Setting a referenced object to null

[Joe]

I might be overworked so please excuse this stupid question...

Say I do the following:

DataTable table = new DataTable();

myDataAdaptor.F ill(table);

dataGrid1.DataS ource = table;

table = null;

Why does dataGrid1.DataS ource still point to a table?

How can I set an object to null and have all references to that object now
be null?

Thanks,
Joe

[Stoitcho Goutsev \(100\)]

Joe,

dataGrid1.DataS ource = table;

table = null;

Why does dataGrid1.DataS ource still point to a table?
Because DataSource has its own copy of the reference to the table.
How can I set an object to null and have all references to that object now
be null?

You can't. I frankly non of the languages that I know can do that unless the
classes are specially designed to use some kind of indirect reference where
they reference an object that holds the real reference. In this case
modifying the reference in this "proxy" class will cut off all the
references to the real object.
--

Stoitcho Goutsev (100)

[Joanna Carter [TeamB]]

"Joe" <jb*******@noem ail.noemail> a écrit dans le message de news:
Oz************* *@TK2MSFTNGP12. phx.gbl...

|I might be overworked so please excuse this stupid question...
|
| Say I do the following:
|
| DataTable table = new DataTable();
|
| myDataAdaptor.F ill(table);
|
| dataGrid1.DataS ource = table;
|
| table = null;
|
| Why does dataGrid1.DataS ource still point to a table?
|
| How can I set an object to null and have all references to that object now
| be null?

You can't. You can only set references to objects to null.

If you know C tthen you would remember that object references hold the
addresses of objects and you would have to dereference that pointer to get
at the object. You could "delete" the object and then the pointer would be
invalid and would have to be nulled to avoid AVs.

In C#, all objects are garbage collected, you can't "delete" them. When the
last reference to a given object falls out of scope, the object is liable to
collection. You can certainly null as many references as you can find, but
the object will remain alive as long as *any* reference still holds that
object.

In your example, the DataGris's DataSource is holding a reference to the
table, so even though you null the original reference, 'table', the object
that 'table' was pointing to is kept alive by the DataSource property.

Why would you want to null a property to which you have only just assigned a
valid object ?

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Joe]

Hi,

Your answer is what I thought. I just wanted to make sure I wasn't loosing
my mind...

BTW - The example I wrote was just to show a really small and quick example
of what I meant. I don't think anyone would ever do as my example displayed.

-Joe

"Joanna Carter [TeamB]" <jo****@not.for .spam> wrote in message
news:uC******** *****@TK2MSFTNG P12.phx.gbl...

"Joe" <jb*******@noem ail.noemail> a écrit dans le message de news:
Oz************* *@TK2MSFTNGP12. phx.gbl...

|I might be overworked so please excuse this stupid question...
|
| Say I do the following:
|
| DataTable table = new DataTable();
|
| myDataAdaptor.F ill(table);
|
| dataGrid1.DataS ource = table;
|
| table = null;
|
| Why does dataGrid1.DataS ource still point to a table?
|
| How can I set an object to null and have all references to that object
now
| be null?

You can't. You can only set references to objects to null.

If you know C tthen you would remember that object references hold the
addresses of objects and you would have to dereference that pointer to get
at the object. You could "delete" the object and then the pointer would be
invalid and would have to be nulled to avoid AVs.

In C#, all objects are garbage collected, you can't "delete" them. When
the
last reference to a given object falls out of scope, the object is liable
to
collection. You can certainly null as many references as you can find, but
the object will remain alive as long as *any* reference still holds that
object.

In your example, the DataGris's DataSource is holding a reference to the
table, so even though you null the original reference, 'table', the object
that 'table' was pointing to is kept alive by the DataSource property.

Why would you want to null a property to which you have only just assigned
a
valid object ?

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Octavio Hernandez]

Joe,

The statement:

DataTable table = new DataTable();

creates a new DataTable object in the heap, and stores a reference to this
object in the variable 'table'.

