out Parameter And Base Class ?

[dahuzizyd]

Hi all:
I think I had a problem with using out parameter , why the instance of
'SubClass' can't convert to 'BaseClass' ? my code is :
----------------------------------------------
using System;

namespace ConsoleApplicat ion2
{

class Class1
{
[STAThread]
static void Main(string[] args)
{
SubClass y = new SubClass() ;
Test( out y);
}

static void Test( out BaseClass y )
{
Console.WriteLi ne(y.ToString() );
}
}
class BaseClass
{
public BaseClass()
{
Console.WriteLi ne("BaseClass") ;
}
}
class SubClass : BaseClass
{
public SubClass()
{
Console.WriteLi ne("SubClass") ;
}
}
}


thanks in advance
Yadong Zhao

[Nicholas Paldino [.NET/C# MVP]]

Re: out Parameter And Base Class ?

dahuzizyd,

The reason you get this is because y needs to be of type BaseClass. If
you derived another class from BaseClass, say, SubClass2, then the Test
method could assign the parameter y to an instance of SubClass2. Upon
return, the variable y would not be able to be assigned to, because it is of
type SubClass2, not SubClass.

Hope this helps.


--
- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard. caspershouse.co m


"dahuzizyd" <dahuzizyd4@hot mail.com> wrote in message
news:%23oM5H7pB GHA.2704@TK2MSF TNGP15.phx.gbl. ..[color=blue]
> Hi all:
> I think I had a problem with using out parameter , why the instance of
> 'SubClass' can't convert to 'BaseClass' ? my code is :
> ----------------------------------------------
> using System;
>
> namespace ConsoleApplicat ion2
> {
>
> class Class1
> {
> [STAThread]
> static void Main(string[] args)
> {
> SubClass y = new SubClass() ;
> Test( out y);
> }
>
> static void Test( out BaseClass y )
> {
> Console.WriteLi ne(y.ToString() );
> }
> }
> class BaseClass
> {
> public BaseClass()
> {
> Console.WriteLi ne("BaseClass") ;
> }
> }
> class SubClass : BaseClass
> {
> public SubClass()
> {
> Console.WriteLi ne("SubClass") ;
> }
> }
> }
>
>
> thanks in advance
> Yadong Zhao
>
>[/color]

[dahuzizyd]

Re: out Parameter And Base Class ?

Thanks for your help .
My english is so bad that I can't show my problem clearly : - (

I write this code to make the Test method can reusnable. All class which
derived from BaseClass can be a parameter for this method.But if I write
like this:

BaseClass y ;
Test( out y);

static void Test( out BaseClass y )
{
y = new SubClass();
Console.WriteLi ne(1);
}

this code can be compiled . But the BaseClass will mean nothing.. If I don't
use the out word,everything is good.
So I think there are some thing that I never knew with the out word.

class Class1
{
[STAThread]
static void Main(string[] args)
{
DeriveClass y ;
Test( out y);

DeriveClass s = new DeriveClass();
DeriveClass2 s1 = new DeriveClass2();
Test1(s);
Test1(s1);
}
static void Test( out BaseClass y )
{
y = new DeriveClass();
Console.WriteLi ne(1);
}
static void Test1 ( BaseClass y )
{
Console.WriteLi ne(2);
}
}
class BaseClass
{
}
class DeriveClass : BaseClass
{

}
class DeriveClass2 : BaseClass
{

}


thanks in advance
Yadong Zhao

[Jon Skeet [C# MVP]]

Re: out Parameter And Base Class ?

dahuzizyd <dahuzizyd4@hot mail.com> wrote:[color=blue]
> Thanks for your help .
> My english is so bad that I can't show my problem clearly : - ([/color]

No, I think you showed your problem fine.
[color=blue]
> I write this code to make the Test method can reusnable. All class which
> derived from BaseClass can be a parameter for this method.But if I write
> like this:
>
> BaseClass y ;
> Test( out y);
>
> static void Test( out BaseClass y )
> {
> y = new SubClass();
> Console.WriteLi ne(1);
> }
>
> this code can be compiled . But the BaseClass will mean nothing.. If I don't
> use the out word,everything is good.[/color]

Indeed.
[color=blue]
> So I think there are some thing that I never knew with the out word[/color]

It's exactly as Nick said. You can't use a variable of a different type
(even if it's a subclass) for an "out" argument. Your Test method (in
your first post) could create a new instance of BaseClass (or a
different subclass) instead of SubClass - and at that stage, when the
method compiled, the variable wouldn't have the right kind of value.

