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C# 2.0: Protected member access and generics

  • Artikkeli

Some time back I had posted about why protected member of a base class cannot be directly called from a derived class. The same thing holds for generic classes as well. However, protected member access rule has been updated in the C#2.0 spec to accomodate for Generics.

Consider the following

 class BClass {    protected int x;}class AClass : DClass {}class DClass : BClass {    void Foo()    {        //BClass b = new BClass();        //b.x = 5;        DClass d = new DClass();        d.x = 10;         AClass a = new AClass();        a.x = 13;     }}

The above code compiles fine because a class is allowed to access protected member of its base class (not directly though) and also of any other class (AClass) that derives from it. Obviously you cannot access the protected member of any other class.

In case of generic classes the rule got slightly modified (2.0 spec Section 20.1.7) as follows

Within a generic class G, access to an inherited protected instance member M using a primary-expression of
the form E.M is permitted if the type of E is a class type constructed from G or a class type inherited from a
class type constructed from G.

What this means is that atleast for protected member access, types constructed from the same generic class are considered the same. Contrast this with the fact that if a generic class has a static member, a seperate copy (not shared) of it used for each constructed type. The following code compiles fine

 class B<T> {    protected T x;}class E<T> : D<T>{}class D<T> : B<T> {    void F()    {        D<int> di = new D<int>();        D<string> ds = new D<string>();        di.x = 123;         ds.x =   "test"  ;          E<string> e = new E<string>();        e.x =   "test2"  ;     }} D <int> d = newD<int>();

So in the above code even though D<string> and D<int> are different closed constructed types they can access each other protected members. Same thing holds for E<string>.

Comments

  • Anonymous
    January 04, 2006
    Sorry. Wrong posting to comment on - it was addressed to http://blogs.msdn.com/gkhanna/archive/2006/01/03/509140.aspx

    But to your posting - why there are two 'x' protected members ? This does not add clarity to example.
    In your AClass example you are access x from BClass - not from sub-class - so your statement "and also of any other class (AClass) that derives from it" is wrong.

    BTW, If you would like to post more - can you comment on this bug report ?
    http://lab.msdn.microsoft.com/productfeedback/viewfeedback.aspx?feedbackid=FDBK42667
  • Anonymous
    January 04, 2006
    The comment has been removed
  • Anonymous
    January 04, 2006
    I just took a look at the feedback report. A quick look into the 2.0 spec Section 25.1 I think this is indeed a bug...