Using a Strategy and Factory Pattern with Dependency Injection
When applying Dependency Injection, you define all the class’s dependencies as required arguments in the constructor. This practice is called Constructor Injection. This pushes the burden of creating the dependency from the class to its consumer. The same rule, however, applies to the class’s consumers as well. They as well need to define their dependencies in their constructor. This goes all the way up the call stack, and this means that so called 'object graphs' can become quite deep at some points.
Dependency Injection causes the responsibility of creating classes all the way up to the entry point of the application; the Composition Root. This does mean, however, that the entry point needs to know about all the dependencies. In case you use DI without a DI Container—a practice called Pure DI—it means that at this point all dependencies must be created in plain old C# code. In case you use a DI Container you still have to tell the DI container about all dependencies.
Sometimes however, you can make use of a technique called batch or Auto-Registration, where the DI Container will use reflection over your projects and register types using Convention over Configuration. This saves you the burden of registering all types one by one and often prevents you from making changes to the Composition Root every time a new class is added to the system.
Do these interfaces/implementations need to be registered with the entry point (the Program class) and passed down through the constructors?
Absolutely.
As a result I find myself declaring all possible interfaces in the service entry point, and passing them down through the application. As a result, the entry point must be changed for new and various strategy class implementations.
The application's entry point is the most volatile part of the system (which is implied by the Stable-Dependencies Principle). It always is, even without DI. But with DI you can make the rest of the system much less volatile. Again, you can reduce the amount of code changes you need to make at the entry point by applying Auto-Registration.
I am wondering if there are best practices to using DI with the factory and strategy patterns?
I would say the best practice concerning factories is to have as little of them as possible, as explained in this article. As a matter of fact, your factory interface is redundant and only complicates the consumers that require it (as explained in the article). Your application can easily do without and you can instead inject a IShippingStrategy directly, since this is the only thing the consumer is interested in: to get the shipping cost for an order. It doesn't care whether there is one or dozens of implementations behind it. It just wants to get the shipping cost and go on with its job:
public int DoTheWork(Order order)
{
// assign properties just as an example
order.ShippingMethod = "Fedex";
order.OrderTotal = 90;
order.OrderWeight = 12;
order.OrderZipCode = 98109;
return shippingStrategy.CalculateShippingCost(order);
}
This means, however, that the injected shipping strategy must now be something that can decide how to calculate the cost based on the Order.Method property. But there's a pattern for this called the Proxy pattern. Here's an example:
public class ShippingStrategyProxy : IShippingStrategy
{
private readonly DHLShippingStrategy _dhl;
private readonly UPSShippingStrategy _ups;
//...
public ShippingStrategyProxy(
DHLShippingStrategy dhl, UPSShippingStrategy ups, ...)
{
_dhl = dhl;
_ups = ups;
//...
}
public int CalculateShippingCost(Order order) =>
GetStrategy(order.Method).CalculateShippingCost(order);
private IShippingStrategy GetStrategy(string method)
{
switch (method)
{
case "DHL": return dhl;
case "UPS": return ups:
//...
default: throw InvalidOperationException(method);
}
}
}
This proxy internally acts a bit like a factory, but there are two important differences here:
- It does not define a different interface. This allows the consumer to only take a dependency on 1 concept: the
IShippingStrategy. - It does not create the strategies itself; they are still injected into it.
This proxy simply forwards the incoming call to an underlying strategy implementation that does the actual work.
There is a variety of ways to implement such proxy. For instance, you could still create the dependencies here manually—or you could forward the call to the container, who will create the dependencies for you. Also the way you inject the dependencies can differ based on what's best for your application.
And even though such proxy might internally work like a factory, the important thing is that there is no factory abstraction here; that would only complicate the consumers.
Everything discussed above is discussed in more detail in the book Dependency Injection Principles, Practices, and Patterns, by Mark Seemann and myself. For instance:
- Composition Root is discussed in § 4.1,
- Constructor Injection in § 4.2,
- abuse of Abstract Factories in § 6.2, and
- Auto-Registration in chapter 12.
