AttributeIocExample.cs

MIT License

Copyright (c) 2016 James Bascle

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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The above copyright notice and this permission notice shall be included in all
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Web.Mvc;
using Autofac;

namespace AttributeIocExample
{
    class Program
    {
        static void Main(string[] args)
        {
            //create container
            var cb = new ContainerBuilder();
            cb.RegisterType<ConstructedByContainer>().AsSelf();
            var container = cb.Build();

            //Magic happens here
            AutoImplementFactoryFuncs(container);

            //create filter and execute normal filter method
            var filter = new ExampleFilterAttribute();
            filter.OnActionExecuted(null);
            Console.Read();
        }

        private static void AutoImplementFactoryFuncs(IContainer container)
        {
            //find all static properties tagged with "AutoImpl" attribute that are of type Func<T>
            var factoryFuncsToAutoImplement =
                AppDomain.CurrentDomain.GetAssemblies()
                    .SelectMany(c => c.DefinedTypes)
                    .SelectMany(c => c.GetProperties(BindingFlags.Public | BindingFlags.Static))
                    .Where(e => e.CustomAttributes.Any(a => a.AttributeType == typeof(AutoImplAttribute)))
                    .Where(
                        p =>
                            p.PropertyType.IsGenericType &&
                            p.PropertyType.GetGenericTypeDefinition() == typeof(Func<>))
                    .ToList();

            //get a MethodInfo for the AutoFac resolve method
            var resolveMethod = typeof(ResolutionExtensions).GetMethod("Resolve", new[] { typeof(IComponentContext), typeof(Type) });

            foreach (var prop in factoryFuncsToAutoImplement)
            {
                //find the type that the factory function actually needs to create
                var typeForFactoryToCreate = prop.PropertyType.GenericTypeArguments[0];
                //create an expression that will call the resolve method on the correct container with the correct ServiceType
                var methodInvoke = Expression.Call(null, resolveMethod, Expression.Constant(container), Expression.Constant(typeForFactoryToCreate));
                //convert the output of the method invokation to the asked for type, and populate the body of the lambda
                var f = Expression.Lambda(Expression.Convert(methodInvoke, typeForFactoryToCreate));
                //set the newly created lambda to the expression on the AutoImpl'd property
                prop.SetValue(null, f.Compile());
            }
        }
    }

    [AttributeUsage(AttributeTargets.Property)]
    public class AutoImplAttribute : Attribute
    {

    }
    public class ExampleFilterAttribute : ActionFilterAttribute
    {
        private ConstructedByContainer _cbc;
        
        [AutoImpl]
        public static Func<ConstructedByContainer> ConstructedByContainerFactoryFunc { get; set; }
        
        public ExampleFilterAttribute()
        {
            _cbc = ConstructedByContainerFactoryFunc();
        }
        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            _cbc.Proof();
        }
    }

    public class ConstructedByContainer
    {
        public void Proof() => Console.WriteLine("It works!");
    }
}