=======================
Interception Extensions
=======================

**Adding interception abilities to Simple Injector.**

.. code-block:: c#

    using System;
    using System.Diagnostics;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Reflection;
    using System.Runtime.Remoting.Messaging;
    using System.Runtime.Remoting.Proxies;

    using SimpleInjector;

    public interface IInterceptor
    {
        void Intercept(IInvocation invocation);
    }

    public interface IInvocation
    {
        object InvocationTarget { get; }
        object ReturnValue { get; set; }
        object[] Arguments { get; }
        void Proceed();
        MethodBase GetConcreteMethod();
    }

    // Extension methods for interceptor registration
    // NOTE: These extension methods can only intercept interfaces, not abstract types.
    public static class InterceptorExtensions
    {
        public static void InterceptWith<TInterceptor>(
            this Container container, Func<Type, bool> predicate)
            where TInterceptor : class, IInterceptor =>
            container.InterceptWith<TInterceptor>(null, predicate);

        public static void InterceptWith<TInterceptor>(
            this Container container,
            Lifestyle lifestyle,
            Func<Type, bool> predicate)
            where TInterceptor : class, IInterceptor
        {
            lifestyle = lifestyle ?? container.Options.LifestyleSelectionBehavior
                .SelectLifestyle(typeof(TInterceptor));

            var producer =
                lifestyle.CreateProducer<TInterceptor, TInterceptor>(container);

            var interceptWith = new InterceptionHelper
            {
                BuildInterceptorExpression = e => producer.BuildExpression(),
                Predicate = type => predicate(type)
            };

            container.ExpressionBuilt += interceptWith.OnExpressionBuilt;
        }

        public static void InterceptWith(
            this Container container,
            Func<IInterceptor> interceptorCreator,
            Func<Type, bool> predicate)
        {
            var interceptWith = new InterceptionHelper()
            {
                BuildInterceptorExpression =
                    e => Expression.Invoke(Expression.Constant(interceptorCreator)),
                Predicate = type => predicate(type)
            };

            container.ExpressionBuilt += interceptWith.OnExpressionBuilt;
        }

        public static void InterceptWith(
            this Container container,
            Func<ExpressionBuiltEventArgs, IInterceptor> interceptorCreator,
            Func<Type, bool> predicate)
        {
            var interceptWith = new InterceptionHelper()
            {
                BuildInterceptorExpression = e => Expression.Invoke(
                    Expression.Constant(interceptorCreator),
                    Expression.Constant(e)),
                Predicate = type => predicate(type)
            };

            container.ExpressionBuilt += interceptWith.OnExpressionBuilt;
        }

        public static void InterceptWith(
            this Container container,
            IInterceptor interceptor,
            Func<Type, bool> predicate)
        {
            var interceptWith = new InterceptionHelper()
            {
                BuildInterceptorExpression = e => Expression.Constant(interceptor),
                Predicate = predicate
            };

            container.ExpressionBuilt += interceptWith.OnExpressionBuilt;
        }

        private class InterceptionHelper
        {
            internal Func<ExpressionBuiltEventArgs, Expression> BuildInterceptorExpression;
            internal Func<Type, bool> Predicate;

            private static readonly MethodInfo NonGenericInterceptorCreateProxyMethod = (
                from method in typeof(Interceptor).GetMethods()
                where method.Name == nameof(Interceptor.CreateProxy)
                where method.GetParameters().Length == 3
                select method)
                .Single();

            [DebuggerStepThrough]
            public void OnExpressionBuilt(object sender, ExpressionBuiltEventArgs e)
            {
                if (this.Predicate(e.RegisteredServiceType))
                {
                    ThrowIfServiceTypeNotInterface(e);
                    e.Expression = this.BuildProxyExpression(e);
                }
            }

            [DebuggerStepThrough]
            private static void ThrowIfServiceTypeNotInterface(ExpressionBuiltEventArgs e)
            {
                // NOTE: We can only handle interfaces, because
                // System.Runtime.Remoting.Proxies.RealProxy only supports interfaces.
                if (!e.RegisteredServiceType.IsInterface)
                {
                    throw new NotSupportedException("Can't intercept type " +
                        e.RegisteredServiceType.Name + " because it is not an interface.");
                }
            }

            [DebuggerStepThrough]
            private Expression BuildProxyExpression(ExpressionBuiltEventArgs e)
            {
                var expr = this.BuildInterceptorExpression(e);

                // Create call to
                // (ServiceType)Interceptor.CreateProxy(Type, IInterceptor, object)
                var proxyExpression =
                    Expression.Convert(
                        Expression.Call(NonGenericInterceptorCreateProxyMethod,
                            Expression.Constant(e.RegisteredServiceType, typeof(Type)),
                            expr,
                            e.Expression),
                        e.RegisteredServiceType);

                if (e.Expression is ConstantExpression && expr is ConstantExpression)
                {
                    return Expression.Constant(CreateInstance(proxyExpression),
                        e.RegisteredServiceType);
                }

                return proxyExpression;
            }

            [DebuggerStepThrough]
            private static object CreateInstance(Expression expression)
            {
                var instanceCreator = Expression.Lambda<Func<object>>(expression,
                    new ParameterExpression[0])
                    .Compile();

                return instanceCreator();
            }
        }
    }

    public static class Interceptor
    {
        public static T CreateProxy<T>(IInterceptor interceptor, T realInstance) =>
            (T)CreateProxy(typeof(T), interceptor, realInstance);

        [DebuggerStepThrough]
        public static object CreateProxy(
            Type serviceType, IInterceptor interceptor, object realInstance)
        {
            var proxy = new InterceptorProxy(serviceType, realInstance, interceptor);
            return proxy.GetTransparentProxy();
        }

        private sealed class InterceptorProxy : RealProxy
        {
            private static readonly MethodBase GetTypeMethod =
                typeof(object).GetMethod(nameof(object.GetType));

            private readonly object realInstance;
            private readonly IInterceptor interceptor;

            [DebuggerStepThrough]
            public InterceptorProxy(Type classToProxy, object obj, IInterceptor interceptor)
                : base(classToProxy)
            {
                this.realInstance = obj;
                this.interceptor = interceptor;
            }

            public override IMessage Invoke(IMessage msg)
            {
                if (msg is IMethodCallMessage message)
                {
                    return object.ReferenceEquals(message.MethodBase, GetTypeMethod)
                        ? this.Bypass(message)
                        : this.InvokeMethodCall(message);
                }

                return msg;
            }

            private IMessage InvokeMethodCall(IMethodCallMessage msg)
            {
                var i = new Invocation { Proxy = this, Message = msg, Arguments = msg.Args };
                i.Proceeding = () =>
                    i.ReturnValue = msg.MethodBase.Invoke(this.realInstance, i.Arguments);
                this.interceptor.Intercept(i);
                return new ReturnMessage(i.ReturnValue, i.Arguments,
                    i.Arguments.Length, null, msg);
            }

            private IMessage Bypass(IMethodCallMessage msg)
            {
                object value = msg.MethodBase.Invoke(this.realInstance, msg.Args);
                return new ReturnMessage(value, msg.Args, msg.Args.Length, null, msg);
            }

            private class Invocation : IInvocation
            {
                public Action Proceeding { get; set; }
                public InterceptorProxy Proxy { get; set; }
                public object[] Arguments { get; set; }
                public IMethodCallMessage Message { get; set; }
                public object ReturnValue { get; set; }
                public object InvocationTarget => this.Proxy.realInstance;
                public void Proceed() => this.Proceeding();
                public MethodBase GetConcreteMethod() => this.Message.MethodBase;
            }
        }
    }