'ref' question

[Priyesh]

public class stringtemp
{
String k ;
public stringtemp(ref String inp)
{
k = inp ;
}
public void Change()
{
k = k.Remove(0, 1) ;
}
};

String stringtochange = new String("Hello". ToCharArray()) ;
stringtemp t = new stringtemp(ref stringtochange) ;
t.Change() ;

Why isnt stringtochange changed to 'ello' here? When I try a similar sample
with an ArrayList instead of a String, my original variable gets reflected.
So my question is: How can i make sure i am operating on a reference?
Documentation says all class types are passed by references. Could someone
elaborate?

TIA

[mdb]

"Priyesh" <pr*****@donotr eply.com> wrote in
news:uf******** ******@TK2MSFTN GP15.phx.gbl:

Why isnt stringtochange changed to 'ello' here? When I try a similar
sample with an ArrayList instead of a String, my original variable
gets reflected. So my question is: How can i make sure i am operating
on a reference? Documentation says all class types are passed by
references. Could someone elaborate?

The "string" class isn't a reference type - its a value type, so in:

public stringtemp(ref String inp)
{
k = inp ;
}

even though you are ref'fing inp, the 'k=inp' is still done as a value
assignment.

In order to do what you want, you'll have to put the 'string inp' as a
member of another class, and pass the class instance instead of the string
itself.

--
-mdb

[Bruce Wood]

public stringtemp(ref String inp)

says that you can change "inp" _in the constructor_ and the
corresponding argument variable will change on the outside.

String k;

says what? Does it say "k is a reference to a string"? No, it says, "k
is a string". So, when you say

this.k = inp;

what you're saying is, "k should be the same string as is contained in
inp*". Since you never avail yourself of the opportunity to change inp
_within the constructor_, you just change k, but never inp,
"stringtochange " is never changed.

Just because you declare a parameter as "ref" does not mean that its
"refness" is sticky and follows that string value wherever it goes.
Now, if C# were allow you to say something like:

public class stringtemp
{
ref String k;

public stringtemp(ref String inp)
{
this.k = inp;
}
...
}

then we might have something going here. However "ref String k" is not
a valid field declaration, so you're out of luck.

By the time you get to the Change method, the fact that "inp" was
originally declared "ref" is completely irrelevant. It's only relevant
within the constructor.

One of the design goals of C# and .NET was to avoid the kind of pointer
mayhem that can occur in C and C++. That's why you can't declare
variables that hold references: they would just be pointers under
another name, and open the door to all kinds of madness. Take another
look at your example, and think of it like this:

String stringtochange = "Hello";
stringtemp t = new stringtemp(ref stringtochange) ;
... 200 lines of complex code here ...
t.Change() ;

suddenly, without any warning or any indication in the code that it
might happen, the variable "stringtochange " *magically* changes after
the call to t.Change(). How easy is it to maintain that code? This is
exactly what C# seeks to avoid.
* Technically, this is a bit of a lie. k is, in fact, a reference to a
string, since strings are held by reference in string variables.
However, "inp" is a reference to a reference to a string, which is what
you need in order to change the argument passed in on the call.
Correctly, I should have said that there is no way in C# to declare k
as a private member that is a reference to a reference to a string,
which is what you would need in order to create the behaviour you want
to see. However, I rephrased it all in order to avoid confusion. In
this case you can think of strings as values, even though they're
really references. For the purposes of this explanation the distinction
is irrelevant.

[Bob Grommes]

Not so. Strings are indeed a reference type. After all, System.String is a
class, not a struct. However, because strings are immutable, they have very
value-like semantics, because any change to a string results in a completely
new string being created "under the hood". In most situations this makes a
string "behave" superficially as if it were a value type. For example:

myString = "foo" + someOtherString ;

.... really results in the compiler emitting code that works more like this:

myString = new String("foo" + someOtherString )

.... which will not change the original myString reference if it exists in
the caller's context, which makes this *appear* to behave like a value type.
But, all that is passed into a conventional string parameter of a method is
a *reference* to a string, nonetheless.

