Protected access for instance members

[Nathan Wiegman]

Hi,

Why must derived classes access protected base members through an instance
of a derived class? The following code would work in C++, why doesn't it in
C#?

public class A
{
protected int x;
}

public class B: A
{
static void F(A a)
{
a.x = 1; // Generates compile error...must access through instance of B
}
}

Maybe someone needs to revisit the C# language specification...

Thanks,
Nate

[Tom Porterfield]

Nathan Wiegman wrote:

Hi,

Why must derived classes access protected base members through an instance
of a derived class? The following code would work in C++, why doesn't it in
C#?

public class A
{
protected int x;
}

public class B: A
{
static void F(A a)
{
a.x = 1; // Generates compile error...must access through instance of B
}
}

???

class A
{
protected:
int x;
};

class B : A
{
public:
static void F(A a)
{
a.x = 1;
}
};

error C2248: 'A::x' : cannot access private member declared in class 'A'
--
Tom Porterfield

[Nathan Wiegman]

Ah...looks like I was wrong. You can't do that, even in C++. But, still, my
question is:

Why Not?

It's almost as if I need a new access modifier...perhaps
"allow_derived_access".

Thanks,
Nate

"Tom Porterfield" wrote:

Nathan Wiegman wrote:

Hi,

Why must derived classes access protected base members through an instance
of a derived class? The following code would work in C++, why doesn't it in
C#?

public class A
{
protected int x;
}

public class B: A
{
static void F(A a)
{
a.x = 1; // Generates compile error...must access through instance of B
}
}

???

class A
{
protected:
int x;
};

class B : A
{
public:
static void F(A a)
{
a.x = 1;
}
};

error C2248: 'A::x' : cannot access private member declared in class 'A'
--
Tom Porterfield

[Chris Jobson]

"Nathan Wiegman" <Na***********@discussions.microsoft.com> wrote in message
news:8B**********************************@microsof t.com...

Ah...looks like I was wrong. You can't do that, even in C++. But, still,
my
question is:

Why Not? I was surprised too, but I found the explanation in Stroustrup's "The C++
programming Language":
"A derived class can access a base class' protected members only for objects
of its own type. ... This prevents subtle errors that would otherwise occur
when one derived class corrupts data belonging to other derived classes."

This seems to make sense. If you have classes B and C both derived from
class A (which has a protected member x), it's not reasonable to allow a
member function of class B to access a class C's x value.

Chris Jobson
It's almost as if I need a new access modifier...perhaps
"allow_derived_access".

Thanks,
Nate

"Tom Porterfield" wrote:

Nathan Wiegman wrote:

> Hi,
>
> Why must derived classes access protected base members through an
> instance
> of a derived class? The following code would work in C++, why doesn't
> it in
> C#?
>
> public class A
> {
> protected int x;
> }
>
> public class B: A
> {
> static void F(A a)
> {
> a.x = 1; // Generates compile error...must access through
> instance of B
> }
> }

???

class A
{
protected:
int x;
};

class B : A
{
public:
static void F(A a)
{
a.x = 1;
}
};

error C2248: 'A::x' : cannot access private member declared in class 'A'
--
Tom Porterfield