Possible to override static member in base class?

[Dave Booker]

It looks like the language is trying to prevent me from doing this sort of
thing. Nevertheless, the following compiles, and I'd like to know why it
doesn't work the way it should:

public class ComponentA
{
static string s_name = "I am the root class.";
public string Name { get {return s_name;} }
}

public class ComponentB : ComponentA
{
new static string s_name = "I am a derived class.";
}

static void Main()
{
ComponentB b = new ComponentB();
Console.WriteLi ne("ComponentB says: " + b.Name);
}

I want this to say "I am a derived class." but it still says "I am the root
class."

My objective is to derive a whole bunch of classes on which I can get a
description both
- Through reflection of the class Type (thus requiring the static member)
- Through an instance of the class using a (virtual) member that is defined
in the root class.

Is this possible?

[Mattias Sjögren]

>My objective is to derive a whole bunch of classes on which I can get a

description both
- Through reflection of the class Type (thus requiring the static member)
- Through an instance of the class using a (virtual) member that is defined
in the root class.

Is this possible?

Why don't you make the Name property virtual and override it in the
derived classes.
Mattias

--
Mattias Sjögren [C# MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

[Jeffrey Tan[MSFT]]

Hi dbooksta,

Thanks for your post!

Yes, C# does not support static virtual member. This is because: to
implement a virtual member, the complier has to place this member as an
entry in the v-table, which attaches to a class instance. Static member
does not attach to any class instance but attaches to the class itself, so
static member can not be virtual and overrided in child class.

You'd better take Mattias' suggestion to mark "Name" instance property as a
virtual one and override it in inherited class. The code snippet below
demonstrates this:

public class ComponentA
{
static string s_name = "I am the root class.";
public virtual string Name { get {return s_name;} }
}

public class ComponentB : ComponentA
{
new static string s_name = "I am a derived class.";
public override string Name { get {return s_name;} }
}

private void button1_Click(o bject sender, System.EventArg s e)
{
ComponentB b = new ComponentB();
Console.WriteLi ne("ComponentB says: " + b.Name);
}
Hope this helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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[Dave Booker]

OK, this will work. But just to confirm: Is there no way to define an
interface, abstract, or base class such that it REQUIRES that a particular
static member (say "s_name") be defined or redefined in a derived class?

If so, it seems like a language shortcoming, doesn't it?
""Jeffrey Tan[MSFT]"" wrote:

Hi dbooksta,

Thanks for your post!

Yes, C# does not support static virtual member. This is because: to
implement a virtual member, the complier has to place this member as an
entry in the v-table, which attaches to a class instance. Static member
does not attach to any class instance but attaches to the class itself, so
static member can not be virtual and overrided in child class.

You'd better take Mattias' suggestion to mark "Name" instance property as a
virtual one and override it in inherited class. The code snippet below
demonstrates this:

public class ComponentA
{
static string s_name = "I am the root class.";
public virtual string Name { get {return s_name;} }
}

public class ComponentB : ComponentA
{
new static string s_name = "I am a derived class.";
public override string Name { get {return s_name;} }
}

private void button1_Click(o bject sender, System.EventArg s e)
{
ComponentB b = new ComponentB();
Console.WriteLi ne("ComponentB says: " + b.Name);
}
Hope this helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
=============== =============== =============== =====
When responding to posts, please "Reply to Group" via your newsreader so
that others may learn and benefit from your issue.
=============== =============== =============== =====
This posting is provided "AS IS" with no warranties, and confers no rights.

[Jeffrey Tan[MSFT]]

Hi dbooksta,

Thanks for your feedback!

No, you can redefine the static member in the child class with "new"
keyword in C#. This will hide the parent class's static member in the child
class. Since C# does not allow us to use class reference to refer the
static member. The correct way of using the redefined static member is
using the child class name. Please see the sample code snippet below:
private void Form1_Load(obje ct sender, System.EventArg s e)
{
ComponentB b = new ComponentB();
MessageBox.Show ("ComponentB says: " + ComponentB.s_na me);//you can not use
b.s_name to access the redefined static member, C# compiler prohibits this
way.
}

public class ComponentA
{
public static string s_name = "I am the root class.";
public string Name { get {return s_name;} }
}

public class ComponentB : ComponentA
{
public new static string s_name = "I am a derived class.";
}

Does this meet your need?

