1. Finalization 17: Finalization
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Describe options for handling unmanaged resources
17: Finalization
Objectives
Describe options for handling unmanaged resources
destructor / Finalize
Dispose / IDisposable / using
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. Resources Program may use many different unmanaged resources disk file
17: Finalization
Resources
Program may use many different unmanaged resources
disk file
mutex lock
socket connection
database connection
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. Unmanaged resource Typical to wrap unmanaged resource in class
17: Finalization
Unmanaged resource
Typical to wrap unmanaged resource in class
wrapper deals with complexities
client code uses convenient methods
class File {
public File(string name) { ... }
public char Read () { ... }
public void Write(char c) { ... }
public void Close() { ... } }
open
access
close
handle details of
resource interaction
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Use of unmanaged resource
17: Finalization
Use of unmanaged resource
Must arrange for release of unmanaged resource
CLR does not handle automatically
responsibility placed on client
error prone since client could forget
void Process(string name) {
File f = new File(name);
char c = f.Read();
...
f.Close(); }
must call
close method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. Destructor Class can supply destructor to release unmanaged resource
17: Finalization
Destructor
Class can supply destructor to release unmanaged resource
same name as class preceded by ~
no argument
no return type
no access modifier
cannot be overloaded
class File {
~File()
if (IsOpen())
Close(); }
...
destructor
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. Destructor invocation
17: Finalization
Destructor invocation
Destructor called automatically during garbage collection
not called immediately when object no longer in use
cannot be called explicitly
void Process(string name) {
File f = new File(name);
char c = f.Read();
... }
destructor not called
until garbage collection
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. Destructors and inheritance
17: Finalization
Destructors and inheritance
Destructor automatically calls base class destructor
call occurs at end of destructor body
ensures each class has chance to release resources
class File {
~File() { ... }
... }
class BufferedFile : File {
~BufferedFile()
... }
calls ~File
automatically
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. Finalize C# destructor compiled into override of object.Finalize()
17: Finalization
Finalize
C# destructor compiled into override of object.Finalize()
ensures compatibility with other .NET languages
includes chain to base Finalize
call to base put in finally block to make code exception safe
C# code not allowed to override Finalize directly
class BufferedFile : File {
protected override void Finalize()
try
Flush(); }
finally
base.Finalize();
...
class BufferedFile : File {
~BufferedFile()
Flush(); }
...
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. Destructor limitations
17: Finalization
Destructor limitations
Destructors typically not sufficient to release resources
not called in specific order
may not be called in timely manner
may not ever be called in some circumstances
Common to supply method to do resource release
many programmers avoid destructor due to its many limitations
class File {
~File() { ... }
public void Close() { ... }
... }
destructor
release method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. Client responsibility
17: Finalization
Client responsibility
Clients typically release unmanaged resources explicitly
allows control over timing of resource release
not common to rely on destructor even if one is provided
void Process(string name) {
File f = new File(name);
...
f.Close(); }
client code should
release resource
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. IDisposable Client release of unmanaged resource formalized
17: Finalization
IDisposable
Client release of unmanaged resource formalized
implement IDisposable.Dispose method
simplifies client's task by defining standard method signature
class File : IDisposable {
public void Dispose()
if (IsOpen())
Close(); }
...
Dispose
void Process(string name) {
File f = new File(name);
...
f.Dispose(); }
client invokes
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. Exception safety Clients should call Dispose even if exception thrown
17: Finalization
Exception safety
Clients should call Dispose even if exception thrown
ensures cleanup in all cases
void Process(string name) {
File f = new File(name);
...
f.Dispose(); }
might be missed
if exception thrown
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. Using A using statement automatically calls Dispose
17: Finalization
Using
A using statement automatically calls Dispose
syntax: keyword using, object to use, code block
ensures timely clean up of unmanaged resource
called even if exception thrown
using (File f = new File()) {
... }
Dispose called
File f = new File();
using (f) {
... }
Dispose called
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. Destructor and Dispose
17: Finalization
Destructor and Dispose
No need to call destructor if Dispose has been called
resource already released
calling destructor would just waste time
Can tell garbage collector to skip finalization of object
use GC.SuppressFinalize()
typically call from Dispose
class File : IDisposable {
public void Dispose()
...
GC.SuppressFinalize(this); }
suppress
destructor
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

16. Owning managed resource
17: Finalization
Owning managed resource
Object may own a managed resource
fields may be references to other objects
class Department {
File employees = new File("Employees.txt");
... }
owned
object
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

17. Eligibility for finalization
Object and owned resources typically eligible for finalization at same time
void Pay() {
Department it = new Department();
... }
owns a file
Department and
File objects both
eligible for finalization
Department
object
File
object it
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

18. 17: Finalization
Order of finalization
Finalization order of separate objects not guaranteed
Department
object
File
object
may be finalized
in any order
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

19. Destructor and managed resource
17: Finalization
Destructor and managed resource
Should not access managed object in destructor
resource may already have been finalized
class Department {
File employees = new File("Employees.txt");
~Department()
employees.Close(); }
...
should not
access in
destructor
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

20. Owned object requiring cleanup
17: Finalization
Owned object requiring cleanup
Should arrange for cleanup of owned objects that need it
provide method user must call such as Close or Dispose
ok to access owned resource since called while object still live
class Department {
File employees = new File("Employees.txt");
public void Close()
employees.Close(); }
...
close file
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

21. Summary Management of non-memory resources problematic
17: Finalization
Summary
Management of non-memory resources problematic
language and library offer some help
but no foolproof way to always release
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003