This is specified in the C# language spec as:
<quote>
When a formal parameter is an output parameter, the corresponding
argument in a method invocation must consist of the keyword out
followed by a variable-reference of the same type as the formal
parameter.
</quote>

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

--
Jon Skeet - <skeet@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

[Joanna Carter [TeamB]]

Re: out Parameter And Base Class ?

"dahuzizyd" <dahuzizyd4@hot mail.com> a ecrit dans le message de news:
%23oM5H7pBGHA.2 704@TK2MSFTNGP1 5.phx.gbl...

| Hi all:
| I think I had a problem with using out parameter , why the instance of
| 'SubClass' can't convert to 'BaseClass' ? my code is :

I think this could be a misleading message; there is definitely a problem
with the following :

| static void Test( out BaseClass y )
| {
| Console.WriteLi ne(y.ToString() );
| }

You cannot call any methods on y until you have assigned a valid instance to
it.

The out parameter means that the reference passed in *has* to be initialised
as it is assumed to be null. The purpose of the out modifier is to allow the
same funtionality as returning a reference from a function and should be
treated as such. Its purpose is to return references from a method not to
pass references into it.

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Jon Skeet [C# MVP]]

Re: out Parameter And Base Class ?

Joanna Carter [TeamB] <joanna@not.for .spam> wrote:[color=blue]
> "dahuzizyd" <dahuzizyd4@hot mail.com> a ecrit dans le message de news:
> %23oM5H7pBGHA.2 704@TK2MSFTNGP1 5.phx.gbl...
>
> | I think I had a problem with using out parameter , why the instance of
> | 'SubClass' can't convert to 'BaseClass' ? my code is :
>
> I think this could be a misleading message; there is definitely a problem
> with the following :[/color]

It's not a misleading message - it's just that there are multiple
things wrong with the code.
[color=blue]
> | static void Test( out BaseClass y )
> | {
> | Console.WriteLi ne(y.ToString() );
> | }
>
> You cannot call any methods on y until you have assigned a valid instance to
> it.
>
> The out parameter means that the reference passed in *has* to be initialised
> as it is assumed to be null.[/color]

No - it's not assumed to be null. It's not definitely assigned. Big
difference :) If it were assumed to be null, you wouldn't get a
compile-time error, you'd get a NullReferenceEx ception at run-time.
[color=blue]
> The purpose of the out modifier is to allow the
> same funtionality as returning a reference from a function and should be
> treated as such. Its purpose is to return references from a method not to
> pass references into it.[/color]

Indeed. The reason the OP is getting the error message, however, is
that the actual argument must be a variable of exactly the same type as
the output parameter.

--
Jon Skeet - <skeet@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

[Joanna Carter [TeamB]]

Re: out Parameter And Base Class ?

"Jon Skeet [C# MVP]" <skeet@pobox.co m> a écrit dans le message de news:
MPG.1e146c048af ae85698cb93@msn ews.microsoft.c om...

| No - it's not assumed to be null. It's not definitely assigned. Big
| difference :) If it were assumed to be null, you wouldn't get a
| compile-time error, you'd get a NullReferenceEx ception at run-time.

You are right, it's early and my brain isn't yet communicating clearly :-)

| > The purpose of the out modifier is to allow the
| > same funtionality as returning a reference from a function and should be
| > treated as such. Its purpose is to return references from a method not
to
| > pass references into it.
|
| Indeed. The reason the OP is getting the error message, however, is
| that the actual argument must be a variable of exactly the same type as
| the output parameter.

Agreed. I think the problem is that the OP does not yet understand what the
out modifier actually does.

private void TestOut(out BaseClass value)
{
value.Test();
}

This code actually raise two errors :

Error 1 Use of unassigned out parameter 'value' F:\Test\Form1.c s 31 7 Test
Error 2 The out parameter 'value' must be assigned to before control leaves
the current method F:\Test\Form1.c s 29 18 Test

So the OP's examples definitely won't compile for other reasons than the
faulty parameter type. These errors make it clear that an out parameter
*must* be assigned something and not used to pass something in.

Can I wake up now ? :-)

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Jon Skeet [C# MVP]]

Re: out Parameter And Base Class ?

