How can I override a virtual method, but still invoke the base class version in C#

Question

I have a simple class hierarchy where I have a virtual method that is overriden. But at certain callsites I want to call the base class version of this method rather than the virtual method.

For example:

public class A {
    public virtual void Foo() {...}
}

public class B : A {
    public override void Foo() {...}
}

public class Program {
    public void SomeMethod()
    {
       ...

       //  ListofA is type IEnumerable<A>
       foreach (var item in ListofA)
       {
           // I want this to call A.Foo(), rather than B.Foo()
           // But everything I've tried, which has really just been casting, has resulted in B.Foo()
           item.Foo();
       }
    }
}

Answer 1

You can't on an override. Overrides replace the original (from the standpoint of the caller). An overridden method may call the base, but you can't externally.

Answer 2

@James is right. To build on his answer, since you can call the base version from the overridden one, you could send some kind of a flag into the method to tell it whether to execute its own implementation or the overridden one. Something like this:

public override void Foo(bool useBaseImplementation)
{
    if(useBaseImplementation)
    {
        base.Foo(useBaseImplementation);
    }
    else
    {
        //other stuff here
    }
}

In order for it to work, you'd have to have the flag as a parameter for the base one as well, but you could just ignore it there. Not elegant, maybe downright ugly, but does what you're looking for.

Answer 3

If this is the case then what you want isn't actually an override; you should rethink your design. If instead of override you use new in the redefinition the method won't be polymorphic, but again, this is highly indicative of a flawed design. You would also need foreach (A item in ListofA)

Answer 4

I don't like my solution, beacause it sorta uglifies abstration, but maybe you could do the following:

public class A {
  public virtual void Foo() {...}
}

public class B : A {
  public override void Foo() {...}
  public override void parentFoo(){
    base.Foo();
  }
}

public class Program {
  public void SomeMethod(){
    ...

     //  ListofA is type IEnumerable<A>
     foreach (var item in ListofA){
       item.Foo(); //calls B.Foo()
       item.parentFoo(); //calls B.parentFoo() == A.Foo()
     }
  }
}

Answer 5

If var item = new A(), it will call A.Foo().

Answer 6

You can call base implementation using the base keyword INSIDE the class, but if You are trying to call it from the other class I am pretty sure You are violating a Liskov Substitution Principle in Your code, as Any subclass shoudl provide such an implementation to virutal methods to be completely substitutable for superclass. Revisit YOUR design.

Answer 7

You can achieve that with using new rather than override. It is very well explained here: http://msdn.microsoft.com/en-us/library/ms173153%28v=vs.80%29.aspx

Answer 8

One possible workaround is not to make the method virtual, but replace it using the new keyword:

   class Program
{
    static void Main(string[] args)
    {
        List<A> list = new List<A>
        {
            new B(),
            new B(),
            new B(),
            new B()
        };

        foreach (A a in list)
        {
            a.Foo();
        }

        foreach (B b in list)
        {
            b.Foo();
        }

        Console.ReadLine();
    }
}

public class A
{
    public void Foo()
    {
        Console.WriteLine("Base");
    }
}

public class B : A
{
    public new void Foo()
    {
        Console.WriteLine("Derived");
    }
}

This will only call the B.Foo() implementation if the reference you use is of type B. I wouldn't encourage you to do this, since I've never came accross a situation where the new keyword made sense.