Garbage Collection is a technique introduced in Microsoft .NET that manages memory automatically. This article discusses the concepts of Garbage Collection and the strategies adopted by Microsoft .NET for handling managed memory efficiently. It also discusses the methods and properties of the System.GC class, the class that is responsible for controlling the garbage collector in the .NET environment.

What is Garbage Collection?

The Common Language Runtime (CLR) requires that you create objects in the managed heap, but you do not have to bother with cleaning up the memory once the object goes out of the scope or is no longer needed. This is unlike the strategies adopted in programming languages like C and C++ where you needed to cleanup the heap memory explicitly using a free function of C and delete operator of C++. Garbage collection refers to the strategy adapted by Microsoft .NET to free unused objects or objects that go out of the scope automatically.

A "garbage" object is one that is no longer needed, is unreachable from the root or goes out of the scope in which it is created. Microsoft .NET uses the information in the metadata to trace the object graph and detect the objects that need to be garbage collected. Objects that are not reachable from the root are referred to as garbage objects and are marked for garbage collection. It is to be noted here that there is a time gap between the time when an object is identified as garbage and the time when the object is actually collected. It is also to be noted that objects in the managed heap are stored in sequential memory locations. This is unlike C and C++ and makes allocation and de-allocation of objects faster.

Strong and Weak References

The garbage collector can reclaim only objects that have no references. An object that is reachable cannot be garbage collected by the garbage collector. Such a reference is known as a strong reference. An object can also be referred to as a weak reference; another term for a weak reference is the target. An object is eligible for garbage collection if it does not contain any strong references, irrespective of the number of weak references it contains. Weak references are of the following types:

· Short Weak Reference

· Long Weak Reference

A short weak reference does not track resurrection while a long weak reference tracks resurrection. The primary advantage of maintaining weak references to an object is that it allows the garbage collector to collect or reclaim memory of the object if it runs out of memory in the managed heap.

The System.GC class

The System.GC class represents the garbage collector and contains many of methods and properties that are described in this section.

GC.Collect Method

This method is used to force a garbage collection of all the generations. It can also force a garbage collection of a particular generation passed to it as a parameter. The signatures of the overloaded Collect methods are:

public static void Collect();
public static void Collect(Integer int);

GC.GetTotalMemory Method

This method returns the total number of bytes that is allocated in the managed memory. This method accepts a boolean parameter. If the parameter is true, it indicates that it should wait for the garbage collector to finish.

GC.KeepAlive Method

This method extends the life time of an object passed to it as a parameter. The signature of this method is as follows:

public static void KeepAlive(object objToKeepAlive);

GC.ReRegisterForFinalize Method

This method re-registers an object for finalization, i.e., makes an object eligible for finalization. The method signature is as follows:

public static void ReRegisterForFinalize(objectobjToRegister);

GC.SupressFinalize Method

This method suppresses the finalization on an object. The prototype of this method is:

public static void SupressFinalize(object obj);

GC.GetGeneration Method

This method returns the current generation of an object or the same of the target of the weak reference. The signature of this overloaded method is:

System.GC.GetGeneration(object obj);
System.GC.GetGeneration(WeakReferenceweakReference);

GC.MaxGeneration Property

This property returns the maximum number of generations available.

GC.WaitForPendingFinalizers Method

This method blocks the current thread till the execution of all the pending finalizers is over. The signature of this method is:

public static void WaitForPendingFinalizers();

The Mark and Compact Strategy

The most commonly used strategy involves the mark and compact algorithm. This occurs in two phases, Mark and Compact.

Mark

The garbage collector identifies the reachable objects by starting from the application’s root and builds a graph of the reachable objects. In order to handle cycles, the garbage collector ignores adding objects to the graph that have already been added. The objects that are not reachable from the root are considered garbage.

Compact

In this phase the garbage collector scans the managed heap, moves the free or available memory to the top and the objects to the bottom of the heap. The memory holes or free spaces are properly compacted and the object references updated as necessary. A pointer to the next object in the managed heap is set up that then indicates that the next object will be created at that memory location in the heap.

Finalization

When an object of a class is created in the heap that implements a finalize method, a pointer to the object is stored in the finalization queue. The garbage collector periodically scans this finalization queue to get the pointers. When it identifies one, it removes the same from the finalization queue and adds the pointer to the reachable queue. Then the finalize method is called on the object and the reachable queue is emptied.

Generations

A generational garbage collector collects the short-lived objects more frequently than the longer lived ones. Short-lived objects are stored in the first generation, generation 0. The longer-lived objects are pushed into the higher generations, 1 or 2. The garbage collector works more frequently in the lower generations than in the higher ones.

When an object is first created, it is put into generation 0. When the generation 0 is filled up, the garbage collector is invoked. The objects that survive the garbage collection in the first generation are promoted onto the next higher generation, generation 1. The objects that survive garbage collection in generation 1 are promoted onto the next and the highest generation, generation 2. This algorithm works efficiently for garbage collection of objects, as it is fast. Note that generation 2 is the highest generation that is supported by the garbage collector.

Suggested Readings

For further references on this topic please refer to the following links:

http://msdn.microsoft.com/msdnmag/issues/1100/gci/

http://msdn.microsoft.com/msdnmag/issues/1200/GCI2/

http://www.codeproject.com/dotnet/garbagecollection.asp

Conclusion

Garbage Collection is one of the most striking features introduced in Microsoft .NET. It is however, not advisable to implement the finalize method unless it is mandatory. An object that has the finalize method implemented has to undergo two generations before it gets garbage collected from the managed heap. Hence, it slows down the operations.