Destructor method

14y2kVisual Basic .NET

-What is the purpose of having destructor

-It is said that if you do not explicitly create a destructor method for a class, C# automatically provides one. If C# will automatically provide one what is purpose of creating it.

Anyone knows please explain

18 Replies

Know the answer? Post it — somebody with the same question will find it here.

14y ago
Thank you Sam
Sam Hobbs14y ago
Also try putting a breakpoint on the "}" at the end of the Main function then debugging (F5); you will probably be able to see the output that way, except you might need to click on the program in the taskbar to see it.
14y ago
Great under expert setting it is producing all four output even no need for GC.Collect(); and Console.ReadKey(); to retain the output screen. Thank you very much for your help.
Vulpes14y ago
Under the Tools menu, click on Settings and then on Expert Settings.

Your Express edition will then look a lot more like the 'paid for' versions and will have the 'Start Without Debugging' option under the Debug menu.

Incidentally, what you're seeing (or rather not seeing) at the moment is a good demonstration of object destruction being non-deterministic in .NET. You just can't be sure when the destructor will be called and, here the Friend class destructor is getting called after the call to Console.ReadKey. It didn't do that when I ran it from the command line but I'm not attaching a debugger which slows things down.
14y ago
I use Microsoft Visual C# 2010 Express which is free download from Microsoft. This version does not have 'start without debugging' instead it has "Start Debugging". I use this only first three lines are coming in the output.
Vulpes14y ago
Do all four lines display if you 'start without debugging' ?
14y ago
When I inserted your code only first three lines are coming as an output last line "Friend's destructor being called" is not coming.
Vulpes14y ago
I actually write most of the short programs I post in this forum using Notepad or QuickSharp and then run them from the command line. I don't then need to add Console.Readkey() or similar to see the output.

However, Sam's right that if you're using VS, then 'Start Without Debugging' (Ctrl-F5) will prevent the console from closing before you've read the output.

The addition of Console.ReadKey() doesn't help here in debug mode (i.e. the console is blank) because the CLR hasn't called any of the destructors when that line is executed. However, if you insert the highlighted line before Console.ReadKey(), then it will force an immediate garbage collection and you should then see the output:

static void Main()
{
Friend f = new Friend();
Child c = new Child(f);
c = null;
GC.Collect();
Console.ReadKey();
}
Sam Hobbs14y ago
When I build and execute the source code that Vulpes shows, it produces the same output that Vulpes says it does.

You can execute the program without debugging using Ctrl-F5 or by using "Debug | Start without debugging". That way you do not need to have the Console.ReadKey(). I doubt that Console.ReadKey(); is the problem, however. Also, instead of using Console.ReadKey(); I just put a breakpoint (F9) at the end of Main and then I have the opportunity to look at the output.

Something else you can do using the debugger is to single-step through the program. If you do not know how to use breakpoints, then find articles about the debugger that explains about breakpoints. "Single-stepping" means that you execute one statement of source code at a time so you can watch what is happening. In this case, however, the debugger might not help so much; so you need to put breakpoints in the lines where Vulpes has the calls to Console.WriteLine and that way you can verify that the code is executed.

The important thing is that you need to check at least one more time to ensure that you have accurately coped all of Vulpes's code.
14y ago
I am talking about above Vulpes program When I inserterted Console.ReadKey();to retain the output screen and see the output the program is producing empty screen.
Sam Hobbs14y ago
Somehow I did not see all that is in this thread. I don't know why I did not see everything. I did not see the sample code. I would not have said everything that I said if I had seen the code first. I will try to look at the code later.
14y ago
Thank you very much for your explanation Sam Hobbs.
14y ago
When I insert Console.ReadKey(); as follows to retain the output screen and see the output the program is producing empty screen. I don't know why

static void Main()
{

Friend f = new Friend();
Child c = new Child(f);
c = null;

Console.ReadKey();

}
Sam Hobbs14y ago
The details of what a destructor and a finalizer is are quite complicated. It is a little complicated just to explain why thy are complicated. So you need to find a good book about C# or at least a good article on the subject.

An over-simplified explanation is that beginners learning C# don't need to know about finalizers or destructors; the garbage collection in .Net and C# will do everything for you automatically. Do not try to do it yourself until you have had the opportunity to learn about how garbage collection, destructors and a finalizers work in .Net and C#.
Vulpes14y ago
Here's a simple example:

using System;

class DestructorExample
{
static void Main()
{
Friend f = new Friend();
Child c = new Child(f);
c = null;
}
}

class Child : Parent
{
private Friend f;

public Child(Friend f)
{
this.f = f;
}

~Child()
{
f.Dispose();
f = null;
Console.WriteLine("Child's destructor being called");
}
}

class Parent
{
~Parent()
{
Console.WriteLine("Parent's destructor being called");
}
}

class Friend : IDisposable
{
public void Dispose()
{
Console.WriteLine("Friend's Dispose method being called");
}

~Friend()
{
Console.WriteLine("Friend's destructor being called");
}
}

The output should be:

Friend's Dispose method being called
Child's destructor being called
Parent's destructor being called
Friend's destructor being called

The explanation is as follows:

1. When 'c' is set to null, it becomes eligible for garbage collection.

2. Just before the Child object is destroyed, the CLR calls its destructor.

3. The Child destructor calls its Friend object's Dispose method (first line in output) and then sets 'f' to null so the Friend object becomes eligible for garbage collection.

4. The Child destructor then prints to the console (2nd line in output) and implicitly calls its base class destructor which also prints to the console (3rd line in output).

5. Finally, the Friend object's destructor gets called and prints to the console (4th line in output). Technically, it then implicitly calls System.Object's destructor (as it has no other base class) but this does nothing and so nothing is printed for that.
14y ago
If it is possible. Please support with a example.
14y ago
Thank you very much for your reply.
Vulpes14y ago
The purpose of having a destructor is to execute some clean-up code prior to an object being destroyed by the garbage collector.

A better name for it would have been a finalizer because in fact the C# compiler translates it to an override of the virtual protected Finalize() method which all classes inherit from System.Object. It also inserts a call to the base class's Finalize() method if it has one.

C# doesn't in fact provide a destructor if you don't provide one yourself. Possibly, you are thinking of an instance constructor?

The destructor is called automatically by the runtime. It cannot be called directly from C#.