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GC Class

Controls the system garbage collector, a service that automatically reclaims unused memory.

For a list of all members of this type, see GC Members.

System.Object
   System.GC

[Visual Basic]
NotInheritable Public Class GC
[C#]
public sealed class GC
[C++]
public __gc __sealed class GC
[JScript]
public class GC

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Remarks

The methods in this class influence when an object is garbage collected and when resources allocated by an object are released. Properties in this class provide information about the total amount of memory available in the system and the age category, or generation, of memory allocated to an object.

The garbage collector tracks and reclaims objects allocated in managed memory. Periodically, the garbage collector performs garbage collection to reclaim memory allocated to objects for which there are no valid references. Garbage collection happens automatically when a request for memory cannot be satisfied using available free memory. Alternatively, an application can force garbage collection using the Collect method.

Garbage collection consists of the following steps:

  1. The garbage collector searches for managed objects that are referenced in managed code.
  2. The garbage collector attempts to finalize objects that are not referenced.
  3. The garbage collector frees objects that are not referenced and reclaims their memory.

During a collection, the garbage collector will not free an object if it finds one or more references to the object in managed code. However, the garbage collector does not recognize references to an object from unmanaged code, and might free objects that are being used exclusively in unmanaged code unless explicitly prevented from doing so. The KeepAlive method provides a mechanism that prevents the garbage collector from collecting objects that are still in use in unmanaged code.

Other than managed memory allocations, implementations of the garbage collector do not maintain information about resources held by an object, such as file handles or database connections. When a type uses unmanaged resources that must be released before instances of the type are reclaimed, the type can implement a finalizer.

In most cases, finalizers are implemented by overriding the Object.Finalize method; however, types written in C# or C++ implement destructors, which compilers turn into an override of Object.Finalize. In most cases, if an object has a finalizer, the garbage collector calls it prior to freeing the object. However, the garbage collector is not required to call finalizers in all situations. Also, the garbage collector is not required to use a specific thread to finalize objects, or guarantee the order in which finalizers are called for objects that reference each other but are otherwise available for garbage collection.

In scenarios where resources must be released at a specific time, classes can implement the IDisposable interface, which contains the IDisposable.Dispose method that performs resource management and cleanup tasks. Classes that implement Dispose must specify, as part of their class contract, if and when class consumers call the method to clean up the object. The garbage collector does not, by default, call the Dispose method; however, implementations of the Dispose method can call methods in the GC class to customize the finalization behavior of the garbage collector.

It is recommended, but not required, that garbage collectors support object aging using generations. A generation is a unit of measure of the relative age of objects in memory. The generation number, or age, of an object tells which generation an object belongs to. Objects created more recently are part of newer generations, and have lower generation numbers than objects created earlier in the application life cycle. Objects in the most recent generation are in generation zero.

Notes to Implementers:  This implementation of the garbage collector supports three generations.

MaxGeneration is used to determine the maximum generation number supported by the system. Object aging allows applications to target garbage collection at a specific set of generations rather than requiring the garbage collector to evaluate all generations.

Example

[Visual Basic] 
Imports System

Namespace GCCollectInt_Example
    Class MyGCCollectClass
        Private maxGarbage As Long = 10000

        Public Shared Sub Main()
            Dim myGCCol As New MyGCCollectClass

            'Determine the maximum number of generations the system
            'garbage collector currently supports.
            Console.WriteLine("The highest generation is {0}", GC.MaxGeneration)

            myGCCol.MakeSomeGarbage()

            'Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol))

            'Determine the best available approximation of the number 
            'of bytes currently allocated in managed memory.
            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(False))

            'Perform a collection of generation 0 only.
            GC.Collect(0)

            'Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol))

            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(False))

            'Perform a collection of generation 2 only.
            GC.Collect(2)

            'Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol))
            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(False))
            Console.Read()

        End Sub


        Sub MakeSomeGarbage()
            Dim vt As Version

            Dim i As Integer
            For i = 0 To maxGarbage - 1
                'Create objects and release them to fill up memory
                'with unused objects.
                vt = New Version
            Next i
        End Sub
    End Class
End Namespace

[C#] 
using System;

namespace GCCollectIntExample
{
    class MyGCCollectClass
    {
        private const long maxGarbage = 1000;
      
        static void Main()
        {
            MyGCCollectClass myGCCol = new MyGCCollectClass();

            // Determine the maximum number of generations the system
        // garbage collector currently supports.
            Console.WriteLine("The highest generation is {0}", GC.MaxGeneration);
            
            myGCCol.MakeSomeGarbage();

            // Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol));
            
            // Determine the best available approximation of the number 
        // of bytes currently allocated in managed memory.
            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(false));
            
            // Perform a collection of generation 0 only.
            GC.Collect(0);
            
            // Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol));
            
            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(false));
            
            // Perform a collection of generation 2 only.
            GC.Collect(2);
            
            // Determine which generation myGCCol object is stored in.
            Console.WriteLine("Generation: {0}", GC.GetGeneration(myGCCol));
            Console.WriteLine("Total Memory: {0}", GC.GetTotalMemory(false));
            Console.Read();
        }

        void MakeSomeGarbage()
        {
            Version vt;

            for(int i = 0; i < maxGarbage; i++)
            {
                // Create objects and release them to fill up memory
        // with unused objects.
                vt = new Version();
            }
        }
    }
}

[C++] 
#using <mscorlib.dll>

using namespace System;

const long  maxGarbage = 1000;

__gc class MyGCCollectClass {
public:
   void MakeSomeGarbage() {
      Version* vt;
      for (int i = 0; i < maxGarbage; i++) {
         // Create objects and release them to fill up memory
         // with unused objects.
         vt = new Version();
      }
   }
};

int main() {
   MyGCCollectClass* myGCCol = new MyGCCollectClass();

   // Determine the maximum number of generations the system
   // garbage collector currently supports.
   Console::WriteLine(S"The highest generation is {0}", __box(GC::MaxGeneration));

   myGCCol->MakeSomeGarbage();

   // Determine which generation myGCCol object is stored in.
   Console::WriteLine(S"Generation: {0}", __box(GC::GetGeneration(myGCCol)));

   // Determine the best available approximation of the number
   // of bytes currently allocated in managed memory.
   Console::WriteLine(S"Total Memory: {0}", __box(GC::GetTotalMemory(false)));

   // Perform a collection of generation 0 only.
   GC::Collect(0);

   // Determine which generation myGCCol object is stored in.
   Console::WriteLine(S"Generation: {0}", __box(GC::GetGeneration(myGCCol)));

   Console::WriteLine(S"Total Memory: {0}", __box(GC::GetTotalMemory(false)));

   // Perform a collection of generation 2 only.
   GC::Collect(2);

   // Determine which generation myGCCol object is stored in.
   Console::WriteLine(S"Generation: {0}", __box(GC::GetGeneration(myGCCol)));
   Console::WriteLine(S"Total Memory: {0}", __box(GC::GetTotalMemory(false)));
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework

Assembly: Mscorlib (in Mscorlib.dll)

See Also

GC Members | System Namespace

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