What strategies and tools are useful for finding memory leaks in .net?

Question

I wrote C++ for 10 years. I encountered memory problems, but they could be fixed with a reasonable amount of effort.

For the last couple of years I've been writing C#. I find I still get lots of memory problems. They're difficult to diagnose and fix due to the non-determinancy, and because the c# philosophy is that you shouldn't have to worry about such things when you very definitely do.

One particular problem I find is that I have to explicitly dispose and cleanup everything in code. If I don't, then the memory profilers don't really help because there is so much chaff floating about you can't find a leak within all the data they're trying to show you. I wonder if I've got the wrong idea, or if the tool I've got isn't the best.

What kind of strategies and tools are useful for tackling memory leaks in .net?

Answer 1

I use Scitech's MemProfiler when I suspect a memory leak.

So far, I have found it to be very reliable and powerful. It has saved my bacon on at least one occasion.

The GC works very well in .NET IMO, but just like any other language or platform, if you write bad code, bad things happen.

Answer 2

You still need to worry about memory when you are writing managed code unless your application is trivial. I will suggest two things: first, read CLR via C# because it will help you understand memory management in .NET. Second, learn to use a tool like CLRProfiler (Microsoft). This can give you an idea of what is causing your memory leak (e.g. you can take a look at your large object heap fragmentation)

Answer 3

We've used Ants Profiler Pro by Red Gate software in our project. It works really well for all .NET language-based applications.

We found that the .NET Garbage Collector is very "safe" in its cleaning up of in-memory objects (as it should be). It would keep objects around just because we might be using it sometime in the future. This meant we needed to be more careful about the number of objects that we inflated in memory. In the end, we converted all of our data objects over to an "inflate on-demand" (just before a field is requested) in order to reduce memory overhead and increase performance.

EDIT: Here's a further explanation of what I mean by "inflate on demand." In our object model of our database we use Properties of a parent object to expose the child object(s). For example if we had some record that referenced some other "detail" or "lookup" record on a one-to-one basis we would structure it like this:

class ParentObject
   Private mRelatedObject as New CRelatedObject
   public Readonly property RelatedObject() as CRelatedObject
      get
         mRelatedObject.getWithID(RelatedObjectID)
         return mRelatedObject
      end get
   end property
End class

We found that the above system created some real memory and performance problems when there were a lot of records in memory. So we switched over to a system where objects were inflated only when they were requested, and database calls were done only when necessary:

class ParentObject
   Private mRelatedObject as CRelatedObject
   Public ReadOnly Property RelatedObject() as CRelatedObject
      Get
         If mRelatedObject is Nothing
            mRelatedObject = New CRelatedObject
         End If
         If mRelatedObject.isEmptyObject
            mRelatedObject.getWithID(RelatedObjectID)
         End If
         return mRelatedObject
      end get
   end Property
end class

This turned out to be much more efficient because objects were kept out of memory until they were needed (the Get method was accessed). It provided a very large performance boost in limiting database hits and a huge gain on memory space.

Answer 4

The best thing to keep in mind is to keep track of the references to your objects. It is very easy to end up with hanging references to objects that you don't care about anymore. If you are not going to use something anymore, get rid of it.

Get used to using a cache provider with sliding expirations, so that if something isn't referenced for a desired time window it is dereferenced and cleaned up. But if it is being accessed a lot it will say in memory.

Answer 5

One of the best tools is using the Debugging Tools for Windows, and taking a memory dump of the process using adplus, then use windbg and the sos plugin to analyze the process memory, threads, and call stacks.

You can use this method for identifying problems on servers too, after installing the tools, share the directory, then connect to the share from the server using (net use) and either take a crash or hang dump of the process.

Then analyze offline.

Answer 6

If the leaks you are observing are due to a runaway cache implementation, this is a scenario where you might want to consider the use of WeakReference. This could help to ensure that memory is released when necessary.

However, IMHO it would be better to consider a bespoke solution - only you really know how long you need to keep the objects around, so designing appropriate housekeeping code for your situation is usually the best approach.

Answer 7

Just for the forgetting-to-dispose problem, try the solution described in this blog post. Here's the essence:

    public void Dispose ()
    {
        // Dispose logic here ...

        // It's a bad error if someone forgets to call Dispose,
        // so in Debug builds, we put a finalizer in to detect
        // the error. If Dispose is called, we suppress the
        // finalizer.
#if DEBUG
        GC.SuppressFinalize(this);
#endif
    }

#if DEBUG
    ~TimedLock()
    {
        // If this finalizer runs, someone somewhere failed to
        // call Dispose, which means we've failed to leave
        // a monitor!
        System.Diagnostics.Debug.Fail("Undisposed lock");
    }
#endif

Answer 8

I have found a few really good articles that have been usefull to me when looking at memory issues in .NET and I have kept a reference to them so I have them around.

Debugging Memory Problems (MSDN)

Debugging Tools for Windows

SOS Debugging Extensions

These have all been very useful. I come from a C++ background too so I know what you mean. In the end there is a lot of overlap in the tools that you use to look at these problems. Hope this helps.

Answer 9

Are you using unmanaged code? If you are not using unmanaged code, according to Microsoft, memory leaks in the traditional sense are not possible.

Memory used by an application may not be released however, so an application's memory allocation may grow throughout the life of the application.

From How to identify memory leaks in the common language runtime at Microsoft.com

A memory leak can occur in a .NET Framework application when you use unmanaged code as part of the application. This unmanaged code can leak memory, and the .NET Framework runtime cannot address that problem.

Additionally, a project may only appear to have a memory leak. This condition can occur if many large objects (such as DataTable objects) are declared and then added to a collection (such as a DataSet). The resources that these objects own may never be released, and the resources are left alive for the whole run of the program. This appears to be a leak, but actually it is just a symptom of the way that memory is being allocated in the program.

For dealing with this type of issue, you can implement IDisposable. If you want to see some of the strategies for dealing with memory management, I would suggest searching for IDisposable, XNA, memory management as game developers need to have more predictable garbage collection and so must force the GC to do its thing.

One common mistake is to not remove event handlers that subscribe to an object. An event handler subscription will prevent an object from being recycled. Also, take a look at the using statement which allows you to create a limited scope for a resource's lifetime.

Answer 10

You may want to check out dotTrace by JetBrains (makers of Resharper). Fantastic tool!

http://www.jetbrains.com/profiler/

Answer 11

Big guns - Debugging Tools for Windows

This is an amazing collection of tools. You can analyze both managed and unmanaged heaps with it and you can do it offline. This was very handy for debugging one of our ASP.NET applications that kept recycling due to memory overuse. I only had to create a full memory dump of living process running on production server, all analysis was done offline in WinDbg. (It turned out some developer was overusing in-memory Session storage.)

"If broken it is..." blog has very useful articles on the subject.

Answer 12

This blog has some really wonderful walkthroughs using windbg and other tools to track down memory leaks of all types. Excellent reading to develop your skills.