Data binding concept in C#

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3.00/5 (2 votes)

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class A
{
public virtual void WhoAreYou() { Console.WriteLine("I am an A"); }
}
class B : A
{
public override void WhoAreYou() { Console.WriteLine("I am a B"); }
}
class C : B
{
public new virtual void WhoAreYou() { Console.WriteLine("I am a C"); }
}
class D : C
{
public override void WhoAreYou() { Console.WriteLine("I am a D"); }
}

C c = new D();
c.WhoAreYou();// "I am a D"
A a = new D();
a.WhoAreYou();// "I am a B" !!!!
Can any one explain Whats going On?

Posted 2-Dec-13 11:35am

Member 10440333

Updated 2-Dec-13 12:53pm

Sergey Alexandrovich Kryukov

v2

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db7uk 2-Dec-13 18:50pm

Apart from being a very confusing post, one with no detail, what you have tried and are trying to achieve etc I can tell you that you need to look more at object inheritance. a good example is http://stackoverflow.com/a/160095. Is this an exam question per chance?

Sergey Alexandrovich Kryukov 2-Dec-13 19:21pm

Why? Nothing confusing for those how has any idea of OOP. OP just wants to understand how things work but got lost a bit. This is a nice code sample to demonstrate how things work. I answered the question in full, to certain level of detail. Please see Solution 1.

Better ask yourself if you understand how things work, in particular, in OP's sample, as this is very important for every software developer.

—SA

db7uk 2-Dec-13 20:14pm

Ok Granted and very valid. The code supplied was actually better than some other posts however I guess I was confused by the "whats going on". It took me a couple of mins to figure out the actual question hence my response. It also seems you worked out the question much quicker than I so hats of to you. I like your solution. Clear and I very much doubt I would do nearly as well.

I guess my point was that the OP question was not specific enough and may not have looked into the issue that much before posting. Not a bad thing really, just one of those things.

Sergey Alexandrovich Kryukov 2-Dec-13 20:30pm

I understand. I was a bit confused at first, too, but after more thorough reading to the very end it became clear.
Unfortunately, the fact that most of the questions we receive are quite pointless can cause some false-positive detection of bad questions...
—SA

BillWoodruff 2-Dec-13 21:12pm

SAK wrote: "Unfortunately, the fact that most of the questions we receive are quite pointless can cause some false-positive detection of bad questions."

Which, imho, is balanced out by the number of questions which are meaningful, but which are perceived as false-negatives :)

Sergey Alexandrovich Kryukov 3-Dec-13 0:08am

I'm not sure I was clear enough. The question cannot be false/true-negative/positive, the assertion can be. By false positive, I meant detection of bad questions. Say, I mistakenly mark a question as making no sense, but it really makes sense, but a bit vague, so it only resembles the question which does not make sense. Such mistakes are likely due to too many non-sensible posts.

Probably, this is what you meant. I don't know how anything can be balanced. If, say, you have 10 false-positives + 10 false-negatives, you have 20 mistakes, not 0.

Anyway, an inquirer usually shares at least part of responsibility for such false-positives. If the question actually makes sense but this sense cannot be seen due to ugly formulation, it's hard to provide and get help. I would advise anyone to be more accurate in wording. Reading some literature helps to be understood.

Cheers,
—SA

Sergey Alexandrovich Kryukov 2-Dec-13 19:25pm

Despite of naive look at the problem and insufficient effort (I think), I up-voted this question with my 4, because OP at least tries to understand how things work and provided a clear short code sample with effective demonstration of the main OOP mechanism. Instead of asking "how to fix it" without any understanding. I would only advise to put more effort and investigate things independently, by reading on OOP and using the debugger.
—SA

Sergey Alexandrovich Kryukov 2-Dec-13 19:56pm

And where do you see data binding? "Data binding" is like virus expression: the whole crowd started to call everything "binding". And hope that it will solve problems. Instead of doing programming. :-)
—SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-12-02T13:19:00

What's going on is called "the most fundamental idea of OOP, without which OOP is a not really OOP but just a meaningless name". (For many our inquirers, OOP is a meaningless name anyway. :-()

When you create a virtual function (which could be abstract), you create a slot in Virtual Method Table (VMT). If you don't know how what is VMT and how it works, you should stop reading this answer and also stop doing any programming, as otherwise continuing with either of the two would be a waste of time. The methods are called via VMT indirectly, so when you override a method, the slot in VMT is replaced with a new version of the method. The method is called according to the run-time type, not a design-time type (declared type) of an object. If you don't understand the distinction between run-time and design-time types… see above. That's why in first WhoAreYou call you have "I am a D". It would be the same if the declared type was B instead of C.

Now, so far, everything seems to be clear. Until you declare the identical object a with the same run-time type D as A. Here you need to understand that you have two totally unrelated VMT slots. Let's see:

"WhoAreYou"-slot 1: A.WhoAreYou —> B.WhoAreYou
"WhoAreYou"-slot 2: C.WhoAreYou —> D.WhoAreYou

They are unrelated, only have the same name. This happened because in C you did not override "slot1"-WhoAreYou with "override", but reintroduced a totally new method under the same name, again, a virtual one. These two names addressing two different unrelated slots hide each other. If your compile-time declaration is A or B, slot 1 is implied, if C or D — slot 2.

In second case, "slot1"-WhoAreYou gives you the latest overridden version of A.WhoAreYou, which was defined by B.WhoAreYou, hence "I am a B".

You need to learn how virtual methods work very deeply, as a conscious developer. In other words, always remember who are you. :-)

This can help to understand things on the OOP level:
http://en.wikipedia.org/wiki/Dynamic_dispatch[^],
http://en.wikipedia.org/wiki/Virtual_table[^].

—SA