Accessing instance-level custom attributes in c#

Question

Imagine I have the following code:

class A
{
    [UsefulAttribute("foo")]
    B var1;
    [UsefulAttribute("bar")]
    B var2;

    ...
}


class B
{
    public string WriteSomethingUseful()
    {
        ?????
    }
}

My question is, what code do it need to put in the ????? such that, when I call var1.WriteSomethingUseful I get an output of foo, and when I call var2.WriteSomethingUseful I get an output of bar?

I've got a feeling this is quite a straightforward question, I think my main issue is that I have worked myself into a state of confusion by thinking about it for too long!!!

Seriously, I have defined UsefulAttribute and realise that part of the code must be a GetCustomAttributes(typeof(UsefulAttribute)...) call. Where I'm getting confused is how to pull these values out on the actual instance, rather than at the type level.

Many thanks, Pete

I agree with the other answers that in its current form the problem you've stated is not possible. The pattern you're going after, however, is not unapproachable and can be quite useful. I believe there is a usable solution if WriteSomethingUseful is passed context regarding its owner, something like WriteSomethingUseful(FieldInfo attributeContext). I'd work out a full solution for you but I don't have a Windows dev box at the moment. Regardless, best of luck. — Slipp D. Thompson
– Slipp D. Thompson, Commented Feb 15, 2014 at 23:45

Chris Shain · Accepted Answer · 2011-09-09 14:45:37Z

6

This isn't possible. For starters, what if multiple different instances of A have references to the same B? Or what if the same instance of B is referenced by both var1 and var2?

When you set the attribute on the field, you are attaching that attribute to the type of class A, not the instance of class B stored in the field var1.

The normal way to go about this is to store the data as a property of B, set it either via a property setter or a constructor parameter, and then access the property from the WriteSomethingUseful method.

answered Sep 9, 2011 at 14:45

Chris Shain

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4 Comments

PeteH Over a year ago

thanks Chris, you're absolutely right that makes perfect sense. You know when sometimes you can't see the wood for the trees....

Slipp D. Thompson Over a year ago

Re Q1: Then they would produce different results when called. Re Q2: Again, different result per call site/context. Clearly the “?????” code would have to be dynamic, producing different results in different situations. While I agree it's not possible with the exact code above, it's not an unsolvable problem— WriteSomethingUseful simply needs to be passed context information regarding the owner.

Chris Shain Over a year ago

@SlippD.Thompson it is possible to write code that produces the output that the OP wants, as I explain in the last sentence of my answer. It is not possible to do so via attributes, because Attributes are static data. They are (in this case) attached to the property definition, not the instance of the class stored in that property, and the function being called (WriteSomethingUseful) does not have metadata that tells it which property is referenced by the caller.

Slipp D. Thompson Over a year ago

@ChrisShain I don't agree.

BrokenGlass · Accepted Answer · 2011-09-09 14:48:16Z

3

Since your WriteSomethingUseful() method is within the type B, but your attributes are declared within type A you will not be able to access them based on an instance - you simply don't have a reference to A.

The current B instance might not be related to A at all, and without being able to retrieve "the type of the class instance (if any) that contains the current B instance" - which is not possible in C# - there is no general way to do this.

answered Sep 9, 2011 at 14:48

BrokenGlass

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1 Comment

PeteH Over a year ago

cheers BG. On reading the answers it is obvious, I'd just got myself confused somewhere along the line.

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