Using 'ref' on a reference type

[Edward Diener]

Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

[Barry Kelly]

Edward Diener <ediener@no_spam_incomm.comwrote:

Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

Sure.

Both reference types and value types, when passed as parameters, are
passed by value - the difference is, with reference types, it's a
reference to the value (i.e. the object on the heap) that's passed by
value.

So, given the following, where Foo is a reference type:

void F(Foo x)
{
x = null; // ok because 'x' is a reference passed by value
}

When you understand this, it's obvious that ref / out are fine with
reference types. They simply cause the reference to the value on the
heap to be passed by reference itself. I.e.:

void F(out Foo x)
{
x = null; // affects caller
}

Foo a = new Foo();
F(out a);
Debug.Assert(a == null); // is true

-- Barry

--
http://barrkel.blogspot.com/

[Laura T]

I'm afraid not. ref modifier really says "don't copy this", not "get the
reference of this".
"Edward Diener" <ediener@no_spam_incomm.comha scritto nel messaggio
news:e9**************@TK2MSFTNGP03.phx.gbl...

Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

[Jon Skeet [C# MVP]]

Edward Diener <ediener@no_spam_incomm.comwrote:

Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

Well, you use ref to pass a parameter by reference. That's not quite
the same as creating *a* reference in the normal .NET sense of a
reference. However, you can certainly pass reference type parameters by
reference too.

See http://www.pobox.com/~skeet/csharp/parameters.html for more
information.

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

[Ignacio Machin \( .NET/ C# MVP \)]

Hi,
"Barry Kelly" <ba***********@gmail.comwrote in message
news:8g********************************@4ax.com...

Edward Diener <ediener@no_spam_incomm.comwrote:

>Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

Sure.

Both reference types and value types, when passed as parameters, are
passed by value - the difference is, with reference types, it's a
reference to the value (i.e. the object on the heap) that's passed by
value.

If I understand correctly this is not what the OP wants, he wants to keep in
a variable the reference to a reference type.

like this:

object o = new object(); //reference type

ref_o = &o; // C syntax.

in C now ref_o has a reference to o , not to the instance being reference
by o;

So the correct answer is no, you cannot have this.

--
--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

[veera sekhar kota]

Yes, we can use..

http://msdn2.microsoft.com/en-us/library/s6938f28.aspx

Hope this helps

Veera.

"Edward Diener" wrote:

Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

[Barry Kelly]

"Laura T" <laura.t@_yahoo.comwrote:

I'm afraid not. ref modifier really says "don't copy this", not "get the
reference of this".

I think you're talking about something else? 'ref' strictly means "pass
by reference rather than pass by value". It's implemented with managed
pointers, using the 'ldloca' instruction (load local address).

-- Barry

--
http://barrkel.blogspot.com/

[Edward Diener]

Barry Kelly wrote:

Edward Diener <ediener@no_spam_incomm.comwrote:

>Can one use 'ref' ( or 'out' ) on a reference type to create a reference
to a reference in C#. I know one can use it on a value type to create a
reference to that value.

Sure.

Both reference types and value types, when passed as parameters, are
passed by value - the difference is, with reference types, it's a
reference to the value (i.e. the object on the heap) that's passed by
value.

So, given the following, where Foo is a reference type:

void F(Foo x)
{
x = null; // ok because 'x' is a reference passed by value
}

When you understand this, it's obvious that ref / out are fine with
reference types. They simply cause the reference to the value on the
heap to be passed by reference itself. I.e.:

void F(out Foo x)
{
x = null; // affects caller
}

Foo a = new Foo();
F(out a);
Debug.Assert(a == null); // is true

Thanks ! What you wrote is what I thought should be the result of
passing a reference type by reference. It is obviously useful when one
wants to return more than one value from a function and the values are
reference types, or the normal return value is a value type and at least
one of the others is a reference type. I am glad to see that MS got this
right in C#.

>
-- Barry

[Barry Kelly]

"Ignacio Machin \( .NET/ C# MVP \)" <ignacio.machin AT dot.state.fl.us>
wrote:

So the correct answer is no, you cannot have this.

According to the OP's himself (parallel to your own), it appears I was
right :)

-- Barry

--
http://barrkel.blogspot.com/

[Jon Skeet [C# MVP]]

Laura T <laura.t@_yahoo.comwrote:

I'm afraid not. ref modifier really says "don't copy this", not "get the
reference of this".

Well, only sort of. It means "pass the parameter by reference instead
of by value". It's certainly valid to pass a reference-type parameter
by reference.

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