I am implementing a command handler pattern using Autofac and am using it's decorator facility handle cross cutting concerns such as logging, authentication etc.
I also have dependencies that I only want scoped to the lifetime of the request / response pipeline.
I have an example implementation below:
public class Program
{
public static void Main()
{
var builder = new ContainerBuilder();
builder.RegisterAssemblyModules(typeof(HandlerModule).Assembly);
builder.RegisterType<LifetimeScopeTester>().AsSelf()
.InstancePerMatchingLifetimeScope("pipline");
var container = builder.Build();
using(var scope = container.BeginLifetimeScope("pipline")) {
var pingHandler = scope.Resolve<IHandle<PingRequest, PingResponse>>();
pingHandler.Handle(new PingRequest());
}
}
}
public class HandlerModule : Autofac.Module
{
protected override void Load(ContainerBuilder builder)
{
builder.RegisterAssemblyTypes(ThisAssembly)
.As(type => type.GetInterfaces()
.Where(interfaceType => interfaceType.IsClosedTypeOf(typeof (IHandle<,>)))
.Select(interfaceType => new KeyedService("IHandle", interfaceType)));
builder.RegisterGenericDecorator(
typeof(SecondDecoratorHandler<,>),
typeof(IHandle<,>),
"IHandle"
)
.Keyed("SecondDecoratorHandler", typeof(IHandle<,>));
builder.RegisterGenericDecorator(
typeof(FirstDecoratorHandler<,>),
typeof(IHandle<,>),
"SecondDecoratorHandler"
);
}
}
public class LifetimeScopeTester {}
public interface IHandle<in TRequest, out TResponse>
where TRequest : class, IRequest<TResponse>
{
TResponse Handle(TRequest request);
}
public interface IRequest<TResponse> {
}
public class PingRequest : IRequest<PingResponse> {
}
public class PingResponse {
}
public class PingHandler : IHandle<PingRequest, PingResponse> {
public PingResponse Handle(PingRequest request) {
Console.WriteLine("PingHandler");
return new PingResponse();
}
}
public class FirstDecoratorHandler<TRequest, TResponse> : IHandle<TRequest, TResponse>
where TRequest : class, IRequest<TResponse>
{
private readonly IHandle<TRequest, TResponse> _decoratedHandler;
private readonly LifetimeScopeTester _lifetimeScopeTester;
public FirstDecoratorHandler(IHandle<TRequest, TResponse> decoratedHandler,
LifetimeScopeTester lifetimeScopeTester)
{
_decoratedHandler = decoratedHandler;
_lifetimeScopeTester = lifetimeScopeTester;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("FirstDecoratorHandler - LifetimeScopeTester[{0}]",
_lifetimeScopeTester.GetHashCode());
return _decoratedHandler.Handle(request);
}
}
public class SecondDecoratorHandler<TRequest, TResponse> : IHandle<TRequest, TResponse>
where TRequest : class, IRequest<TResponse>
{
private readonly IHandle<TRequest, TResponse> _decoratedHandler;
private readonly LifetimeScopeTester _lifetimeScopeTester;
public SecondDecoratorHandler(IHandle<TRequest, TResponse> decoratedHandler, LifetimeScopeTester lifetimeScopeTester)
{
_decoratedHandler = decoratedHandler;
_lifetimeScopeTester = lifetimeScopeTester;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("SecondDecoratorHandler - LifetimeScopeTester[{0}]", _lifetimeScopeTester.GetHashCode());
return _decoratedHandler.Handle(request);
}
}
As you can see, I wrap the pipleine in a scope named pipeline which means that everytime I resolve LifetimeScopeTester which is scope to pipeline, I get the same instance.
I as thinking that I might be able to replace
using(var scope = container.BeginLifetimeScope("pipline")) {
var pingHandler = scope.Resolve<IHandle<PingRequest, PingResponse>>();
pingHandler.Handle(new PingRequest());
}
with
var pingHandler = scope.Resolve<IHandle<PingRequest, PingResponse>>();
pingHandler.Handle(new PingRequest());
by creating another decorator that does that same thing.
My first instinct was:
public class LifetimeScopeDecoratorHandler<TRequest, TResponse> : IHandle<TRequest, TResponse>
where TRequest : class, IRequest<TResponse>
{
private readonly ILifetimeScope _scope;
private readonly IHandle<TRequest, TResponse> _decoratedHandler;
public LifetimeScopeDecoratorHandlerAttempt1(ILifetimeScope scope,
IHandle<TRequest, TResponse> decoratedHandler)
{
_scope = scope;
_decoratedHandler = decoratedHandler;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("LifetimeScopeDecoratorHandler");
TResponse response;
using (_scope.BeginLifetimeScope("pipeline"))
{
response = _decoratedHandler.Handle(request);
}
return response;
}
}
But the decoratedHandler would have already been resolved by the time it's injected so that won't work.
So I tried:
public class LifetimeScopeHandler<TRequest, TResponse> : IHandle<TRequest, TResponse>
where TRequest : class, IRequest<TResponse>
{
private readonly ILifetimeScope _scope;
private readonly Func<IHandle<TRequest, TResponse>> _decoratedHandlerFactory;
public LifetimeScopeHandler(ILifetimeScope scope,
Func<IHandle<TRequest, TResponse>> decoratedHandlerFactory)
{
_scope = scope;
_decoratedHandlerFactory = decoratedHandlerFactory;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("LifetimeScopeDecoratorHandler");
TResponse response;
using (_scope.BeginLifetimeScope("pipeline"))
{
var decoratedHandler = _decoratedHandlerFactory();
response = decoratedHandler.Handle(request);
}
return response;
}
}
However this repeated infinitely as calling _decoratedHandlerFactory() tries to wrap the inner handler with a LifetimeScopeHandler decorator again.
