Importing Dependencies from the Target Object

The principal reason why aspects are believed to be difficult to test is that they are statically scoped by default, i.e. aspect objects are stored in static fields. However, any aspect can be made instance-scoped if it implements the IInstanceScopedAspect interface. See Understanding Aspect Lifetime and Scope for more information about aspect scopes.

Instance-scoped aspects can consume dependencies from the objects to which they are applied. They can also add dependencies to the target objects.

For instance, an aspect can consume a service ILogger using the following procedure:

To consume a service from an instance-scoped aspect:

1. Add a public property of name Logger and type ILogger to the aspect and add the IntroduceMemberAttribute custom attribute. This will cause the aspect to add a property to the target class. Use the parameter MemberOverrideAction.Ignore to ignore the property if it already exists in the target type of if it has been added by another aspect.

2. Add two custom attributes ImportAttribute and CopyCustomAttributesAttribute to the Logger property. This will cause the aspect to add the [Import] custom attribute to the Logger property added to the target class.

3. Add a public field of name LoggerProperty and type Property<ILogger> to the aspect class and add the ImportMemberAttribute custom attribute to this field, with "Logger" as the parameter value. This will allow the aspect to read the Logger property even if it has been defined from outside the aspect.

4. The aspect can now consume the dependency by calling this.LoggerProperty.Get().

The procedure is illustrated in the next example.

Example: testable logging aspect that consumes the dependency from the target object

The following code snippet shows a logging aspect and how it could be used in production code:

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.ComponentModel.Composition.Primitives;
using System.ComponentModel.Design;
using System.Reflection;
using PostSharp.Aspects;
using PostSharp.Aspects.Advices;
using PostSharp.Extensibility;
using PostSharp.Reflection;
using PostSharp.Serialization;

namespace DependencyResolution.InstanceScoped
{
    public interface ILogger
    {
        void Log(string message);
    }


    [PSerializable]
    public class LogAspect : OnMethodBoundaryAspect, IInstanceScopedAspect
    {
        [IntroduceMember(Visibility = Visibility.Family, OverrideAction = MemberOverrideAction.Ignore)]
        [CopyCustomAttributes(typeof (ImportAttribute))]
        [Import(typeof(ILogger))]
        public ILogger Logger { get; set; }

        [ImportMember("Logger", IsRequired = true)] 
        public Property<ILogger> LoggerProperty;

        public override void OnEntry(MethodExecutionArgs args)
        {
            this.LoggerProperty.Get().Log("OnEntry");
        }

        object IInstanceScopedAspect.CreateInstance(AdviceArgs adviceArgs)
        {
            return this.MemberwiseClone();
        }

        void IInstanceScopedAspect.RuntimeInitializeInstance()
        {
        }
    }


    [Export(typeof (MyServiceImpl))]
    internal class MyServiceImpl
    {
        [LogAspect]
        public void LoggedMethod()
        {
            Console.WriteLine("Hello, world.");
        }
    }

    internal class Program
    {
        private static void Main(string[] args)
        {
            AssemblyCatalog catalog = new AssemblyCatalog(typeof (Program).Assembly);
            CompositionContainer container = new CompositionContainer(catalog);
            MyServiceImpl service = container.GetExport<MyServiceImpl>().Value;
            service.LoggedMethod();
        }
    }


    [Export(typeof (ILogger))]
    internal class ConsoleLogger : ILogger
    {
        public void Log(string message)
        {
            Console.WriteLine(message);
        }
    }
}

The following code snippet shows how the logging aspect can be tested:

using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Text;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace DependencyResolution.InstanceScoped.Test
{
    [TestClass]
    public class TestLogAspect
    {
        [TestMethod]
        public void TestMethod()
        {
            TypeCatalog catalog = new TypeCatalog(typeof (TestLogger), typeof (TestImpl));
            CompositionContainer container = new CompositionContainer(catalog);
            TestImpl service = container.GetExport<TestImpl>().Value;
            TestLogger.Clear();
            service.TargetMethod();
            Assert.AreEqual("OnEntry" + Environment.NewLine, TestLogger.GetLog());
        }

        [Export(typeof (TestImpl))]
        private class TestImpl
        {
            [LogAspect]
            public void TargetMethod()
            {
            }
        }
    }

    [Export(typeof (ILogger))]
    internal class TestLogger : ILogger
    {
        public static readonly StringBuilder stringBuilder = new StringBuilder();

        public void Log(string message)
        {
            stringBuilder.AppendLine(message);
        }

        public static string GetLog()
        {
            return stringBuilder.ToString();
        }

        public static void Clear()
        {
            stringBuilder.Clear();
        }
    }
}