Protected method access from derived class

Question

I have a protected method in base class :

public class BaseClass
{
  protected virtual void Foo(){}
}

The method is overrided one of the derived classes:

public class Derived1 : BaseClass
{
   protected override void Foo()
    {
     //some code...
    }
}

Another derived class has an instance of the first derived class.
When I try to access the Foo method (existing in base class, as mentiond) I get an error :

 public class DerivedClass2 : BaseClass
    {
     BaseClass instance = new DerivedClass1();
     instance.Foo(); // Here I get an error
    }

The error I get:

Error CS1540: Cannot access protected member 'BaseClass.Foo' via a qualifier of type 'BaseClass';   
the qualifier must be of type 'DerivedClass2' (or derived from it)

I understand that protected members should not give up their value to any other instance, even an instance derived from the same type,
but is there a way not to modify the method as public?

If it is acceptable to make it public only within that assembly, you can mark it as internal — Suresh Kumar Veluswamy
– Suresh Kumar Veluswamy, Commented Mar 24, 2014 at 9:57

Suresh Kumar Veluswamy · Accepted Answer · 2014-03-24 12:23:09Z

9

You can make the Foo method declaration as protected internal....

public class BaseClass
{
  protected internal virtual void Foo(){}
} 

public class Derived1 : BaseClass
{
   protected internal override void Foo()
    {
     //some code...
    }
}

Here "protected internal" means the member is visible to any class inheriting the base class, whether it's in the same assembly or not. The member is also visible via an object declared of that type anywhere in the same assembly.

edited Mar 24, 2014 at 12:23

answered Mar 24, 2014 at 10:03

Suresh Kumar Veluswamy

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3 Comments

Aftab Ahmed Over a year ago

he is accessing Foo() with the instance of base class that's why he is getting this exception.

Suresh Kumar Veluswamy Over a year ago

@Aftab:Though the variable is of type base class the instance it holds is of type derived type. Hence it works!

user3165438 Over a year ago

@SureshKumarVeluswamy, Thanks, it works perfectly! Can you please add some other explanation so that I can understand properly why does internal protected did the work? Thanks in advance!

Romulus Crisan · Accepted Answer · 2014-03-24 11:50:58Z

1

Update: I've just seen that both of the classes are derived from BaseClass. In the initial sample no base class was specified. In this case one way to solve this problem is to create a new AbstractDerived which would be the base class for those 2 classes. Something like this:

public abstract AbstractDevired: BaseClass
{
   protected override Foo()
   {
      //put implementation from Derived1 here
   }
}

public Derived1: AbstractDevired
{
  //no need to override here because it is the same logic in AbstractDerived class
}

public DerivedClass2: Derived1
{
   protected override Foo()
   {
      base.Foo();
      //some code here
   }
}

Old answer: You should have DerivedClass2 derived from Derived1 and use base keyword. You can find more information and examples about base keyword here: http://msdn.microsoft.com/en-us/library/hfw7t1ce.aspx In your case it will look something like this:

public class DerivedClass2 : Derived1
{
   protected override void Foo()
   {
      base.Foo();
      //some code
   }
}

edited Mar 24, 2014 at 11:50

answered Mar 24, 2014 at 10:16

Romulus Crisan

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1 Comment

user3165438 Over a year ago

But then I cannot use the override method. Any ideas? Thanx

Jake Rote · Accepted Answer · 2014-03-24 09:56:34Z

0

You can try this it hides the base version of the method and makes the overridden version public.

public class Derived1 : BaseClass
{
    public new void Foo()
    {
        //some code...
    }
}

answered Mar 24, 2014 at 9:56

Jake Rote

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2 Comments

user3165438 Over a year ago

Thanks. My purpose was to still set the method protected along the inheritance hierarchy. Is there a way?

Jake Rote Over a year ago

then make the method on the base class public if it needs to be accessed from no inherited classes

Pratik Bhoir · Accepted Answer · 2014-03-24 10:11:22Z

0

You can use internal access modifier, It will work same as public, but wont work with different assemblies.

More info on MSDN : http://msdn.microsoft.com/en-us/library/wxh6fsc7.aspx

So, just replace protected with internal. Hope, it helps you. :)

answered Mar 24, 2014 at 10:11

Pratik Bhoir

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