The statement:

dataGrid1.DataS ource = table;

COPIES the reference stored in the 'table' var to dataGrid1.DataS ource.

The statement:

table = null;

assigns null to 'table', so 'table' does not point anymore to the
dynamically created object; but dataGrid1.DataS ource still points at it.

There is no direct way to know from what different places an object might be
referenced. The CLR keeps track of that, and will automatically reclaim the
memory used by the object when no references points at it.

Regards - Octavio
"Joe" <jb*******@noem ail.noemail> escribió en el mensaje
news:Oz******** ******@TK2MSFTN GP12.phx.gbl...

I might be overworked so please excuse this stupid question...

Say I do the following:

DataTable table = new DataTable();

myDataAdaptor.F ill(table);

dataGrid1.DataS ource = table;

table = null;

Why does dataGrid1.DataS ource still point to a table?

How can I set an object to null and have all references to that object now
be null?

Thanks,
Joe

[Joanna Carter [TeamB]]

"Joe" <jb*******@noem ail.noemail> a écrit dans le message de news:
OT************* *@TK2MSFTNGP14. phx.gbl...

| BTW - The example I wrote was just to show a really small and quick
example
| of what I meant. I don't think anyone would ever do as my example
displayed.

Just checking :-)

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Jon Skeet [C# MVP]]

Joe <jb*******@noem ail.noemail> wrote:

I might be overworked so please excuse this stupid question...

Say I do the following:

DataTable table = new DataTable();

myDataAdaptor.F ill(table);

dataGrid1.DataS ource = table;

table = null;

Why does dataGrid1.DataS ource still point to a table?

How can I set an object to null and have all references to that object now
be null?

The important thing is that you're not setting an object to null.
You're not even setting a reference to null, really.

You're changing the variable's value to null. That doesn't affect
anything other than the value of that variable - so the fact that
dataGrid1.DataS ource happened to have a reference value which was the
same as the value of table is irrelevant.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Chris Dunaway]

Question:

If I create an instance of a value type object (int for example), that
value is stored directly on the stack is it not? And my variable
points directly at the location in the stack where the integer value is
stored? Is that correct?

If I create an instance of a reference type, the object itself is
created on the heap correct? And the variable holds the address of
that object on the heap. Where is the value of the variable stored?
Is it stored on the heap as well? Or is it stored on the stack?

[Jon Skeet [C# MVP]]

Chris Dunaway <du******@gmail .com> wrote:

Question:

If I create an instance of a value type object (int for example), that
value is stored directly on the stack is it not?
Not always - it depends on the type of the variable.
And my variable
points directly at the location in the stack where the integer value is
stored? Is that correct?
If it's a local variable, yes.
If I create an instance of a reference type, the object itself is
created on the heap correct?
Yes.
And the variable holds the address of that object on the heap.
Well, that's the value of the variable.
Where is the value of the variable stored?
Is it stored on the heap as well? Or is it stored on the stack?

It depends on whether the variable is a local variable, an instance
variable etc. See http://www.pobox.com/~skeet/csharp/memory.html

Where a variable's value is stored does *not* depend on the type of the
variable (in terms of whether it's a reference type or a value type).
It only depends on what kind of variable it is (local, instance, static
etc).

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Deleo]

You can force the garbageCollecto r to free up the memory.
But you HAVE to remove ANY refenrence to the table object. If the table
object has to references it is eligble for GarbageCollecto r.

GarbageCollecto r is a complicated matter, but you SELDOM need to call it
manually. It is tweaked at the best to release memory when needed.

Your answere is that it is doable, but the time and effort to do it is not
worth it.
"Joe" <jb*******@noem ail.noemail> wrote in message
news:Oz******** ******@TK2MSFTN GP12.phx.gbl...

I might be overworked so please excuse this stupid question...

Say I do the following:

DataTable table = new DataTable();

myDataAdaptor.F ill(table);

dataGrid1.DataS ource = table;

table = null;

Why does dataGrid1.DataS ource still point to a table?

How can I set an object to null and have all references to that object now
be null?

Thanks,
Joe