Attribute programming

[James Lapalme]

Hy,
I understand that attribute programming is used to add metadata to the code,
but since they are implemented with a class and we can extend that class, we
should be able to do any normal class thing with attributes like:

-Have methods
-Use any kind of code in the constructor and the methods (ex. Use a
MessageBox)

But the run time will not execute a MessageBox in the constructor?? why?
Where is it defiened that and attribute can only have fields (properties)
and that the constructor is used only to init these
fields....???

James

[Jon Skeet]

Re: Attribute programming

James Lapalme <james.lapalme@ videotron.ca> wrote:[color=blue]
> I understand that attribute programming is used to add metadata to the code,
> but since they are implemented with a class and we can extend that class, we
> should be able to do any normal class thing with attributes like:
>
> -Have methods
> -Use any kind of code in the constructor and the methods (ex. Use a
> MessageBox)
>
> But the run time will not execute a MessageBox in the constructor?? why?
> Where is it defiened that and attribute can only have fields (properties)
> and that the constructor is used only to init these
> fields....???[/color]

An attribute can have any kind of constructor you like, and can have
any methods you like. However, when you *specify* an attribute for a
field/type/method etc, you're limited as to which types can be passed
in the constructor.

See section 22.3 of partition 2 of the ECMA CLI spec (ECMA 335) for
more details.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet/

If replying to the group, please do not mail me too

[Mikael]

Re: Attribute programming

Hi James,

Attributes can have methods and use any kind of code in the constructor.

This compiles/runs just fine on my machine.

using System;

public class MyAttribute : Attribute
{
public MyAttribute()
{
System.Windows. Forms.MessageBo x.Show("MyAttri bute Constructor");
}
public void Hello()
{
System.Windows. Forms.MessageBo x.Show("Hello from MyAttribute");
}
}

[MyAttribute()]
class AttributeTest
{
static void Main()
{
Type type = typeof(Attribut eTest);

// check if attribute exists
if(type.IsDefin ed(typeof(MyAtt ribute), true))
{
// create an instance of the attribute
MyAttribute tmp =
type.GetCustomA ttributes(false )[0] as MyAttribute;

// call method on attribute
tmp.Hello();
}
}
}


/Mikael

"James Lapalme" <james.lapalme@ videotron.ca> skrev i meddelandet
news:%23WJiobCR DHA.3700@tk2msf tngp13.phx.gbl. ..[color=blue]
> Hy,
> I understand that attribute programming is used to add metadata to the[/color]
code,[color=blue]
> but since they are implemented with a class and we can extend that class,[/color]
we[color=blue]
> should be able to do any normal class thing with attributes like:
>
> -Have methods
> -Use any kind of code in the constructor and the methods (ex. Use a
> MessageBox)
>
> But the run time will not execute a MessageBox in the constructor?? why?
> Where is it defiened that and attribute can only have fields (properties)
> and that the constructor is used only to init these
> fields....???
>
> James
>
>[/color]