There are a few ways of doing this, but the way I prefer is to inject a list of available strategies into your factory, and then filtering them to return the one(s) you're interested in.
Working with your example, I'd modify IShippingStrategy to add a new property:
public interface IShippingStrategy
{
int CalculateShippingCost(Order order);
string SupportedShippingMethod { get; }
}
Then I'd implement the factory like so:
public class ShippingStrategyFactory : IShippingStrategyFactory
{
private readonly IEnumerable<IShippingStrategy> availableStrategies;
public ShippingStrategyFactory(IEnumerable<IShippingStrategy> availableStrategies)
{
this.availableStrategies = availableStrategies;
}
public IShippingStrategy GetShippingStrategy(Order order)
{
var supportedStrategy = availableStrategies
.FirstOrDefault(x => x.SupportedShippingMethod == order.ShippingMethod);
if (supportedStrategy == null)
{
throw new InvalidOperationException($"No supported strategy found for shipping method '{order.ShippingMethod}'.");
}
return supportedStrategy;
}
}
The main reason I like using it this way is that I never have to come back and modify the factory. If ever I have to implement a new strategy, the factory doesn't have to be changed. If you're using auto-registration with your container, you don't even have to register the new strategy either, so it's simply a case of allowing you to spend more time writing new code.
So I did it like this. I would have preferred to have injected an IDictionary, but because of the limitation with injecting "IEnumerable" into the constructor (this limitation is Unity specific), I came up with a little workaround.
public interface IShipper
{
void ShipOrder(Order ord);
string FriendlyNameInstance { get;} /* here for my "trick" */
}
..
public interface IOrderProcessor
{
void ProcessOrder(String preferredShipperAbbreviation, Order ord);
}
..
public class Order
{
}
..
public class FedExShipper : IShipper
{
private readonly Common.Logging.ILog logger;
public static readonly string FriendlyName = typeof(FedExShipper).FullName; /* here for my "trick" */
public FedExShipper(Common.Logging.ILog lgr)
{
if (null == lgr)
{
throw new ArgumentOutOfRangeException("Log is null");
}
this.logger = lgr;
}
public string FriendlyNameInstance => FriendlyName; /* here for my "trick" */
public void ShipOrder(Order ord)
{
this.logger.Info("I'm shipping the Order with FedEx");
}
..
public class UpsShipper : IShipper
{
private readonly Common.Logging.ILog logger;
public static readonly string FriendlyName = typeof(UpsShipper).FullName; /* here for my "trick" */
public UpsShipper(Common.Logging.ILog lgr)
{
if (null == lgr)
{
throw new ArgumentOutOfRangeException("Log is null");
}
this.logger = lgr;
}
public string FriendlyNameInstance => FriendlyName; /* here for my "trick" */
public void ShipOrder(Order ord)
{
this.logger.Info("I'm shipping the Order with Ups");
}
}
..
public class UspsShipper : IShipper
{
private readonly Common.Logging.ILog logger;
public static readonly string FriendlyName = typeof(UspsShipper).FullName; /* here for my "trick" */
public UspsShipper(Common.Logging.ILog lgr)
{
if (null == lgr)
{
throw new ArgumentOutOfRangeException("Log is null");
}
this.logger = lgr;
}
public string FriendlyNameInstance => FriendlyName; /* here for my "trick" */
public void ShipOrder(Order ord)
{
this.logger.Info("I'm shipping the Order with Usps");
}
}
..