Is what I'm trying to achieve possible.
I have created a dotnetfiddle at https://dotnetfiddle.net/hwujNI demonstrating the issue.
When the Handle method of LifetimeScopeHandler class invoke the decoratedHandlerFactory delegate, it asks Autofac to resolve a IHandle<TRequest, TResponse> which is a LifetimeScopeHandler. That's why you have a StackOverflowException. We can simplify your case to this code sample :
public class Foo
{
public Foo(Func<Foo> fooFactory)
{
this._fooFactory = fooFactory;
}
private readonly Func<Foo> _fooFactory;
public void Do()
{
Foo f = this._fooFactory();
f.Do();
}
}
Even if there is a single instance of Foo you will have a StackOverflowException
In order to resolve this issue, you have to indicate Autofac that the decoratedHandlerFactory delegate of LifetimeScopeHandler should not be a delegate of LifetimeScopeHandler.
You can use the WithParameter to indicate the last decorator to use a specific parameter :
builder.RegisterGenericDecorator(
typeof(LifetimeScopeHandler<,>),
typeof(IHandle<,>),
"FirstDecoratorHandler"
)
.WithParameter((pi, c) => pi.Name == "decoratedHandlerFactory",
(pi, c) => c.ResolveKeyed("FirstDecoratorHandler", pi.ParameterType))
.As(typeof(IHandle<,>));
With this configuration, the output will be
LifetimeScopeHandler
FirstDecoratorHandler - LifetimeScopeTester[52243212]
SecondDecoratorHandler - LifetimeScopeTester[52243212]
PingHandler
By the way, you want LifetimeScopeHandler to be a special kind of decorator that will create the inner IHandler<,> in a special scope.
You can do this by asking the LifetimeScopeHandler to create the correct scope for you and resolve the previous Ihandler.
public class LifetimeScopeHandler<TRequest, TResponse>
: IHandle<TRequest, TResponse> where TRequest : class, IRequest<TResponse>
{
private readonly ILifetimeScope _scope;
public LifetimeScopeHandler(ILifetimeScope scope)
{
this._scope = scope;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("LifetimeScopeDecoratorHandler");
using (ILifetimeScope s = this._scope.BeginLifetimeScope("pipline"))
{
var decoratedHandler =
s.ResolveKeyed<IHandle<TRequest, TResponse>>("FirstDecoratorHandler");
TResponse response = decoratedHandler.Handle(request);
return response;
}
}
}
This implementation will require that LifetimeScopeHandler knows the first decorator on the chain, we can bypass that by sending the name on its constructor.
public class LifetimeScopeHandler<TRequest, TResponse>
: IHandle<TRequest, TResponse> where TRequest : class, IRequest<TResponse>
{
private readonly ILifetimeScope _scope;
private readonly String _previousHandlerName;
public LifetimeScopeHandler(ILifetimeScope scope, String previousHandlerName)
{
this._scope = scope;
this._previousHandlerName = previousHandlerName;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("LifetimeScopeDecoratorHandler");
using (ILifetimeScope s = this._scope.BeginLifetimeScope("pipline"))
{
var decoratedHandler =
s.ResolveKeyed<IHandle<TRequest, TResponse>>(previousHandlerName);
TResponse response = decoratedHandler.Handle(request);
return response;
}
}
}
And you will have to register it like this :
builder.RegisterGenericDecorator(
typeof(LifetimeScopeHandler<,>),
typeof(IHandle<,>),
"FirstDecoratorHandler"
)
.WithParameter("previousHandlerName", "FirstDecoratorHandler")
.As(typeof(IHandle<,>));
We can also bypass everything by not using the RegisterGenericDecorator method.
If we register the LifetimeScopeHandler like this :
builder.RegisterGeneric(typeof(LifetimeScopeHandler<,>))
.WithParameter((pi, c) => pi.Name == "decoratedHandler",
(pi, c) =>
{
ILifetimeScope scope = c.Resolve<ILifetimeScope>();
ILifetimeScope piplineScope = scope.BeginLifetimeScope("pipline");
var o = piplineScope.ResolveKeyed("FirstDecoratorHandler", pi.ParameterType);
scope.Disposer.AddInstanceForDisposal(piplineScope);
return o;
})
.As(typeof(IHandle<,>));
And LifetimeScopeHandler can now look like all decorator :
public class LifetimeScopeHandler<TRequest, TResponse>
: IHandle<TRequest, TResponse> where TRequest : class, IRequest<TResponse>
{
private readonly IHandle<TRequest, TResponse> _decoratedHandler;
public LifetimeScopeHandler(IHandle<TRequest, TResponse> decoratedHandler)
{
this._decoratedHandler = decoratedHandler;
}
public TResponse Handle(TRequest request)
{
Console.WriteLine("LifetimeScopeDecoratorHandler");
TResponse response = this._decoratedHandler.Handle(request);
return response;
}
}
By the way, this solution may have an issue if you use more than one IHandler<,> in a scope and you need to have a single pipline scope. To resolve this, you can see this dotnetfiddle : https://dotnetfiddle.net/rQgy2X but it seems to me over complicated and you may not need it.
来源：https://stackoverflow.com/questions/31230305/autofac-lifetime-scope-decorator