finally block, but after the last use of request
However, it's my belief that since the above code is just shorthand for something like this:
Stream stream=getStream();
Request request=null;
try
{
Stream copy=request.Stream;
// From here on can "request" be garbage collected?
DoStuff1();
DoStuff2(copy);
}
finally
{
if(request!=null)
request.Dispose();
}
Then request
cannot be garbage collected after the call to request.Stream
at it is still reachable from the finally
block.
Also, if it were possible for the garbage collector to collect the object then the finally
block would potentially exhibit undefined behavior as Dispose
would be called on a GC'd object, which doesn't make sense. Likewise, it isn't possible to optimize away the finally
block as an exception could be thrown within the try
/using
block before any garbage collection had taken place, which would require the finally
block to execute.
Ignoring the issue of closing the stream in the finalizer, is it ever possible for the garbage collector to collect the object before the end of the finally
block, and in effect optimize away the logic in the finally
block?
. NET's garbage collector manages the allocation and release of memory for your application. Each time you create a new object, the common language runtime allocates memory for the object from the managed heap.
Object lifetime is the time when a block of memory is allocated to this object during some process of execution and that block of memory is released when the process ends.
The garbage collector serves as an automatic memory manager. You do not need to know how to allocate and release memory or manage the lifetime of the objects that use that memory. An allocation is made any time you declare an object with a “new” keyword or a value type is boxed.
High Object Creation Rate A high garbage collection rate will increase the GC pause time as well. Thus, optimizing the application to create fewer objects is THE EFFECTIVE strategy to reduce long GC pauses. This might be a time-consuming exercise, but it is 100% worth doing.
There's quite a bit going on in this question, so I'll tackle the overarching issues first.
The variable declared in a using
statement will not be garbage collected before the end of the block, for exactly the reason you indicated - a reference is held in order to call Dispose()
in the implicit finally
block.
If you find yourself writing a finalizer in C#, you are probably doing something wrong. If your finalizer in C# calls Stream.Dispose()
, you are definitely doing something wrong. Outside of an implementation of .NET itself, I have seen hundreds of misused finalizers, and exactly 1 finalizer that was actually needed. For more information, see DG Update: Dispose, Finalization, and Resource Management.
ObjectDisposedException
has nothing to do with finalization. This exception generally occurs when code calls Dispose()
on an object (not finalize), and then a later call is made to do something with the object.
Sometimes it's not obvious when code is disposing of a Stream
. One case that surprised me was using StreamContent
as part of sending an HTTP request using HttpClient
. The implementation calls Stream.Dispose()
after sending the request, so I had to write a wrapper Stream
class called DelegatingStream
to preserve our library's original behavior during the conversion from HttpWebRequest
to HttpClient
.
One scenario where you could see an ObjectDisposedException
is if the getStream()
method caches a Stream
instance and returns it for multiple future calls. If Request.Dispose()
disposes of the stream, or if DoStuff2(Stream)
disposes of the stream, then the next time you attempt to use the stream you'll get an ObjectDisposedException
.
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