C# protected field access

Question

(This question is a follow-up to C# accessing protected member in derived class)

I have the following code snippet:

public class Fox
{
    protected string FurColor;
    private string furType;

    public void PaintFox(Fox anotherFox)
    {
        anotherFox.FurColor = "Hey!";
        anotherFox.furType = "Hey!";
    }
}

public class RedFox : Fox
{
    public void IncorrectPaintFox(Fox anotherFox)
    {
        // This one is inaccessible here and results in a compilation error.
        anotherFox.FurColor = "Hey!";
    }

    public void CorrectPaintFox(RedFox anotherFox)
    {
        // This is perfectly valid.
        anotherFox.FurColor = "Hey!";
    }
}

• Now, we know that private and protected fields are private and protected for type, not instance.

• We also know that access modifiers should work at compile time.

• So, here is the question - why can't I access the FurColor field of the Fox class instance in RedFox? RedFox is derived from Fox, so the compiler knows it has access to the corresponding protected fields.

• Also, as you can see in CorrectPaintFox, I can access the protected field of the RedFox class instance. So, why can't I expect the same from the Fox class instance?

Answer 1

Simple reason is:

public void IncorrectPaintFox(Fox anotherFox)
{
    anotherFox = new BlueFox();

    // This one is inaccessible here and results in a compilation error.
    anotherFox.FurColor = "Hey!";
}

Now you're not accessing the protected field from within BlueFox, therefore since the compiler doesn't know what the runtime type is, it has to always make this an error.