Joanna Carter [TeamB] <joanna@not.for .spam> wrote:[color=blue]
> | No - it's not assumed to be null. It's not definitely assigned. Big
> | difference :) If it were assumed to be null, you wouldn't get a
> | compile-time error, you'd get a NullReferenceEx ception at run-time.
>
> You are right, it's early and my brain isn't yet communicating clearly :-)[/color]

No problem. I suspected you probably realised that, but thought it
would be worth clarifying it for the sake of everyone else :)
[color=blue]
> | > The purpose of the out modifier is to allow the
> | > same funtionality as returning a reference from a function and should be
> | > treated as such. Its purpose is to return references from a method not
> to
> | > pass references into it.
> |
> | Indeed. The reason the OP is getting the error message, however, is
> | that the actual argument must be a variable of exactly the same type as
> | the output parameter.
>
> Agreed. I think the problem is that the OP does not yet understand what the
> out modifier actually does.[/color]

Quite possibly. The reason you can't use an out argument of a derived
type is probably the most subtle part of it, to be honest.
[color=blue]
> private void TestOut(out BaseClass value)
> {
> value.Test();
> }
>
> This code actually raise two errors :
>
> Error 1 Use of unassigned out parameter 'value' F:\Test\Form1.c s 31 7 Test
> Error 2 The out parameter 'value' must be assigned to before control leaves
> the current method F:\Test\Form1.c s 29 18 Test
>
> So the OP's examples definitely won't compile for other reasons than the
> faulty parameter type. These errors make it clear that an out parameter
> *must* be assigned something and not used to pass something in.[/color]

Yup.
[color=blue]
> Can I wake up now ? :-)[/color]

Yup - go get some coffee :)

--
Jon Skeet - <skeet@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

[dahuzizyd]

Re: out Parameter And Base Class ?

Thanks for your answer .

I think I understand.Befo re I write this code . I never pay attention to
this point of the out keyword.
:-(

Now , I rewrite my code to reach my purpose for reusnable. It's looks better
than before. :-)

My code is :
-----------------------------------------
class Class1
{
[STAThread]
static void Main(string[] args)
{
BaseClass y ;
Test( out y ,typeof(DeriveC lass2) );
y.WriteName();
Console.ReadLin e();
}
static void Test( out BaseClass y ,Type t )
{
y = BaseClass.Creat eInstance(t);
}
}
class BaseClass
{
public static BaseClass CreateInstance( Type deriveClassType )
{
return (BaseClass)Syst em.Activator.Cr eateInstance(de riveClassType);
}
public void WriteName()
{
Console.WriteLi ne(this.ToStrin g());
}
}
class DeriveClass : BaseClass
{
}
class DeriveClass2 : BaseClass
{
}

Thanks and Regards
Yadong Zhao

[Jon Skeet [C# MVP]]

Re: out Parameter And Base Class ?

dahuzizyd <dahuzizyd4@hot mail.com> wrote:[color=blue]
> Thanks for your answer .
>
> I think I understand.Befo re I write this code . I never pay attention to
> this point of the out keyword.
> :-(
>
> Now , I rewrite my code to reach my purpose for reusnable. It's looks better
> than before. :-)[/color]

Good. It's worth avoiding out parameters where you can though,
preferring to use straight return values. That's simpler, and more
obvious.

--
Jon Skeet - <skeet@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

[Joanna Carter [TeamB]]

Re: out Parameter And Base Class ?

"dahuzizyd" <dahuzizyd4@hot mail.com> a ecrit dans le message de news:
ODMZ8btBGHA.628 @TK2MSFTNGP14.p hx.gbl...

| Now , I rewrite my code to reach my purpose for reusnable. It's looks
better
| than before. :-)

In actual fact, what you are doing is implementing the Factory Method design
pattern.

Using an out parameter is not the usual way of doing this, at least for
returning a single instance of a given type.

You could just as well do this :

class Class1
{
[STAThread]
static void Main(string[] args)
{
BaseClass y = BaseClass.Creat eInstance(typeo f(DeriveClass2) );
y.WriteName();
Console.ReadLin e();
}
}

Or if you still feel that you need to have a Factory Method in Class1 as
well, then just use a method that returns an object of BaseClass type.

class Class1
{
static BaseClass Test(Type t )
{
return BaseClass.Creat eInstance(t);
}

[STAThread]
static void Main(string[] args)
{
BaseClass y = Test(typeof(Der iveClass2));
y.WriteName();
Console.ReadLin e();
}
}

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[dahuzizyd]

Re: out Parameter And Base Class ?

This question is from our project . My origin code is an example for
project's code .
The project's framework design and coding is aleary finished . The desinger
and coder of the framework is at another country . So we have no chance to
change it and I think I must know the real reason for this. Thanks for your
help !

Thanks and Regards
Yadong Zhao