Sharing State with Advice

If you need to share compile-time state between different pieces of advice, or between your implementation of the BuildAspect method and between advice, a few strategies are available to you.

Note

If you need to share run-time state with advice, you have to choose another strategy, for instance introducing a field in the target type and using it from several advice methods.

Warning

DO NOT share state with an aspect field if that state depends on the target declaration of the aspect. In case of inherited aspects, the same instance of the aspect class will be reused for all inherited targets.

This is the most straightforward way to pass values from your BuildAspect method to a template method, but it works only with method templates. For details, see Template Parameters and Type Parameters.

For event, properties or field templates, compile-time template parameters are not available. The simplest alternative is to use tags. Tags are arbitrary values assigned to arbitrary names.

To define and use tags:

In your implementation of the BuildAspect method, when adding the advice by calling a method of the IAdviceFactory interface, pass the tags as an anonymous object to the tags argument like this: args: new { A = 5, B = "x", C = builder.Target.DeclaringType } where A, B and C are three arbitrary names..
In your template method, the tags are available under the meta.Tags dictionary. You would for instance use the meta.Tags["A"] expression to access the tag named A that you defined in the previous step.

Example

Aspect Code
Target Code
Transformed Code

using Metalama.Framework.Aspects;
using Metalama.Framework.Code;
using System;

namespace Doc.Tags
{
    internal class TagsAspect : MethodAspect
    {
        public override void BuildAspect( IAspectBuilder<IMethod> builder )
        {
            builder.Advice.Override(
                builder.Target,
                nameof(this.OverrideMethod),
                tags: new { ParameterCount = builder.Target.Parameters.Count } );
        }

        [Template]
        private dynamic? OverrideMethod()
        {
            Console.WriteLine( $"This method has {meta.Tags["ParameterCount"]} parameters." );

            return meta.Proceed();
        }
    }
}

using System;

namespace Doc.Tags
{
    internal class Foo
    {
        [TagsAspect]
        private void Bar( int a, int b )
        {
            Console.WriteLine( $"Method({a}, {b})" );
        }
    }
}

using System;

namespace Doc.Tags
{
    internal class Foo
    {
        [TagsAspect]
        private void Bar(int a, int b)
        {
            Console.WriteLine($"This method has 2 parameters.");
            Console.WriteLine($"Method({a}, {b})");
            return;
        }
    }
}

You can use the IAspectBuilder.AspectState property to store any aspect state that depends on the target declaration. This object is exposed on the IAspectInstance.AspectState property and is therefore also visible to inheritors and children aspects.

Note

Warning

Sharing state with compile-time template parameters

Sharing state with the Tags property

Example

Sharing state with the State property