How to call a base class virtual method instead - C# Discussion Boards

Re: need code for this app in C#

2-Jul-07 12:55

I voted only 4, since you did not provide NUnit fixture, which is essential
to verify the lot.

Smile | :)

Luc Pattyn

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Re: Here ya go

ruanr2-Jul-07 20:58

ruanr

2-Jul-07 20:58

I lol'd irl

Dave Kreskowiak2-Jul-07 12:53

Dave Kreskowiak

2-Jul-07 12:53

Sure. Fork over about US$10,000 (cash, up front) and I'll do your homework for you. Pity, you won't learn anything so you can actually pass the course.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

How to call a base class virtual method instead

mschuckmann2-Jul-07 10:45

Re: How to call a base class virtual method instead

2-Jul-07 10:45

Given the following class hyerarcy I'm trying to figure out how to explicitly call the Base class implimentation of a virtual method (in this case Name()) instead of the Child class implimentation:

class Base
{
public virtual void Name()
{
Console.Writeline( "Base" );
}
};

class Child : Base
{
public override void Name()
{
Consol.Writeline( "Child" );
}
};

So far the only way I've been able to figure out how to do this is to impliment another method on Child, something like
...
public void BaseName()
{
base.Name();
}
...

But I don't want to have to impliment this method, I want to call the Base class Name method from outside of Child.

In C++ this would be done by using the static binding operator (: Smile | :)

, like so.

Child c;
c.Base::Name();

It seems like there has to be something equivalent in C#.

Thanks
Matt Schuckmann

PIEBALDconsult2-Jul-07 10:54

Re: How to call a base class virtual method instead

2-Jul-07 10:54

mschuckmann wrote:
Child c;
c.Base::Name();

Cast the child to base:

<br />
Child c;<br />
((Base) c).Name();<br />

(I hope that works, I didn't test it.)

Pete O'Hanlon2-Jul-07 10:58

Pete O'Hanlon

2-Jul-07 10:58

Unfortunately, this doesn't work. I did try it.

Please visit http://www.readytogiveup.com/ and do something special today.
Deja View - the feeling that you've seen this post before.

Re: How to call a base class virtual method instead

PIEBALDconsult2-Jul-07 11:37

Re: How to call a base class virtual method instead [modified]

2-Jul-07 11:37

Dang, I put my foot in it again.

From http://msdn2.microsoft.com/en-us/library/aa645767(VS.71).aspx[^]

Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less derived type in which the method is not hidden.

This must be one of those naughty things that C++ allows that C# fixed.

Luc Pattyn2-Jul-07 11:46

Re: How to call a base class virtual method instead

2-Jul-07 11:46

Of course ! that's how you can do it, at the expense of some new method names.

Here is my experiment:

class class1 {
    public virtual void print() { basePrint(); }
    public void basePrint() { log("class1"); }
}

class class2 : class1 {
    public override void print() { log("class2"); }
}
class class3 : class2 {
    public override void print() { log("class3"); }
}

public void Test() {
    class3 a=new class3();
    a.print();
    a.basePrint();
    class1 b=new class3();
    b.print();
    b.basePrint();
}

it does print:

class3
class1
class3
class1

at the expense of 1 additional method (basePrint in the base class)
[EDIT: there is absolutely no need for a second method in class2, what was I thinking?]

I still would prefer something that strips the specialization (but without copying data),
so one could write b.base.print(), which would mean print() needs no sibbling.

Smile | :)

-- modified at 17:55 Monday 2nd July, 2007

Luc Pattyn

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mschuckmann2-Jul-07 11:58

Re: How to call a base class virtual method instead

2-Jul-07 11:58

That's not even right the immediate base of class3 is class2 not class1.
And who wants to pollute their code with a bunch of extra methods when there should a be a way to syntactically statically bind to any method in the inheritance tree.

Yes it's not something you need to do often and it might mean that there is something wrong with your design but there still should be a way to do it, it's not unsafe.

Luc Pattyn2-Jul-07 12:10

Re: How to call a base class virtual method instead

2-Jul-07 12:10

mschuckmann wrote:
the immediate base of class3 is class2 not class1

That is easily fixed by adding a new basePrint() at each level

mschuckmann wrote:
pollute their code

I agree completely; now if its only for one method (Serialize) one might choose to live
with it, lacking an alternative in the given context.

Smile | :)

Luc Pattyn

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mschuckmann2-Jul-07 12:14

Re: How to call a base class virtual method instead

2-Jul-07 12:14

Agreed, bummer.

Weirdoz1210-Nov-09 17:59

Weirdoz12

10-Nov-09 17:59

+ if you want to call print from class1 you should use this syntax:

((class1)b).print();
where class1 is the base class.

Re: How to call a base class virtual method instead

Luc Pattyn2-Jul-07 11:19

Re: How to call a base class virtual method instead

2-Jul-07 11:19

Hi Matt !

I think you cant:

1.
if you have a Base object or a Base-derivative pointed to by a Base reference,
it will do the virtual call, as intended by the virtual keyword.

2.
there is the "base" keyword, but its use is restricted (see paragraph 7.5.8 in the C# ref man)
to a few places, all inside classes that derive from the base class.
So yoy cant do myObject.base.methodname()

Conclusion: the virtual stuff really hides the base methods/properties.
And my first impression is that is how it should be.
Why is it you want to get at the base implementation from outside of the class anyway ?

There is of course a trick to do it, by providing a Clone() method in the base class,
something that takes a Base or Base-derivative and creates a new Base object, equivalent
to the base-part of the object (similar to a copy constructor), stripping off all
specialization.

Smile | :)

Luc Pattyn

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mschuckmann2-Jul-07 11:44

Re: How to call a base class virtual method instead

2-Jul-07 11:44

Thanks for the reply and that's pretty much what I thought that you can't.

This kind of came up from a friend of mine who is trying to impliment something like the boost serialization library in C# in that he has a secondary class that is walking the inheritance tree of the class to serialize by exlicitly serializing the base classes states. It kind of does this in a funny way and I'm not entirely convinced it's the right way.

Beyond that I have run into times where it was handy to do this, usually because something beyond my control was implimented improperly.

Luc Pattyn2-Jul-07 11:50

Re: How to call a base class virtual method instead

2-Jul-07 11:50

I see; in the mean time PIEBALDconsult has come up with a good approach that may well
fit your needs.

Smile | :)

Luc Pattyn

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PIEBALDconsult2-Jul-07 11:58

Re: How to call a base class virtual method instead

2-Jul-07 11:58

Ummm... What? Did I?

Luc Pattyn2-Jul-07 12:06

Re: How to call a base class virtual method instead

2-Jul-07 12:06

Sorry

with a spoonful of lateral thinking I read your sentence "Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less derived type in which the method is not hidden" as "WTF just give the method a name that remains visible...".

Luc Pattyn

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PIEBALDconsult2-Jul-07 13:35

Re: How to call a base class virtual method instead

2-Jul-07 13:35

That wasn't my sentence; it's a quote from the linked document... I need to find a clearer way of indicating such.

Luc Pattyn2-Jul-07 13:49