My example is quite like what you described.
I have a set of attributes, in my case, of AOP advices.
You have different advice weaving behaviors depending on the attribute you
put. E.g. there are AroundAttribute (intercept around the targets),
BeforeAttribute, AfterAttribute, ExceptionAttribute, or even
AnnotateAttribute (annotating the targets with additional attributes). I was
reluctant to put an if-else logic to check these logics, because I want to
have plugin-like capability in which third-party users are able to inherit
from the attributes to provide their own custom behaviors.
As a workaround, I was thinking to have the child-classes to set a property
which holds the type of the behavior-factory. However, I'm not sure if it
works since it implies that the reflection would execute this "setter"
coming from the child-attribute.
E.g.
public MyCustomAdviceAttribute: AdviceAttribute
{
public MyCustomAdviceAttribute: base(typeof(MyAdviceBehaviorFactory))
{
}
}
And via cecil, I would get the value of
AdviceAttribute.Attributes["Factory"]
I haven't tried that but I doubt that would work.
Another thing is, what's the best way to get a certain custom attributes
allowing inheritance with cecil? There's only the property of
CustomAttributes. Does it mean I have to interrogate each of their types,
then traverse all the way up of each Attribute's line of inheritance, as
well as all the way up each Method's line of inheritance and repeat and
rinse?
Cheers
On Sun, Apr 24, 2011 at 9:32 PM, Simon Cropp <[email protected]> wrote:
> sorry. your correction is correct :)
>
> Why do u need to put code in the attribute?
> Might be helpful if u post some code?
>
> On Sun, Apr 24, 2011 at 9:20 PM, Hendry Luk <[email protected]> wrote:
> > Yes, I'm able to get the metadata of the attributes. But i need to
> > instantiate it because I do implement the behavior inside of the
> attribute
> > for extensibility reason.
> > I guess mono-cecil is strictly behaving like .net assembly in the
> > reflection-only mode (although the .net one is even more limited since
> you
> > can't even inspect attribute at all, AFAIK).
> >
> >> The thing u can do with cecil is instantiate and instance of those
> >> attributes at runtime.
> >
> > Did you mean "you can't"?
> >
> > Cheers
> >
> > On Sun, Apr 24, 2011 at 5:26 PM, Simon Cropp <[email protected]>
> wrote:
> >>
> >> Hendry
> >>
> >> Just to be clear Cecil can read attributes and you can even extract
> >> out properties and constructor parameters.
> >> The thing u can do with cecil is instantiate and instance of those
> >> attributes at runtime. You would have to switch back to standard
> >> reflection to do that.
> >>
> >> So what does this mean for you... Just dont write code in your
> >> attributes and you will be fine. Use Attributes soley for "metadata"
> >> then have another class that does something with that metadata.
> >>
> >> For example I have an attribute like this
> >>
> >> public class NotifyPropertyAttribute : Attribute
> >> {
> >> public bool PerformEqualityCheck { get; set; }
> >> public string[] AlsoNotifyFor { get; set; }
> >> }
> >>
> >> At weaving time I extract the PerformEqualityCheck property using this
> >> code
> >>
> >> static bool? GetCheckForEquality(CustomAttribute notifyAttribute)
> >> {
> >> var performEqualityCheck =
> >> notifyAttribute.Properties.FirstOrDefault(x => x.Name ==
> >> "PerformEqualityCheck");
> >> var equalityCheckValue = performEqualityCheck.Argument.Value;
> >> if (equalityCheckValue != null)
> >> {
> >> return (bool)equalityCheckValue;
> >> }
> >> return null;
> >> }
> >>
> >> Would this approach meet your requirements?
> >>
> >>
> >> On Sun, Apr 24, 2011 at 12:52 PM, Hendry Luk <[email protected]>
> wrote:
> >> > Thanks for the response, Jb.
> >> > I was speculating there might be a way to convert (i.e. load) from
> cecil
> >> > object models into the .net runtime.
> >> > I use cecil because the main business my application does is weaving
> >> > assemblies (AOP), and unfortunately in few places I often need to read
> >> > attribute metadata to control the weaving behavior.
> >> > I guess I will need to use both .net reflection and cecil for this,
> and
> >> > somehow swap back and forth between them. Efficiency was my concern.
> >> >
> >> > Thanks again
> >> >
> >> > On Sun, Apr 24, 2011 at 5:55 AM, Jb Evain <[email protected]> wrote:
> >> >>
> >> >> Hi,
> >> >>
> >> >> On Sat, Apr 23, 2011 at 9:30 PM, Hendry Luk <[email protected]>
> >> >> wrote:
> >> >> > Hi,
> >> >> > Is there any easy way in cecil to get a certain custom-attributes
> (of
> >> >> > a
> >> >> > specific type) from a method, and execute a virtual method on it?
> >> >> > Just like it can easily be done using a basic System.Reflection.
> >> >> >
> >> >> > The only way i can think of is to manually load the assembly
> >> >> > containing
> >> >> > that
> >> >> > attributes (that I've discovered using cecil), then instantiate the
> >> >> > attribute using the constructor-arguments information (from cecil),
> >> >> > and
> >> >> > then
> >> >> > copy across all property-values (from cecil). Once you got the
> >> >> > attribute
> >> >> > instance, I can then execute the method. It seems to be a lot of
> work
> >> >> > for
> >> >> > such a _very_ common thing.
> >> >>
> >> >> .net attributes have been designed to be instantiated at runtime.
> >> >> Cecil is not a runtime feature, it just gives you an object model
> >> >> which maps to the serialized metadata. Cecil has obviously no way to
> >> >> instantiate an object based on this metadata.
> >> >>
> >> >> If you want to instantiate attributes, you're looking at the wrong
> >> >> tool.
> >> >>
> >> >> > Oh while I'm here, is there also an easy way to do
> >> >> > type.IsAssignableFrom(type) on cecil? At the moment i'm recursively
> >> >> > checking
> >> >> > all the base-classes within its ancestry line, as well as their
> >> >> > interfaces,
> >> >> > but it feels very inefficient, considering it's also a very common
> >> >> > thing
> >> >> > (checking for types assignability is probably far more common than
> >> >> > checking
> >> >> > for an exact equality). Is there a better way to do this?
> >> >>
> >> >> Not really, I don't think I have such a method around.
> >> >>
> >> >> Jb
> >> >>
> >> >> --
> >> >> --
> >> >> mono-cecil
> >> >
> >> > --
> >> > --
> >> > mono-cecil
> >>
> >> --
> >> --
> >> mono-cecil
> >
> > --
> > --
> > mono-cecil
>
> --
> --
> mono-cecil
--
--
mono-cecil
