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New Message on BDOTNET
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From: SitaramanM
Message 5 in Discussion
Hi Anand's point is very valid. There are basically two cases Case 1: If a
method is declared virtual in as base class and the same method is overridden in the
derived class, Then the Method implementation in the Underlying type of the object
rather than the declaring type of the object will be used. Case 2 :If a method is NOT
declared virtual in as base class and the same method is overridden(shadoewed to be
precise) in the derived class, Then the Method implementation in the declaring type of
the object rather than the Underlying type of the object will be used The reason why
it is behaving the way Anand has posted can be found if you disassemble the code you
will see that the ILDASM output is as follows ... ... L_000c: ldloc.1
L_000d: callvirt A.F
L_0012: ldloc.2
L_0013: callvirt A.F
L_0018: ldloc.3
L_0019: callvirt C.F
L_001e: ldloc.0
L_001f: callvirt C.F
L_0024: call Console.ReadLine
L_0029: pop
L_002a: ret ... ... Note that the calls A.F and B.F have been converted to A.F and
A.F. |||ly the calls C.F and D.F have been converted to C.F and C.F So the first
two calls are A.F calls where it is virtual and so the concrete B.F will get executed.
So it print B.F doe the first 2 calls Similarly the second 2 calls are C.F calls
where again it is virtual and so the concrete D.F will get executed But the
question is : According to MSDN , as noted by Gags and by me in a previous post "When
a virtual method is invoked, the run-time type of the object is checked for an
overriding member. The overriding member in the most derived class is called, which
might be the original member, if no derived class has overridden the member.In a
virtual method invocation, the run-time type of the instance for which theinvocation
takes place determines the actual method implementation to invoke". This means that
Anand's code should print a.F(); => Virtual in A so should print Underlying Type
function i.e. D.F
b.F(); => Concrete in B so should print Declared Type function i.e. B.F
c.F(); => Virtual in C so should print Underlying Type function i.e. D.F
d.F(); => Concrete in D so should print Declared Type function i.e. D.F So the
actual implementation is different from what is specified in MSDN!!!! Why???
To make things more confusing, try a similar code in C++ compile by MS C++Compiler
// Test.cpp : Defines the entry point for the console application.
// #include "stdafx.h"
#include <conio.h>
#include <conio.h> class A
{ public:virtual void F()
{
printf("A.F Called");
}
}; class B:A
{ public: void F()
{
printf("B.F Called");
}
};
class C:B
{ public:virtual void F()
{
printf("C.F Called");
}
}; class D:C
{ public: void F()
{
printf("D.F Called");
}
}; int main(int argc, char* argv[])
{
printf("Hello World!\n");
D d; A *a=(A*)new D;
B *b=(B*)new D;
C *c=(C*)new D;
a->F();;printf("\n");// prints D.F()
b->F();;printf("\n");// prints D.F()
c->F();;printf("\n");// prints D.F()
d.F();;printf("\n");// prints D.F() A *a1=new A;
a1->F();;printf("\n"); // prints A.F().Cool!!!
getch();
return 0;
} This is as expected but for one case B.F() call. As the method is virtual, the
underlying type that is D is used. But the method F of Class B is concrete. So why is
it printing D.F and not B.F??? regards, sr
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