I'm building a program that will be processing messages from various topics. Since the data on each topic is different, I need dedicated classes that consumes data from any given topic. I want to do so using a base class that handles the communicates with the messaging system, and derived classes that implement the handling of the message:
public abstract class MessageConsumer<T>
{
public void Start(){/*omitted*/}
/* Other abstract properties and methods here to support template pattern*/
}
The Start method will tell the MessageConsumer to start pulling in new message. Examples of derived classes are:
public class CreatedMessageConsumer : MessageConsumer<CreatedMessage>
{
/*Implementation omitted*/
}
public class DeletedMessageConsumer : MessageConsumer<DeletedMessage>{}
In the code snippet above, I omitted the required constructor arguments, but it's easy to imagine how a DiContainer can be useful here. I use Autofac. The registration of CreatedMessageConsumer and DeletedMessageConsumer using Autofac works well.
My problem is how to resolve all classes that derives from MessageConsumer<>.
This SO post discusses how to resolve a given derived type, but I want to resolve all types that derive from MessageConsumer. Subsequently I want to call the Start method on all of them, e.g. something like
foreach(var consumer in container.Resolve<IEnumerable<MessageConsumer<??>>())
{
consumer.Start();
}
However, because I cannot provide the generic argument, it will not compile. Any idea how to do this? I'm using Autofac 4.2.1.
If you do not care about the generic argument, (which you don't otherwise you would have specified a generic argument in your last code block), then:
Define an interface (with method Start)
Implement that on your type(s)
Add it to the As chain where you define your Autofac registrations
Resolve the new interface
Updated code:
public interface IStartable {
void Start();
}
public abstract class MessageConsumer<T> : IStartable
{
public void Start(){/*omitted*/}
/* Other abstract properties and methods here to support template pattern*/
}
Caling code:
foreach(var consumer in container.Resolve<IEnumerable<IStartable>>())
{
consumer.Start();
}
Related
In the underlying use case I'm dealing with different serialization formats and depending on the configured format (via DI) a certain factory is used. However, I think I run into a more general problem here, therefore I simplified the following use case.
Suppose I have a simple interface with a generic method:
public interface IProducerFactory
{
public IProducer<T> CreateProducer<T>();
}
Now I have a concrete implementation ProducerFactoryA, which works just fine. Even though BuilderA comes from an external library, I can easily create an instantiation implementing IProducer<T>:
public class ProducerFactoryA : IProducerFactory
{
public IProducer<T> CreateProducer<T>()
{
// some configuration ...
return new BuilderA<T>().Build();
}
}
For ProducerFactoryB, I need to use the external library BuilderB. However, BuilderB has type constraints, which would require me to add them to the interface IProducerFactory and basically everywhere where I want to use it:
public class ProducerFactoryB : IProducerFactory
{
public IProducer<T> CreateProducer<T>()
where T : ISomeConstraint, new()
{
// some configuration ...
return new BuilderB<T>().Build();
}
}
My question is, how can I avoid such a situation? I clearly don't want ISomeConstraint which comes from an external library to be part of my interface and propagate this type throughout my codebase. Furthermore, it would also break ProducerFactoryA.
Is there a way to check these constraints to satisfy the compiler without adding them on the method/interface level?
The IProducerFactory makes a promise that any implementation can produce any kind of object, without restrictions. So if you want restrictions you would need to propagate them or get rid of them.
An alternative would be to declare the generic type as part of the interface:
public interface IProducerFactory<T>
{
public IProducer<T> CreateProducer();
}
public class ProducerFactoryB : IProducerFactory<ISomeConstraint>
{
public IProducer<ISomeConstraint> CreateProducer()
{
...
}
}
This makes a much weaker promise, i.e. if you somehow get a producer factory for a specific type, it can create producers of that type. You could also mix the patterns, and have one interface that can create producers for any type, and one that can only create specific types.
When registering the unbound generic with a class I cannot get the service injected into my objects:
services.AddTransient(typeof(IGetItems<,>), typeof(GetItems));
//Exampe class
public class GetItems : IGetItems<GetResultsParms, GetResults> {}
The error I'm getting is:
requires registering an open generic implementation type
How can I solve this issue?
You can't combine open-generic-interfaces with an closed-generic implementation.
Here's an example of why:
services.AddTransient(typeof(IGetItems<,>), typeof(GetItems));
class GetItems : IGetItems<GetResultsParms, GetResults> {}
class Consumer {
public Consumer( IGetItems<String,Int32> getter )
{
// ...
}
}
Supposing this works, then Consumer will be given IGetItems<GetResultsParms, GetResults>, which is demonstrably incompatible with its request for IGetItems<String,Int32>.
You need to rethink your interface design.
I assume IGetItems is an interface that represents a single data-retrieval operation that's meant to be generic over different types (e.g. one consuming class would request IGetItems<CustomersParams,CustomersResults> and another would do IGetItems<OrdersParams,OrdersResults>).
To do that, you need a custom factory - not a specific implementation type. However you need to provide more detail about how IGetItems is meant to work, and how the values/instances for for the type parameters are created.
Alternatively, if each possible type-argument for the type-parameters is known at compile-time, just register each possible combination:
services.AddTransient(typeof(IGetItems<CustomersParams,CustomersResults>), typeof(CustomersGetter));
services.AddTransient(typeof(IGetItems<OrdersParams,OrdersResults>), typeof(OrdersGetter));
services.AddTransient(typeof(IGetItems<ProductsParams,ProductsResults>), typeof(ProductsGetter));
...which can be done more succintly using the generic overloads:
services.AddTransient< IGetItems< CustomersParams, CustomersResults>, CustomersGetter >();
services.AddTransient< IGetItems< OrdersParams, OrdersResults>, OrdersGetter>();
services.AddTransient< IGetItems< ProductsParams, ProductsResults>, ProductsGetter>();
I have created components containing domain specific information in my application e.g. ImportManager, ExportManager etc
I'd like each component to operate as an isolated unit but I'm coming a little undone by my use of generics - when used with dependency inject (unity).