--Bob

----- Original Message -----
From: "mdb" <m_b_r_a_y@c_t_ i_u_s_a__d0t__c om>
Newsgroups: microsoft.publi c.dotnet.langua ges.csharp
Sent: Wednesday, June 01, 2005 4:09 PM
Subject: Re: 'ref' question

The "string" class isn't a reference type - its a value type, so in:

[Amar]

try rewriting it because the way it was you never actually accesing inp parameter.

public class stringtemp
{
public stringtemp()
{
}
public void Change(ref String inp)
{
inp.Remove(0, 1) ;
}
};
"Priyesh" <pr*****@donotr eply.com> wrote in message news:<uf******* *******@TK2MSFT NGP15.phx.gbl>. ..

public class stringtemp
{
String k ;
public stringtemp(ref String inp)
{
k = inp ;
}
public void Change()
{
k = k.Remove(0, 1) ;
}
};

String stringtochange = new String("Hello". ToCharArray()) ;
stringtemp t = new stringtemp(ref stringtochange) ;
t.Change() ;

Why isnt stringtochange changed to 'ello' here? When I try a similar sample
with an ArrayList instead of a String, my original variable gets reflected.
So my question is: How can i make sure i am operating on a reference?
Documentation says all class types are passed by references. Could someone
elaborate?

TIA

[Priyesh]

Bruce,

Thanks for the reply.

Couple of points that i dont understand still.

String stringtochange = "Hello";
stringtemp t = new stringtemp(ref stringtochange) ;
... 200 lines of complex code here ...
t.Change() ;

suddenly, without any warning or any indication in the code that it
might happen, the variable "stringtochange " *magically* changes after
the call to t.Change(). How easy is it to maintain that code? This is
exactly what C# seeks to avoid.
This does not hold good when i use an ArrayList or another user defined
class. My original variable is *magically*
changed.(See example below) I am failing to draw a common reference
behaviour i can base on from all these.
Correctly, I should have said that there is no way in C# to declare k
as a private member that is a reference to a reference to a string,
I think you are right here when using a string. When i use an ArrayList it
seem to work.

Here's what i tried.

public class arraylistchange
{
ArrayList ar ;

//This is passed in as a reference even if i dont specify ref.
public arraylistchange (ArrayList arIn)
{
//This seems to be copied as a reference. So my member
//variable ar is a reference type even if i dont specify it to be?
ar = arIn ;
}
public void Change()
{
ar.Add("456") ;
}
};

ArrayList ar = new ArrayList(10) ;
ar.Add("123") ;
arraylistchange archange = new arraylistchange (ar) ;
archange.Change () ;
int n = ar.Count ;//is 2

I thought String would do the same thing, but didnt. Any explanation about
this which would help me understand this better would be really helpful.
Thanks for your time.

"Bruce Wood" <br*******@cana da.com> wrote in message
news:11******** **************@ f14g2000cwb.goo glegroups.com.. . public stringtemp(ref String inp)

says that you can change "inp" _in the constructor_ and the
corresponding argument variable will change on the outside.

String k;

says what? Does it say "k is a reference to a string"? No, it says, "k
is a string". So, when you say

this.k = inp;

what you're saying is, "k should be the same string as is contained in
inp*". Since you never avail yourself of the opportunity to change inp
_within the constructor_, you just change k, but never inp,
"stringtochange " is never changed.

Just because you declare a parameter as "ref" does not mean that its
"refness" is sticky and follows that string value wherever it goes.
Now, if C# were allow you to say something like:

public class stringtemp
{
ref String k;

public stringtemp(ref String inp)
{
this.k = inp;
}
...
}

then we might have something going here. However "ref String k" is not
a valid field declaration, so you're out of luck.

By the time you get to the Change method, the fact that "inp" was
originally declared "ref" is completely irrelevant. It's only relevant
within the constructor.

