Conditional Attribute question

Question

I want to make attribute of some fields conditional, as below:

enum AttrType
{
Type1,
Type2
}

public class Shape
{
[MyAttribute(AttrType.Type1)]
mArea;
}

public class Circle:Shape
{
[Make it AttrType.Type2]
mArea;
[Make it AttrType.Type1]
mRadius;
}

If it is base class A, and mRadius is not defined, I want mArea has AttrType.Type1. If it is in derived class B, and mRadius is defined, I want mArea has AttrType.Type2 and mRadius has AttrType.Type1

How to make it work?

Thanks,

Vulpes · Answer

I don't think it is feasible - or if it is it's well above my pay grade :) On the last point you'd need to be able to apply a [NonSerialized] attribute to a field dynamically if AttrType.Type2 were applicable. To the best of my knowledge this isn't possible using reflection unless of course you're creating the whole type dynamically using reflection emit. You might be able to do it using TypeDescriptors if your classes implement the IComponent interface but it seems to me that you'd just be replacing code bulk with serious complexity.

Ken Shen · Answer

Thanks again for your reply Vulpes. I agree that it is easy work that we use abstract properties (e.g.Area) in base class and override it in derived classes. However there are two reasons I don't use in my case: (1) There are many fields whose attributes are to be changed in the problem I am thinking of. Using override properties or fields will make the code bulky. (2) Sometimes we want to use the base class (e.g. Shape) if we don't know the exact kind of shape. In this case we just need to construct and use a Shape instance with mArea value. What I am trying is to create a static information sheet or class which is constructed and collects MyAttribute information of multiple classes only ONCE between compiling and running. This sheet also updates MyAttribute information during its construction based on the following custom attribute [AttributeUsage(AttributeTargets.Class AllowMultiple = true)] public class ResetMyAttribute : Attribute Also I want to realize that: If AttrType.Type1 then this field is Serializable. If AttrType.Type2 then this field is NonSerializable. This is kind of attribute dependency question. Is is feasible? Thanks

Vulpes · Answer

There's no way of doing that to my knowledge. Whilst you could specify that your custom attribute targeted fields and wasn't inherited at all by derived classes I can't think of any way that you could inherit it but change its properties in derived classes. You can't in any case control read/write access of fields using custom attributes - you can only do that using access modifiers. I suppose you could have code which used reflection to check whether a field could be written to by examining its attributes at runtime but that would not be very efficient. If I were coding the Shape/Circle classes I'd make Shape abstract and give it an abstract readonly property called Area. I wouldn't bother giving it an mArea field as this is always going to be calculable from some other field(s) in derived classes. Something like this: using System; public abstract class Shape { public abstract double Area { get; } } public class Circle : Shape { private double mRadius; public double Radius { get { return mRadius; } set { if(value >= 0) mRadius = value; } } public override double Area { get { return Math.PI * mRadius * mRadius; } } public Circle(double radius) { Radius = radius; } } class Test { static void Main() { Shape s = new Circle(2); Console.WriteLine(The area of a circle with a radius of {0} is {1:F4} ((Circle)s).Radius s.Area); Console.ReadKey(); } }

Ken Shen · Answer

Thanks for the information Vulpes. As in my earlier case Shape.mArea can be readable/writable however Circle.mArea is only readable as it can be calculated from Circle.mRadius. So by letting Type1 be 'rw' and Type2 be 'r' we define the attribute to control the accessibility of similar fields. That's the origin of my question. It is true as you mentioned that we can redefine Circle.mArea but it will make derived classes not concise and large for general cases. So the basic question I was trying to ask is that if there is a way to modify attributes of inherited fields in compile time? A method constructor in CustomAttribute or whatever.

Vulpes · Answer

What do you mean when you say mRadius is defined or not defined? A field is either included in a class or it isn't. If it is included then it's inherited by any derived classes even if it's private. However a private inherited field is not accessible in a derived class except that you can redefine it if you want (of course it's not then the same field). So here the private field Shape.mArea could be redefined by Circle.mArea and you could then place a different attribute on it. It's also possible to specify whether a custom attribute is inheritable or not using the AttributeUsage.Inherited property. Normally when people talk about conditional attibutes they mean the ConditionalAttribute class which can be used to decorate methods. The inclusion or not of the method (and calls to it) in an assembly is then conditional on whether specified conditional compilation symbols are defined or not.