I've got a bit of a problem. I'm working in the Castle Windsor IOC Container. Now what i wanted to do is just mess about with some AOP principles and what i specifically want to do is based on a method name perform some logging. I have been looking at Interceptors and at the moment i am using the IInterceptor interface implemented as a class to perform this logging using aspects. The issue is if i want to perform the logging on a specific method then it gets messy as i need to put in some logic into my implemented aspect to check the method name etc...
I have read that you can do all of this using Dynamic Proxies and the IInterceptorSelector interface and the IProxyGenerationHook interface. I have seen a few examples of this done on the net but i am quite confused how this all fits into the Windsor container. I mean i am using the windsor container which in my code is actually a ref to the IWindsorContainer interface to create all my objects. All my configuration is done in code rather than XML.
Firstly does anyone know of a way to perform method specific AOP in the windsor container besides the way i am currently doing it.
Secondly how do i use the Dynamic Proxy in the windsor container ?
Below i have added the code where i am creating my proxy and registering my class with
the interceptors
ProxyGenerator _generator = new ProxyGenerator(new PersistentProxyBuilder());
IInterceptorSelector _selector = new CheckLoggingSelector();
var loggingAspect = new LoggingAspect();
var options = new ProxyGenerationOptions(new LoggingProxyGenerationHook())
{ Selector = _selector };
var proxy = _generator.CreateClassProxy(typeof(TestClass), options, loggingAspect);
TestClass testProxy = proxy as TestClass;
windsorContainer.Register(
Component.For<LoggingAspect>(),
Component.For<CheckLoggingAspect>(),
Component.For<ExceptionCatchAspect>(),
Component.For<ITestClass>()
.ImplementedBy<TestClass>()
.Named("ATestClass")
.Parameters(Parameter.ForKey("Name").Eq("Testing"))
.Proxy.MixIns(testProxy));
The Test Class is below:
public class TestClass : ITestClass
{
public TestClass()
{
}
public string Name
{
get;
set;
}
public void Checkin()
{
Name = "Checked In";
}
}
as for the interceptors they are very simple and just enter a method if the name starts with Check.
Now when i resolve my TestClass from the container i get an error.
{"This is a DynamicProxy2 error: Mixin type TestClassProxy implements IProxyTargetAccessor which is a DynamicProxy infrastructure interface and you should never implement it yourself. Are you trying to mix in an existing proxy?"}
I know i'm using the proxy in the wrong way but as i haven't seen any concrete example in how to use a proxy with the windsor container it's kind of confusing.
I mean if i want to use the LoggingProxyGenerationHook which just tell the interceptors to first for methods that start with the word "check" then is this the correct way to do it or am i completely on the wrong path. I just went down the proxy way as it seems very powerfull and i would like to understand how to use these proxies for future programming efforts.
By using .Interceptors() you already are using Dynamic Proxy. When component has specified interceptors Windsor will create proxy for it, and use these interceptors for it. You can also use method .SelectedWith and .Proxy property to set other options you already know from DynamicProxy.
I just added a website about Windsor AOP to documentation wiki. There's not much there yet, but I (and Mauricio ;) ) will put there all the information you need. Take a look, and let us know if everything is clear, and if something is missing.
Related
I've been running into endless problems attempting to use Windsor with Web API and injecting HttpRequestMessage into downstream dependencies of a controller. Since I've tried all the matching answers on Stackoverflow, I'd like to ask the question in a different way:
In Castle Windsor, how can I resolve a component instance while supplying a value for a downstream dependency? That is, the supplied value is required by a component that is required by the component being resolved.
For context, I'm trying to inject HttpRequestMessage so that I can use it to resolve the request context (primarily to resolve an absolute URL).
Edit I'd also like to point out that I don't currently have a dependency on Web Host / System.Web and I'd rather not change that.
A proper approach is to
Create IMyDesiredRouteParameterProvider
Implement it. Get the current request inside it and get the url
Register it and inject it in the desired dependent class via constructor.
