Problem in Inheritance

Question

My Class Structure is Like This.

Class A
{
}

Class B:A{..}

Class Demo
    {
    //Here Actual Parameter is instance of Class B. But in xyz function accepts
    //  parameter of type Class A. So in Below function can i get the Properties of 
    //Class B by using "((B)a).Property_Name", But i don want to specify the Name 
    //of Class B. So is ther any approach to meet my need.
    public void xyz(A a){ ....Here I Can i get }
    }

So shld i change the design. Can you plz suggest me any best way to accomplish this. — sandeep, Sep 19 '11 at 04:31
Your question makes no sense whatsoever. It's already an `A`. Just call the members of `A` on it. — Andrew Barber, Sep 19 '11 at 04:40
@Andrew Barber, He is saying, while calling he does like this B b = new B() and then he needs to use B's properties. So absolutely down casting — Zenwalker, Sep 19 '11 at 05:46
Andrew Barber@ ya its A but how can we get the property of A using B?. Anyways thanx for ur try.. — sandeep, Sep 19 '11 at 05:49

score 1 · Answer 1 · answered Sep 19 '11 at 04:31

Point you missed is B is derived class of A.

Which means B already has access to A's properties so you don't need cast.

You can just pass B parameter to XYZ function which takes A and then you can just use

to call A's properties.

On the other hand if you are trying to access private properties of A... that is just not possible outside of class A and considered logical error.

Now different scenario is if you are passing B type parameter into XYZ and want to access B's properties then you will have to cast it to B as A was created before B so it has no knowledge of B's specific properties and methods.

score 1 · Accepted Answer · edited May 23 '17 at 11:47

It sounds like you want to use a member of B that doesn't exist in A in a method that takes in A as a parameter. You don't want to move the member to A because it probably does't make sense to do so and you want something special when a B is passed in.

The most straight forward way is to test for it

public void xyz(A a){


     if (a is B) 
     {
          ((B)a).dosomthing()
     } 

 }

However this is bad because maybe you have C that needs this to.

Then maybe its better to introduce an interface e.g.

B: A, ISomthing

public void xyz(A a){


     if (a is ISomthing) 
     {
          ((ISomthing)b)dosomthing()
     } 

 }

Another option is to use Double Dispatch

score 1 · Answer 3 · answered Sep 19 '11 at 04:57

1

No you can't access the members of B from an obj of type A, think of this, if A had properties X and Y, and B had property Z, how would you cast it and provide the value of Z? The CLR would have no idea, you would have to change the method signature to take a param of type B to accomplish this. Alternatively if you absolutely know how all cases of A should cast to B, you could implement your own casting operation to handle that for you.

answered Sep 19 '11 at 04:57

Jason

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"how would you cast it and provide the value of Z" -- like this `((B)a).Property_Name` – Conrad Frix Sep 19 '11 at 05:42
Yes you are correct it will compile and *may* work, however, the method takes a type A, there is no guarantee that the type passed will be of type B (if there is then I'd think he should change the type to B instead). If he absolutely wanted to do it that way, imho, using the "as" cast operator and doing a null check or doing the interface implementation (as you suggested in the solution) would actually be better than the former – Jason Sep 19 '11 at 16:08
That's a good point about `as` however in this case I would probably go with adding `if (a == null) throw new ArgumentNullException("a");` So as not to conflate a null argument with a unable to cast. Typically I reserve `as` for when I want to treat nulls and unable to cast to type as the same. – Conrad Frix Sep 19 '11 at 16:33

Problem in Inheritance

3 Answers3