C# custom attribute

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I have a AAA class with a custom attribute XXX, is it possible to get the AAA instance and the Property where the attribute is used in the attribute constructor?

Class where is Attribute used.

public class AAA
{
    int _property;
    [XXX]
    public int Property { get { return _property; } set { _property = value; } }


}

Attribute constructor - is it possible to get access the AAA class instance and the property where the attribute is used?

public class XXX : JsonAttribute
{

    public XXX()
    {
        //???



    }


}

What I have tried:

I tried going through the object this in the constructor in the debugg mod to see if I can find any mention of what I'm looking for.

Posted 12-Oct-21 3:21am

KarNullKa

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PIEBALDconsult 12-Oct-21 9:27am

Pretty sure not. In fact, the Attribute could be used on something static and there won't be an instance.

Maybe in the constructor for AAA, it could access its XXX and introduce itself?

KarNullKa 12-Oct-21 9:51am

Okay. How about reflecting the AAA class to find the attribute XXX in the AAA constructor and then replacing it with another one? Because I would already have access to the instance data in the constructor.

PIEBALDconsult 12-Oct-21 10:07am

Uhhh... what?
If I follow that, no, that won't work.
You can definitely get an instance of XXX from the AAA's constructor, but you can't _replace_ it -- not in a way which will cause other pieces of code to see the replacement. Attributes don't work that way.

KarNullKa 12-Oct-21 10:25am

I was wondering if it's possible to make a simple attribute that only has e.g. true/false information. After calling the constructor of the AAA class, go through all the properties using reflection and if it has the XXX attribute set and the value set to true, add another attribute to the properties such as [JsonIgnore]. This results in the [JsonIgnore] attribute, but it depends on what data the AAA class contains and the setting of the XXX attribute.

PIEBALDconsult 12-Oct-21 10:56am

Yeah, no. Attributes are limited in what you can do with them. You would simply write the class with the Attributes you want. Attributes are set when you write them. Any other class can query the definition to get the set of Attributes, but can't see any instances of the class which may exist already.

Potentially, you could write an Attribute by which your AAA instances could let all other AAA instances know about each other, but non-AAA classes wouldn't know about it.

lmoelleb 12-Oct-21 10:07am

You can't replace the attribute for an existing class. This is set at compile time. You could in theory generate a new class (using Roslyn or Reflection.Emit), but this sounds too much like an XY problem. An XY problem is: You have problem X. You think of solution Y. You can't implement Y so you ask for help finding a way to implement Y. You should ask for X instead, as there might be a better solution than Y. So what is the problem you are trying to solve?

KarNullKa 12-Oct-21 10:37am

I'm trying to make an attribute that distinguishes between Serialization and Deserialization, as opposed to [JsonIgnore]. There are other ways to solve this than using an attribute, but again, constantly writing more code [JsonIgnoreSerialize]/[JsonIgnoreDeserialize] would be nicer.

BillWoodruff 12-Oct-21 23:56pm

I suggest you edit/update your original post to include the information you have now disclosed in comments.

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