I have the following base object defined in a library.
public class ImportManager : IImportManager
{
[Dependency]
public IImportSettings Settings {get;set;}
}
The idea here being that I define a base class that implements standard functionality.
I then create a client-specific implementation which changes the standard behaviour slightly. This class has its own implementation and settings implemented in a different assembly as follows:
public class CustomImportManager : ImportManager, ICustomImportManager
{
}
The difference with this implementation is that I'd like to load ICustomSettings into the CustomImportManager - not ISettings.
I could just register the dependency in my bootstrapper and it would load fine but then I would have to cast the settings object every time I use it in CustomImportManager.
Alternatively, I could define a generic parameter on IImportManager:
public interface IImportManager<TSettings> where TSettings: ISettings
{
[Dependency]
public TSettings Settings {get; set}
}
Unfortunately this will require me to add the generic parameter to every class that defines this interface as a property leading to classes having masses of generic parameters.
In the example below, the facade could potentially implement 10+ components depending on its requirements meaning I'd have to define a TObject for every component - also making it very difficult to use itself.
public class Facade
{
[Dependency] IImportManager ImportManager {get; set; }
}
Does anybody have any ideas on how I may get around this?
Thanks in advance
I'm trying to get into the habit of coding to an interface rather than an implementation and whilst in most cases I can see the reasoning there are a few where I struggle.
Take this really simple example:
public interface IAuditLog
{
void AddLog(string log);
}
public class AuditLog : IAuditLog
{
public void AddLog(string log)
{
//implementation
}
}
To call the audit log class:
public partial class AuditLogPage : System.Web.UI.Page
{
protected void btnAddLog_Click(object sender, EventArgs e)
{
IAuditLog objAuditLog = new AuditLog();
objAuditLog.AddLog("test log");
}
}
I still have to use AuditLog when instantiating, so what's the point? If the AddLog method signature changes i'm still going to have to go through all my pages that use it and amend the code. Am I missing the point?
Thanks for any help in advance,
Wilky.
In the example if you switched out FileAuditLogger() with DatabaseAuditLogger() or EventLogAuditLogger() you can switch implementations without having to rewrite your code.
Typically you'd use an IoC container (Autofac, StructureMap, Unity, etc.) to automatically wire up the object instantiation. So instead of calling new AuditLog() you would call IoC.Container.Resolve<IAuditLog>()
Let me know if you'd like more information.
Let imagine that there there are two AuditLog classes
class AuditLogToDatabase : IAuditLog // writes to database
and another is
class AuditLogToFile : IAuditLog // writes to file
like
protected void btnAddLog_Click(object sender, EventArgs e)
{
IAuditLog objAuditLog = AuditLogFactory.GetAuditLog();
objAuditLog.AddLog("test log");
}
now you can inject any class based on some configuration at run time without changing the actual implementation
This doesn't necessarily mean that you have to actually use a C# interface. An interface in OOP terms is the publicly visible façade of an API. It's a contract and externally visible results of operations should be specified. How exactly it works beneath the surface should be irrelevant so that you can swap out the implementation at any time.
Of course, in that regard an interface is a method of being able to use different implementations, but so is an abstract base class or even a non-abstract class others can derive from.
But more to the exact point of your question: Of course, when instantiating a class its type must be known, but you don't necessarily have to create the class instance there. You could set an IAuditLog from the outside or get it via a factory class, etc. where you wouldn't know, at that exact point in the code, what exact type you're getting (except that it's compatible with IAuditLog).
This is actually useful when you create the AuditLog instance from a method say like a Factory method and you have more than one AuditLogXXX classes derived from the IAuditLog interface.
So, instead of using this code:
IAuditLog objAuditLog = new AuditLog();
You would actually use this code when you program to an interface:
IAuditLog objAuditLog = LogFactory.GetAuditLog(); //This call is programmed to an interface
where GetAuditLog() is an interface typed method defined on the LogFactory class as below:
class LogFactory
{
public IAuditLog GetAuditLog() // This method is programmed to an interface
{
//Some logic to make a choice to return appropriate AuditLogXXX instance from the factory
}
}
I am fairly new to Unity, (well, IoC and DI in general), and am having problems configuring it for my use.
I have two interfaces:
public interface ISample { ... }
and
public interface IDerivedSample : ISample { ... }
and a number of concreate classes simular to:
public class UseSample : IDerivedSample { ... }
I am attempting to configure Unity to resolve these when used like this:
public class UsesSample
{
private ISample _sample;
public UsesSample(ISample sample)
{
_sample = sample;
}
}
My attempts at getting this configured is failing, so I thought I would ask the crowd on how to do so.
EDIT
I have already configured the container to find the different versions of the interfaces by name, so the resulting code for resolving UserSample should be simular to:
ISample = contianer.Resolve<ISample>("derived");
However, it is failing.
The short answer
container.RegisterType<ISample, UseSample>();
The longer answer
The code above works if you only register one mapping from ISample to a concrete class. As I read your question, you have many implementations of ISample, in which case ambiquity arises because the UsesSample provides no hint at how to pick a proper ISample instance.
There are several ways out of such a conundrum. Some are specific to the DI Container (in this case Unity) while others solve the issue by creating a less ambiguous API design.
You may, for example, use an Abstract Factory to select a dependency based on a run-time value.