Why does my .NET Attribute not perform an action?

Question

Hi, I've created a simple Attribute:

[AttributeUsage(AttributeTargets.Method)]
public class InitAttribute : System.Attribute
{
    public InitAttribute()
    {
        Console.WriteLine("Works!");
    }
}

and I apply it to a simple method:

static class Logger
{
    public static string _severity;

    public static void Init(string severity)
    {
        _severity = severity;
    }

    [Init()]
    public static void p()
    {
        Console.WriteLine(_severity);
    }
}

What is going on is pretty streight-forward. Only, I expect the attribute to perform an action (printing Works!), but this does not happen.

Addictionally, printing "Works!" is of course just for debugging purposes: I'd like to access the instance's property _severity (to check if is != null, for example), but everything I keep reading about attributes (that are pretty new to me) is about accessing the class' methods or properties and so on via reflection. Once I've evaluated _severity, how can I modify the behavior of the decorated method (in this case, rise an exception "Logger is not initialized" and do not execute it)?

Any help appreciated.

Answer 1

If you need to perform an action as control enters a method, you should look at aspect-oriented programming and frameworks such as PostSharp. Attributes are not designed to perform anything by themselves. They are just a bunch of data (or metadata if you will) attached to stuff in IL assemblies that can be queried at runtime.

Answer 2

Attributes only allow decoration of types and members, but the attribute itself cannot acces the decorated object. You will have to use the constructor parameters of the attribute to pass in any data you require to work with within the attribute.

If you wish to use attributes to automatically alter the behaviour of their target objects, you will have to look at AOP solutions like PostSharp.

Answer 3

The attribute is never actually instantiated and so its constructor is never called. The attribute remains as meta-data until you use reflection to retrieve it. As has been mentioned previously what you are after is an Aspect Oriented Programming tool. PostSharp works by altering the assembly as a post-build step. If you are using the Castle Windsor or Unity Inversion of Control Containers they both offer AOP capabilities as well.