Are memory leaks possible in managed environments

问题:

In C++ it is easily possible to have a permanent memory leak - just allocate memory and don't release it:

new char; //permanent memory leak guaranteed

and that memory stays allocated for the lifetime of the heap (usually the same as program runtime duration).

Is the same (a case that will lead to a specific unreferenced object never been released while memory management mechanisms are working properly) possible in a C# program?

I've carefully read this question and answers to it and it mentions some cases which lead to getting higher memory consumption than expected or IMO rather extreme cases like deadlocking the finalizer thread, but can a permanent leak be formed in a C# program with normally functioning memory management?

回答1:

It depends on how you define a memory leak. In an unmanaged language, we typically think of a memory leak as a situation where memory has been allocated, and no references to it exist, so we are unable to free it.

That kind of leaks are pretty much impossible to create in .NET (unless you call out into unmanaged code, or unless there's a bug in the runtime).

However, you can get another "weaker" form of leaks: when a reference to the memory does exist (so it is still possible to find and reset the reference, allowing the GC to free the memory normally), but you thought it didn't, so you assumed the object being referenced would get GC'ed. That can easily lead to unbounded growth in memory consumption, as you're piling up references to objects that are no longer used, but which can't be garbage collected because they're still referenced somewhere in your app.

So what is typically considered a memory leak in .NET is simply a situation where you forgot that you have a reference to an object (for example because you failed to unsubscribe from an event). But the reference exists, and if you remember about it, you can clear it and the leak will go away.

回答2:

You can write unmanaged code in .NET if you wish, you have enclose your block of code with unsafe keyword, so if you are writing unsafe code are you not back to the problem of managing memory by yourself and if not get a memory leak?

回答3:

It's not exactly a memory leak, but if you're communicating with hardware drivers directly (i.e. not through a properly-written .net extension of a set of drivers) then it's fairly possible to put the hardware into a state where, although there may or may not be an actual memory leak in your code, you can no longer access the hardware without rebooting it or the PC...

Not sure if this is a useful answer to your question, but I felt it was worth mentioning.

回答4:

GC usually delay the collection of unreachable memory to some later time when an analysis of the references show that the memory is unreachable. (In some restricted cases, the compiler may help the GC and warn it that a memory zone is unreachable when it become so.)

Depending on the GC algorithm, unreachable memory is detected as soon as a collection cycle is ran, or it may stay undetected for a certain number of collection cycles (generational GC show this behavior for instance). Some techniques even have blind spots which are never collected (use of reference counted pointer for instance) -- some deny them the name of GC algorithm, they are probably unsuitable in general purpose context.

Proving that a specific zone will be reclaimed will depend on the algorithm and on the memory allocation pattern. For simple algorithm like mark and sweep, it is easy to give a bound (says till the next collection cycle), for more complex algorithms the matter is more complex (under a scheme which use a dynamic number of generations, the conditions in which a full collection is done are not meaningful to someone not familiar with the detail of the algorithm and the precise heuristics used)

回答5:

A simple answer is that classic memory leaks are impossible in GC environments, as classically a memory leak is leaked because, as an unreferenced block theres no way for the software to find it to clean it up.

On the other hand, a memory leak is any situation where the memory usage of a program has unbounded growth. This definition is useful when analyzing how software might fail when run as a service (where services are expected to run, perhaps for months at a time).

As such, any growable data structure that continues to hold onto references onto unneeded objects could cause service software to effectively fail because of address space exhaustion.

回答6:

Easiest memory leak:

public static class StaticStuff
{
    public static event Action SomeStaticEvent;
}

public class Listener
{
   public Listener() {
      StaticStuff.SomeStaticEvent+=DoSomething;
   }
   void DoSomething() {}
}

instances of Listener will never be collected.

回答7:

If we define memory leak as a condition where a memory that can be used for creating objects, cannot be used or a memory that can be released does not then

Memory leaks can happen in:

• Events in WPF where weak events need to be used. This especially can happens in Attached Properties.
• Large objects

Large Object Heap Fragmentation

http://msdn.microsoft.com/en-us/magazine/cc534993.aspx