Multi level inheritance in c#

Question

I am trying to call a method from class A and apply it to a value in class C:

class A
{
    //my code here
    public virtual string calculatebnft()
    {
        string bnft = "";
        //my code here
        return bnft;
    }
}

class B : A
{
    //my code here
    public override string calculatebnft()
    {
        string bnft = "";
        //my code here
        return bnft;
    }
}

class C : B
{
    //my code here
}

In my Main method:

C c1=new C();
//my code here
string bnft=c1.calculatebnft();
MessageBox.Show(bnft);

When I run program it always runs class B's method calculatebnft(). How can I call calculatebnft() from A instead? The rest of code from B "which is working correctly".

If the runtime type of your object is at least B it will always run the method from B (unless it is declared as new, but then it's actually a different method) — knittl
– knittl, Commented Jan 30, 2014 at 19:08
Even though you can achieve it, you might want to reconsider. I think most folks would (understandably) expect that if your class inherits from B, then it should utilize B's implementation of methods; otherwise, it makes more sense as a parallel implementation of A. — Brian Warshaw
– Brian Warshaw, Commented Jan 30, 2014 at 19:11

MarcinJuraszek · Accepted Answer · 2014-01-30 19:08:39Z

1

You can't do that without changing B code. You have to change calculatebnft method from override to new:

class B : A
{
    //my code here
    public new string calculatebnft()
    {
        string bnft = "";
        //my code here
        return bnft;
    }
}

with that you could do following:

string bngt = ((A)c1).calculatebnft();

answered Jan 30, 2014 at 19:08

MarcinJuraszek

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Comments

JaredPar · Accepted Answer · 2014-01-30 19:09:35Z

1

From the outside there is no way to call a base class instance of a virtual method. It is simply not possible because it is forbidden by the CLR (possible, but not verifiable). The only way to get access to it is for A or B to give you another method on which to call the functionality. For example

class A {
  public string callMe() { 
   return ...;
  }

  public virtual string calculatebnft() {
    return callMe();
  }
}

Now a caller who wants the A version of calculatebnft can use callMe instead.

In general though I'd consider this bad practice. If there is a situation where you really wanted the base class version of a virtual method then it's probably not the best method to be virtual.

answered Jan 30, 2014 at 19:09

JaredPar

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4 Comments

Sriram Sakthivel Over a year ago

+1 exactly what I was about to post. Can you elaborate a bit on possible, but not verifiable?

JaredPar Over a year ago

@SriramSakthivel at an IL level you can issue a .call (non virtual method of calling) on A::calculatebnft and it would do the trick. This essentially how a base.calculatebnft would be issued. But when done from outside the class hierarchy it's considered unverifiable code

Sriram Sakthivel Over a year ago

What you mean by unverifiable here? CLR will throw InvalidProgramException in that case? or something else?

JaredPar Over a year ago

@SriramSakthivel tools like peverify will say that it's incorrect. It can be loaded in the CLR under full trust but I doubt it would load under say partial trust

Martin Volek · Accepted Answer · 2014-01-30 19:14:50Z

0

I don't think that's possible this way.

But for example you can do something like this:

class A
{
    //my code here
    public virtual string calculatebnft()
    {
        string bnft = "A";
        //my code here
        return bnft;
    }
}

class B : A
{
    public string calculatebnftFromA()
    {
        return base.calculatebnft();
    }

    //my code here
    public override string calculatebnft()
    {
        string bnft = "B";
        //my code here
        return bnft;
    }
}

answered Jan 30, 2014 at 19:14

Martin Volek

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