One of the design goals of C# and .NET was to avoid the kind of pointer
mayhem that can occur in C and C++. That's why you can't declare
variables that hold references: they would just be pointers under
another name, and open the door to all kinds of madness. Take another
look at your example, and think of it like this:

String stringtochange = "Hello";
stringtemp t = new stringtemp(ref stringtochange) ;
... 200 lines of complex code here ...
t.Change() ;

suddenly, without any warning or any indication in the code that it
might happen, the variable "stringtochange " *magically* changes after
the call to t.Change(). How easy is it to maintain that code? This is
exactly what C# seeks to avoid.
* Technically, this is a bit of a lie. k is, in fact, a reference to a
string, since strings are held by reference in string variables.
However, "inp" is a reference to a reference to a string, which is what
you need in order to change the argument passed in on the call.
Correctly, I should have said that there is no way in C# to declare k
as a private member that is a reference to a reference to a string,
which is what you would need in order to create the behaviour you want
to see. However, I rephrased it all in order to avoid confusion. In
this case you can think of strings as values, even though they're
really references. For the purposes of this explanation the distinction
is irrelevant.

[Priyesh]

Amar,

Thanks for the reply. The point i was trying to put forward is elaborated in
my reply to Bruce.
"Amar" <ma******@gmail .com> wrote in message
news:8a******** *************** ***@posting.goo gle.com...

try rewriting it because the way it was you never actually accesing inp
parameter.

public class stringtemp
{
public stringtemp()
{
}
public void Change(ref String inp)
{
inp.Remove(0, 1) ;
}
};
"Priyesh" <pr*****@donotr eply.com> wrote in message
news:<uf******* *******@TK2MSFT NGP15.phx.gbl>. ..

public class stringtemp
{
String k ;
public stringtemp(ref String inp)
{
k = inp ;
}
public void Change()
{
k = k.Remove(0, 1) ;
}
};

String stringtochange = new String("Hello". ToCharArray()) ;
stringtemp t = new stringtemp(ref stringtochange) ;
t.Change() ;

Why isnt stringtochange changed to 'ello' here? When I try a similar
sample
with an ArrayList instead of a String, my original variable gets
reflected.
So my question is: How can i make sure i am operating on a reference?
Documentation says all class types are passed by references. Could
someone
elaborate?

TIA

[C# Learner]

Priyesh <pr*****@donotr eply.com> wrote:

This does not hold good when i use an ArrayList or another user defined
class. My original variable is *magically*
changed.
That's because ArrayList is mutable, while String isn't.

Consider:

1 {
2 string original = "foo";
3 string copy = original;
4
5 copy = "bar";
6 }

After the assignment in line three, both original and copy reference the
same string. However, after line five, copy now references a new string.

Replace line five with (from your example)

copy = original.Remove (0, 1)

and you get the same situation: copy now references a whole new string.

This happens because strings are immutable (their state cannot be changed).

The same thing happens here, for example:

myString = myString.Replac e("foo", "bar");

After this, myString now references a whole new string which looks like the
old one but has all occurences of "foo" replaced with "bar".
//This is passed in as a reference even if i dont specify ref.
public arraylistchange (ArrayList arIn)

Note that this is a mistake -- if that parameter were of type string, a
reference would be passed just the same. The only difference here is that
strings are immutable, while ArrayLists aren't.

Also, a 'ref' parameter is different from a reference type parameter. See
<http://www.yoda.arachs ys.com/csharp/parameters.html >.

[Jon Skeet [C# MVP]]

Amar <ma******@gmail .com> wrote:

try rewriting it because the way it was you never actually accesing inp parameter.

public class stringtemp
{
public stringtemp()
{
}
public void Change(ref String inp)
{
inp.Remove(0, 1) ;
}
};

That won't change the value either. You'd need:

inp = inp.Remove (0, 1);

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

[Jon Skeet [C# MVP]]

Priyesh <pr*****@donotr eply.com> wrote:

String stringtochange = "Hello";
stringtemp t = new stringtemp(ref stringtochange) ;
... 200 lines of complex code here ...
t.Change() ;

suddenly, without any warning or any indication in the code that it
might happen, the variable "stringtochange " *magically* changes after
the call to t.Change(). How easy is it to maintain that code? This is
exactly what C# seeks to avoid.

This does not hold good when i use an ArrayList or another user defined
class. My original variable is *magically*
changed.(See example below) I am failing to draw a common reference
behaviour i can base on from all these.

The value of your variable isn't changed at all. The value of your
variable is a reference to an object. The reference doesn't change, but
the ArrayList it refers to does. Now, that can't happen with a string,
because strings are immutable. None of the methods you can call on a
string actually change its contents.

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