Note: your original sample code result is expected. You did not override
Name property in the child class, then when you are using b.Name to access
"Name" property, you are referring the parent class's "Name" property,
which returns parent class's s_name static member to you. In this scenario,
there is no Polymorphism at all.

Below is some more comment on OOP Polymorphism with static member:

In Object Oriented Programming area, static member is a property of the
class itself, it has nothing to do with any instance of that class.
Polymorphism in OOP means initializing a child class instance and assigning
it to the parent class pointer/reference, like this:
ParentClass obj=new ChildClass();

Then we can override the parent class member behavior in the child class.
This is called Polymorphism in OOP. As we can see, in Polymorphism of OOP,
this concept only applies to the class member, which requires initializing
an instance of the class. This concept does not apply to the class itself.
So Polymorphism concept has nothing to do with static member. Also, neither
C++ nor C# support virtual static member.(Because it does not make sense
based on OOP concept)

Hope this information helps!

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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[Jeffrey Tan[MSFT]]

Hi dbooksta,

Does my reply make sense to you? If you still have any concern or
questions, please feel free to tell me, thanks!

Best regards,
Jeffrey Tan
Microsoft Online Community Support
=============== =============== =============== =====
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that others may learn and benefit from your issue.
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[Dave Booker]

FYI to all, I also realized that I could accomplish my objective here by
tagging my classes with an Inheritable Attribute.

[Mark Wilden]

Why do Microsoft employees write messages like this? I mean, they respond to
perhaps 1/1000th of the messages posted here, lard those responses with
useless boilerplate, then write equally useless "followups" like this. It's
like they think newsgroups are email support (except that they ignore most
requests).

Sorry - just had to vent.

///ark

""Jeffrey Tan[MSFT]"" <je***@online.m icrosoft.com> wrote in message
news:PA******** ******@TK2MSFTN GXA01.phx.gbl.. .

Hi dbooksta,

Does my reply make sense to you? If you still have any concern or
questions, please feel free to tell me, thanks!

Best regards,
Jeffrey Tan
Microsoft Online Community Support
=============== =============== =============== =====
When responding to posts, please "Reply to Group" via your newsreader so
that others may learn and benefit from your issue.
=============== =============== =============== =====
This posting is provided "AS IS" with no warranties, and confers no
rights.

[Jeffrey Tan[MSFT]]

Hi dbooksta,

Thanks for your feedback!

Can you be specific regarding your finding? Please forgive my ignorance,
based on my research, I can not find any "Inheritabl e Attribute" in C#
language specification. Can you share your sample code with us? Thanks :-)

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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that others may learn and benefit from your issue.
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[Dave Booker]

Basically I define an Attribute like this:

[AttributeUsage( AttributeTarget s.Class, Inherited = true)]
public class ComponentTypeAt tribute : Attribute
{
public string ComponentType;
public ComponentTypeAt tribute(string c)
{
ComponentType = c;
}
}

Now the root class gets:

[ComponentTypeAt tribute("BaseTy pe")]
public class BaseComponent
{
public string ComponentType { get { return
((ComponentType Attribute)Attri bute.GetCustomA ttribute(this.G etType(),
typeof(Componen tTypeAttribute) )).ComponentTyp e; } }
}

A little verbose, but the net effect is that a call to .ComponentType member
of ANY derived class will return "BaseType," unless they redefine the
attribute like this:

[ComponentTypeAt tribute("Specia lType")]
public class Special : BaseComponent {...}

So when we call Special.Type we get "SpecialTyp e"!

Also, we don't have to instantiate any of these classes to find out what
their ComponentType, because the Attribute is defined on the Type.

Hence, we have the effect of a static property that can be both inherited
and overridden!