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Posted 2 Mar 2004

Controlling object life time and garbage collection in .NET

, 2 Mar 2004
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This article discusses scenarios where we have to take care of an object's life time and cannot rely on JIT's marking of objects for garbage collection.


Managed applications and garbage collection advertise that the developer need not care about memory allocations and object lifetime, leaving them to the mercy of Garbage Collector. The JIT compiler has a mind of its own and optimizes the code under the hood. This veils the developer from the object's moment of abrogation. Although we do not know the moment of an object's destruction, careful planning and design are required to orchestrate with the Garbage Collector to avoid pitfalls.

The Problem

Guessing when and where the object is marked for garbage collection is tricky. Consider the following code :

namespace NS
    class A
        public static void Main()
            System.String s;
            s = "some test string"; 
            // <- is s marked for collection after this ?
            System.Int32 i;
            i = 10;
        }    // <- is s marked for collection here ? 

The place where string s is marked for garbage collection by JIT is not known exactly. This is especially a problem when we try to control objects which are required throughout the life of an application.

Sample solution

A common example that is quoted is a mutex which is used to make sure that only one instance of the application is running. The following code illustrates this:

namespace NS
    class SomeFormClass:System.Windows.Forms.Form
        public static void Main()
            bool created;
            System.Threading.Mutex m = new 
                System.Threading.Mutex(true, "OnlyOneApp", out created);
            if (! created)
            // <- mutex may be marked for garbage collection here ! 
            // if the application runs for long and mutex
            // is garbage collected, the design fails
            System.Windows.Forms.Application.Run(new SomeFormClass()); 
            // <- this will keep the mutex alive till this point 

If the GC.KeepAlive() statement is missing, the JIT compiler might optimize and mark the mutex for garbage collection before the Application.Run() statement. The GC.KeepAlive() statement keeps the object alive upto that statement. In the above scenario, we know the exact place where the mutex is created and till where it is required. Another point is that the variable is accessible at both these known points. Consider an example where an object is required throughout the application but its instantiation point is unknown. Also, it is assumed that no single "live" object may be holding a reference to this required object throughout the life of the application. This scenario prompts the JIT to mark the object for garbage collection at the earliest point where the object goes out of scope. The code below illustrates a design to keep this kind of objects alive and also points out how to handle the situation in Console and Windows applications. It is most important to use the GC.SuppressFinalize() method at the end when the application exists, otherwise the application will go into an infinite loop and hangs.

namespace GCDemo
    // class used to test the scenario
    class StartTest
        // following delegate and event are required by console apps
        // In Windows apps, this will be taken care by ApplicationExit event
        internal delegate void ApplicationExit();
        static internal event ApplicationExit AppExit;
        public static void Main()
            for (int i=0; i<3; i++)
                new GCDemo(); // <- creating 3 objects here as a test case
            // <- we expect the JIT to mark for
            // garbage collect these objects atleast here
            // following lines are required for a console app,
            // Application.ApplicationExit event is raised by windows apps
            if (AppExit != null) 
    // this class object should be kept alive for the entire application life
    public class GCDemo
        public GCDemo()
            // register with the application exit event
            // AppExit is my own event for console applications
            // See the declaration in StartTest class above
            StartTest.AppExit += new StartTest.ApplicationExit(AppExiting);

            // for windows app use the line below by 
            // implementing a EventHandler delegate
            // Application.ApplicationExit += new EventHandler(eventhandler);

            // ReRegisterForFinalize will put the entry back in the queue
            // and resurrect the object 
        void AppExiting()
            // suppressing the finalization will
            // stop the application from hanging 

Sometimes, it is worth calling GC.Collect() and GC.WaitForPendingFinalizers() like shown below before allocating huge memory, especially when there is a crunch on memory.

for (int i=0; i<1000; i++)
        somelist.Add(new someobject());


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Sriram Chitturi
United States United States
No Biography provided

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Comments and Discussions

GeneralYipes! Don't rely on finalization Pin
Anthony Berglas16-Mar-04 15:03
memberAnthony Berglas16-Mar-04 15:03 
GeneralRe: Yipes! Don't rely on finalization Pin
Nathan Holt at CCEI22-Mar-04 6:04
memberNathan Holt at CCEI22-Mar-04 6:04 
GeneralLisp/Java/.Net is NOT C++ Pin
Anthony Berglas22-Mar-04 12:18
memberAnthony Berglas22-Mar-04 12:18 
GeneralRe: Lisp/Java/.Net is NOT C++ Pin
gnk26-Jun-07 16:12
membergnk26-Jun-07 16:12 
GeneralPoint noted Pin
Sriram Chitturi10-Mar-04 6:13
memberSriram Chitturi10-Mar-04 6:13 
GeneralYes, I mostly agree with zucchini. Pin
Keith Vinson10-Mar-04 5:13
sussKeith Vinson10-Mar-04 5:13 
It would seem to me, by definition at as long as a object reference is still in scope the GC will not destroy the object. If this is not true, then what's the point of scope?

Secondly, yes, The GC will not run on its own unless the system begins to run out of resources.

Thirdly, I will take issue with the idea of never calling GC.Collect(). If the program is creating new objects as it runs, eventually the program will become a pig on the system and burn all of its resources. Calling GC.Collect() after major tasks or periodically when the program is idling, will keep the program from dragging down the whole system. Would you create a c program that never called free, but kept a reference to all allocated memory and only would free something when malloc failed? That is how the GC works. It doesn't free stuff until necessary (tries not to burn CPU cycles unless necessary), but that doesn't mean you shouldn't call GC.Collect now and again to clue the GC that its ok to burn a few CPU cycles...

Overall, I believe you should use program structure to control how the GC will behave not by making some obscure calls into the GC....


Keith V, just another lunatic from the fringe....
GeneralRe: Yes, I mostly agree with zucchini. Pin
Sriram Chitturi11-Mar-04 3:17
memberSriram Chitturi11-Mar-04 3:17 
GeneralRe: Yes, I mostly agree with zucchini. Pin
Keith Vinson11-Mar-04 5:44
memberKeith Vinson11-Mar-04 5:44 
GeneralRe: Yes, I mostly agree with zucchini. Pin
zucchini11-Mar-04 4:22
memberzucchini11-Mar-04 4:22 
GeneralRe: Yes, I mostly agree with zucchini. Pin
Keith Vinson11-Mar-04 5:31
sussKeith Vinson11-Mar-04 5:31 
GeneralRe: Yes, I mostly agree with zucchini. Pin
Sriram Chitturi13-Mar-04 4:10
memberSriram Chitturi13-Mar-04 4:10 
QuestionI'm skeptical about the last example; have you tested this? Pin
zucchini10-Mar-04 3:50
memberzucchini10-Mar-04 3:50 
AnswerRe: I'm skeptical about the last example; have you tested this? Pin
Sriram Chitturi11-Mar-04 3:34
memberSriram Chitturi11-Mar-04 3:34 

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