I made myself such an implementation and I can say that this way it works fine. You can make Web.Infrastructure assembly and put the implementation there. Or put both the interface and the implementation there if you are going to reference it from another web module.
using System;
using System.Web;
namespace RouteParameterProvider
{
interface IMyRouteParameterProvider
{
string GetRouteParameter();
}
public class ControllerActionMethodRouteParameterProvider : IMyRouteParameterProvider
{
public string GetRouteParameter()
{
string Parameter = HttpContext.Current.Request.RequestContext.RouteData.Values["controller"] as string;
if (string.IsNullOrEmpty(Parameter))
{
throw new InvalidOperationException();
}
return Parameter;
}
}
}
You can get every possible thing that the Request Context contains from :
HttpContext.Current.Request.RequestContext
And it will be better if you rethink your design decision :
I need HttpRequestMessage to be regstered prior to creating each
instance of SomethingController so that it will be available down at
the LinkGenerator layer.
Containers are to be initialized at runtime and then used to resolve.
I need HttpRequestMessage to be regstered prior to creating each
instance of SomethingController so that it will be available down at
the LinkGenerator layer.
It sounds like you want to register an item with the container at runtime, post-startup. In general, this is not a good practice--registration should be a discrete event that happens when the app is fired up, and the container's state should not be changed during runtime.
Dependency Injection is about resolving service components, not runtime state--state is generally passed via methods (method injection). In this case it sounds like your LinkGenerator component needs access to the ambient state of the request.
I'm not that familiar with HttpRequestMessage, but this answer seems to show that it is possible to retreive it from HttpContext.Current. You could make this a method on your LinkGenerator class, or wrap this call in a separate component that gets injected into LinkGenerator (HttpRequestMessageProvider?). The latter would be my preferred method, as it allows LinkGenerator to be more testable.
Given the lack of a clean way of doing this and Web API not providing information as to the hosted endpoint beyond per-request context objects, I ended up injecting the base url from configuration.
Is this library by Mark Seemann the answer? In the description he writes explicitly :
This approach enables the use of Dependency Injection (DI) because the
request can be injected into the services which require it.
Then gives an example :
// Inside an ApiController
var uri = this.Url.GetLink(a=> a.GetById(1337));
By which you can then pass the URL down the road in the service that you have injected in the controller.
UPDATE :
Mark Seemann wrote about the same exact problem here:
"Because HttpRequestMessage provides the context you may need to
compose dependency graphs, the best extensibility point is the
extensibility point which provides an HttpRequestMessage every time a
graph should be composed. This extensibility point is the
IHttpControllerActivator interface:..."
This way you can pass request context information to a component deep in the object graph by getting from the HttpRequestMessage and passing it to the DI container.
Just take a look at the interface of IHttpControllerActivator.
The WEB API framework gets the IHttpControllerActivator through DependencyResolver. You probably already replaced it by your CastleWindsorDependencyResolver. Now you have to implement and register your HttpControllerActivator and register it.
When the WEB API framework gets IHttpControllerActivator from DependencyResolver (your Castle Windsor DR) and calls IHttpControllerActivator.Create() it will pass you the HttpRequestMessage. You can get your info from there and pass it to the your CastleDR before you call Resolve(typeof(MyController)) which will resolve the whole object graph - that means you will have MyHttpContextInfo to inject in your XYZComponent deep in the resolution stack.
This way tou are passing the arguments in the last possible moment but it is still possible. In Castle Windsor I make such passing of arguments though CreationContext.AdditionalArguments["myArgument"];.
I have created the following classes for sharing images. They implement an interface, but I need a way of switching between them with user interaction. I've done it the following way:
As you can see, service 1 and service 2 implement iSharingServices, and inherit from PolimorphSharing.
PolimorphSharing is simply and an abstract class that implements the methods I want public from Service 1 and Service 2. Those methods will then be overridden on the Service 1 and Service 2.
