c# - A method that executes any time a class property is accessed (get or set)?

Question

C# - .net 3.5

I have a family of classes that inherit from the same base class. I want a method in the base class to be invoked any time a property in a derrived class is accessed (get or set). However, I don't want to write code in each and every property to call the base class... instead, I am hoping there is a declarative way to "sink" this activity into the base class.

Adding some spice to the requirement, I do need to determine the name of the property that was accessed, the property value and its type.

I imagine the solution would be a clever combination of a delegate, generics, and reflection. I can envision creating some type of array of delegate assignments at runtime, but iterating over the MemberInfo in the constructor would impact performance more than I'd like. Again, I'm hoping there is a more direct "declarative" way to do this.

Any ideas are most appreciated!

Answer 1

You can't do it automatically, but you can pretty much get 95% for free. This is a classic case for aspect-oriented programming. Check out PostSharp, which has the OnFieldAccessAspect class. Here's how you might solve your problem:

[Serializable]
public class FieldLogger : OnFieldAccessAspect {
    public override void OnGetValue(FieldAccessEventArgs eventArgs) {
        Console.WriteLine(eventArgs.InstanceTag);
        Console.WriteLine("got value!");
        base.OnGetValue(eventArgs);
    }

    public override void OnSetValue(FieldAccessEventArgs eventArgs) {
        int i = (int?)eventArgs.InstanceTag ?? 0;
        eventArgs.InstanceTag = i + 1;
        Console.WriteLine("value set!");
        base.OnSetValue(eventArgs);
    }

    public override InstanceTagRequest GetInstanceTagRequest() {
        return new InstanceTagRequest("logger", new Guid("4f8a4963-82bf-4d32-8775-42cc3cd119bd"), false);
    }
}

Now, anything that inherits from FieldLogger will get the same behavior. Presto!