Because I need a way to switch the service in runtime, I've created a gateway class that inherits from PolimorphSharing. I can then call it the following way:
private var sharingService:PolimorphSharing = new SharingServicesGW('svc1').createService();
This all works flawlessly, and I can now switch between services with no problem whatsoever. However, I feel there's something wrong about it, so I would like to ask you guys for some advice on how to better implement this.
Any opinions here would be appreciated. I feel like I'm kind of implementing the factory pattern here the hard way.
UPDATE:
Just adding some more insight to this. Basically the idea here is for my client to be able to upload images with various different public sharing services such as imageshack, imgur etc. I want my client to be able to select the service in which the image is to be published to (hence the "switching between them with user interaction" bit of the question.
The method that does the uploading bit, is requestShareImage(), processResults() simply turns whatever gets returned to a unique format, so my client can read off it always the same way. getObject() is my accessor, and onIOError will handle exceptions with any of the public API's
Thanks all in advance,
SharingServicesGW IS a factory. However, there's no need for it to - and it shouldn't - inherit from PolimorphSharing. Also you're doing it a bit skewed. The client should be using objects of the interface type, not the abstract type.
Your interface should be defining the public API, not your abstract base class. In fact in AS3 interfaces can only define public members, while pseudo abstract classes can enforce implementation of protected members.
-- EDIT --
here's a UML diagram of how I would do it
do any other .NET IoC containers provide equivalent functionality to the typed factory facility in Castle Windsor?
e.g. if I am using an abstract factory pattern in a WPF application:
public class MyViewModel
{
private IAnotherViewModelFactory factory;
public void ShowAnotherViewModel()
{
viewController.ShowView(factory.GetAnotherViewModel());
}
}
I don't want to have to create a manual implementation of IAnotherViewModelFactory for every type of ViewModel I wish to show, I want the container to take care of this for me.
AutoFac has a feature called Delegate Factories, but as far as I can tell, it works only with delegates, and not interfaces.
I haven't encountered anything similar to Castle's Typed Factory Facility in neither StructureMap nor Unity, but that doesn't necessarily mean that they're not there...
The only way I can imagine that something like this could be implemented for interfaces is via a dynamic proxy. Since Castle Windsor has a Dynamic Proxy, but few other containers have anything similar, this might go a long way to explain why this feature isn't ubiquitous.
Unity also offers interception capabilities, so it must have some sort of dynamic proxy implementation, but I'm pretty sure it doesn't have anything equivalent to Typed Factories. Compared to other containers, Unity is rather basic.
In Autofac you can implement typed factories on top of the delegate approach Mark mentions. E.g.
class AnotherViewModelFactory : IAnotherViewModelFactory {
Func<AnotherViewModel> _factory;
public AnotherViewModelFactory(Func<AnotherViewModel> factory) {
_factory = factory;
}
public AnotherViewModel GetAnotherViewModel() {
return _factory();
}
}
If this class is registered with the container, along with AnotherViewModel Autofac will provide the Func<AnotherViewModel> implementation implicitly:
builder.RegisterType<AnotherViewModel>();
builder.RegisterType<AnotherViewModelFactory>()
.As<IAnotherViewModelFactory>();
Practically any interface you can implement using Typed Factory Facility can be implemented in Autofac using this kind of approach. The primary difference is that the Windsor implementation configures the factory through the component registration API, while in Autofac the factory is a component in its own right.
For more sophisticated examples you might like to look at: http://code.google.com/p/autofac/wiki/RelationshipTypes and http://nblumhardt.com/2010/01/the-relationship-zoo/.
I have recently implemented an equivalent of Castle Windsor Typed Factories for Unity. You can find the project at https://github.com/PombeirP/Unity.TypedFactories, and the NuGet package at http://nuget.org/packages/Unity.TypedFactories.
The usage is the following:
unityContainer
.RegisterTypedFactory<IFooFactory>()
.ForConcreteType<Foo>();
The parameter matching is done by name, which is fine for my needs, although the library could easily be extended to support other needs.
I have a Domain Specific Language, and I would like to register objects that can be instantiated inside.
For instance a class that can do httprequests.
[IoC("HttpRequest", typeof(DslScriptObject), IoCAttribute.IoCLifestyleType.Transient)]
internal class WebRequestDslObj : DslScriptObject
{
[DslNew]
public WebRequestDslObj() : this(null, null)
{}
[DslNew]
public WebRequestDslObj([DslParam("uri")]string uristring, [DslOptionalParam("contenttype")] string contenttype) : this(uristring, null)
{}
}
I then have a class that maps types from my dsl datatypes to c# datatypes (I have them as an IList if that makes any difference), and this works ok, if I do not use Castle to instantiate the object.
But as soon as I want to use IoC to autoregister the various types, then I dont know what to do about the constructors. I have tried to look at setting a CustomComponentActivator, but I got stuck at not being able to find any good example or documentation. Is that a viable path to take? (and will I be able to get around the funny special case for null parameters?)
Anyone have an example of where I can start?
So what are you trying to do with Windsor, because I'm not sure I see where you're going with it...
If you want to affect how component gets register in Windsor, for example rename parameters, you can write custom ComponentModel construction contributor to do it.
Method chaining is the only way I know to build fluent interfaces.
Here's an example in C#:
John john = new JohnBuilder()
.AddSmartCode("c#")
.WithfluentInterface("Please")
.ButHow("Dunno");
Assert.IsNotNull(john);
[Test]
public void Should_Assign_Due_Date_With_7DayTermsVia_Invoice_Builder()
{
DateTime now = DateTime.Now;
IInvoice invoice = new InvoiceBuilder()
.IssuedOn(now)
.WithInvoiceNumber(40)
.WithPaymentTerms(PaymentTerms.SevenDays)
.Generate();
Assert.IsTrue(invoice.DateDue == now.AddDays(7));
}
So how do others create fluent interfaces. How do you create it? What language/platform/technology is needed?
The core idea behind building a fluent interface is one of readability - someone reading the code should be able to understand what is being achieved without having to dig into the implementation to clarify details.
In modern OO languages such as C#, VB.NET and Java, method chaining is one way that this is achieved, but it's not the only technique - two others are factory classes and named parameters.
Note also that these techniques are not mutually exclusive - the goal is to maximize readabilty of the code, not purity of approach.
Method Chaining
The key insight behind method chaining is to never have a method that returns void, but to always return some object, or more often, some interface, that allows for further calls to be made.
You don't need to necessarily return the same object on which the method was called - that is, you don't always need to "return this;".
One useful design technique is to create an inner class - I always suffix these with "Expression" - that exposes the fluent API, allowing for configuration of another class.
This has two advantages - it keeps the fluent API in one place, isolated from the main functionality of the class, and (because it's an inner class) it can tinker with the innards of the main class in ways that other classes cannot.
You may want to use a series of interfaces, to control which methods are available to the developer at a given point in time.
Factory Classes
Sometimes you want to build up a series of related objects - examples include the NHibernate Criteria API, Rhino.Mocks expectation constraints and NUnit 2.4's new syntax.
In both of these cases, you have the actual objects you are storing, but to make them easier to create there are factory classes providing static methods to manufacture the instances you require.
For example, in NUnit 2.4 you can write:
Assert.That( result, Is.EqualTo(4));
The "Is" class is a static class full of factory methods that create constraints for evaluation by NUnit.
In fact, to allow for rounding errors and other imprecision of floating point numbers, you can specify a precision for the test:
Assert.That( result, Is.EqualTo(4.0).Within(0.01));
(Advance apologies - my syntax may be off.)
Named Parameters
In languages that support them (including Smalltalk, and C# 4.0) named parameters provide a way to include additional "syntax" in a method call, improving readability.
Consider a hypothetical Save() method that takes a file name, and permissions to apply to the file after saving:
myDocument.Save("sampleFile.txt", FilePermissions.ReadOnly);
with named parameters, this method could look like this:
myDocument.Save(file:"SampleFile.txt", permissions:FilePermissions.ReadOnly);
or, more fluently:
myDocument.Save(toFile:"SampleFile.txt", withPermissions:FilePermissions.ReadOnly);
You can create a fluent interface in any version of .NET or any other language that is Object Oriented. All you need to do is create an object whose methods always return the object itself.
For example in C#:
public class JohnBuilder
{
public JohnBuilder AddSmartCode(string s)
{
// do something
return this;
}
public JohnBuilder WithfluentInterface(string s)
{
// do something
return this;
}
public JohnBuilder ButHow(string s)
{
// do something
return this;
}
}
Usage:
John = new JohnBuilder()
.AddSmartCode("c#")
.WithfluentInterface("Please")
.ButHow("Dunno");
AFAIK, the term fluent interface does not specify a specific technology or framework, but rather a design pattern. Wikipedia does have an extensive example of fluent interfaces in C♯.
In a simple setter method, you do not return void but this. That way, you can chain all of the statements on that object which behave like that. Here is a quick example based on your original question:
public class JohnBuilder
{
private IList<string> languages = new List<string>();
private IList<string> fluentInterfaces = new List<string>();
private string butHow = string.Empty;
public JohnBuilder AddSmartCode(string language)
{
this.languages.Add(language);
return this;
}
public JohnBuilder WithFluentInterface(string fluentInterface)
{
this.fluentInterfaces.Add(fluentInterface);
return this;
}
public JohnBuilder ButHow(string butHow)
{
this.butHow = butHow;
return this;
}
}
public static class MyProgram
{
public static void Main(string[] args)
{
JohnBuilder johnBuilder = new JohnBuilder().AddSmartCode("c#").WithFluentInterface("Please").ButHow("Dunno");
}
}
Sometime ago I had the same doubts you are having now. I've done some research and now I'm writing a series of blog posts about techinics of designing a fluent interface.
Check it out at:
Guidelines to Fluent Interface design in C# part 1
I have a section there about Chaining X Nesting that can be interesting to you.
In the following posts I will talk about it in a deeper way.
Best regards,
André Vianna
Fluent interface is achieved in object oriented programming by always returning from your methods the same interface that contains the method. Consequently you can achieve this effect in java, javascript and your other favorite object oriented languages, regardless of version.
I have found this technique easiest to accomplish through the use of interfaces:
public interface IFoo
{
IFoo SetBar(string s);
IFoo DoStuff();
IFoo SetColor(Color c);
}
In this way, any concrete class that implements the interface, gets the fluent method chaining capabilities. FWIW.. I wrote the above code in C# 1.1
You will find this technique littered throughout the jQuery API
A couple of things come to mind that are possible in .Net 3.5/C# 3.0:
If an object doesn't implement a fluent interface, you could use Extension Methods to chain your calls.
You might be able to use the object initialization to simulate fluent, but this only works at instantiation time and would only work for single argument methods (where the property is only a setter). This seems hackish to me, but the there it is.
Personally, I don't see anything wrong with using function chaining if you are implementing a builder object. If the builder object has chaining methods, it keeps the object you are creating clean. Just a thought.
This is how I've built my so called fluent interfaces or my only forary into it
Tokenizer<Bid> tkn = new Tokenizer<Bid>();
tkn.Add(Token.LambdaToken<Bid>("<YourFullName>", b => Util.CurrentUser.FullName))
.Add(Token.LambdaToken<Bid>("<WalkthroughDate>",
b => b.WalkThroughDate.ToShortDateString()))
.Add(Token.LambdaToken<Bid>("<ContactFullName>", b => b.Contact.FullName))
.Cache("Bid")
.SetPattern(#"<\w+>");
My example required .net 3.5 but that's only cause of my lambda's. As Brad pointed out you can do this in any version of .net. Although I think lambda's make for more interesting possibilities such as this.
======
Some other good examples are nHibernate's Criteria API, there is also a fluent nhibernate extension for configuring nhibernate but I've never used it
Dynamic keyword in C# 4.0 will make it possible to write dynamic style builders. Take a look at following article about JSON object construction.