public class OrderProcessor : IOrderProcessor
{
private Common.Logging.ILog logger;
//IDictionary<string, IShipper> shippers; /* :( I couldn't get IDictionary<string, IShipper> to work */
IEnumerable<IShipper> shippers;
public OrderProcessor(Common.Logging.ILog lgr, IEnumerable<IShipper> shprs)
{
if (null == lgr)
{
throw new ArgumentOutOfRangeException("Log is null");
}
if (null == shprs)
{
throw new ArgumentOutOfRangeException("ShipperInterface(s) is null");
}
this.logger = lgr;
this.shippers = shprs;
}
public void ProcessOrder(String preferredShipperAbbreviation, Order ord)
{
this.logger.Info(String.Format("About to ship. ({0})", preferredShipperAbbreviation));
/* below foreach is not needed, just "proves" everything was injected */
foreach (IShipper sh in shippers)
{
this.logger.Info(String.Format("ShipperInterface . ({0})", sh.GetType().Name));
}
IShipper foundShipper = this.FindIShipper(preferredShipperAbbreviation);
foundShipper.ShipOrder(ord);
}
private IShipper FindIShipper(String preferredShipperAbbreviation)
{
IShipper foundShipper = this.shippers.FirstOrDefault(s => s.FriendlyNameInstance.Equals(preferredShipperAbbreviation, StringComparison.OrdinalIgnoreCase));
if (null == foundShipper)
{
throw new ArgumentNullException(
String.Format("ShipperInterface not found in shipperProviderMap. ('{0}')", preferredShipperAbbreviation));
}
return foundShipper;
}
}
...
And calling code: (that would be in something like "Program.cs" for example)
Common.Logging.ILog log = Common.Logging.LogManager.GetLogger(typeof(Program));
IUnityContainer cont = new UnityContainer();
cont.RegisterInstance<ILog>(log);
cont.RegisterType<IShipper, FedExShipper>(FedExShipper.FriendlyName);
cont.RegisterType<IShipper, UspsShipper>(UspsShipper.FriendlyName);
cont.RegisterType<IShipper, UpsShipper>(UpsShipper.FriendlyName);
cont.RegisterType<IOrderProcessor, OrderProcessor>();
Order ord = new Order();
IOrderProcessor iop = cont.Resolve<IOrderProcessor>();
iop.ProcessOrder(FedExShipper.FriendlyName, ord);
Logging Output:
2018/09/21 08:13:40:556 [INFO] MyNamespace.Program - About to ship. (MyNamespace.Bal.Shippers.FedExShipper)
2018/09/21 08:13:40:571 [INFO] MyNamespace.Program - ShipperInterface . (FedExShipper)
2018/09/21 08:13:40:572 [INFO] MyNamespace.Program - ShipperInterface . (UspsShipper)
2018/09/21 08:13:40:572 [INFO] MyNamespace.Program - ShipperInterface . (UpsShipper)
2018/09/21 08:13:40:573 [INFO] MyNamespace.Program - I'm shipping the Order with FedEx
So each concrete has a static string providing its name in a strong-typed fashion. ("FriendlyName")
And then I have an instance string-get property which uses the exact same value to keep things in sync. ("FriendlyNameInstance")
By forcing the issue by using a property on the Interface (below partial code)
public interface IShipper
{
string FriendlyNameInstance { get;}
}
I can use this to "find" my shipper out of the collection of shippers.
The internal method "FindIShipper" is the kinda-factory, but removes the need to have a separate IShipperFactory and ShipperFactory interface and class. Thus simplifying the overall setup. And still honors Constructor-Injection and Composition root.
If anyone knows how to use IDictionary<string, IShipper> (and inject via the constructor), please let me know.
But my solution works...with a little razzle dazzle.
...........................
My third-party-dll dependency list. (I'm using dotnet core, but dotnet framework with a semi new version of Unity should work too). (See PackageReference's below)
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp2.0</TargetFramework>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Common.Logging" Version="3.4.1" />
<PackageReference Include="Microsoft.Extensions.Configuration" Version="2.1.1" />
<PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="2.1.1" />
<PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="2.1.1" />
<PackageReference Include="Unity" Version="5.8.11" />
</ItemGroup>
<ItemGroup>
<None Update="appsettings.json">
<CopyToOutputDirectory>Always</CopyToOutputDirectory>
</None>
</ItemGroup>
</Project>
APPEND:
Here is the autofac version:
(using all the same interfaces and concretes above )
Program.cs
namespace MyCompany.ProofOfConcepts.AutofacStrategyPatternExample.DemoCommandLineInterfaceOne
{
using System;
using System.Text;
using Autofac;
using Autofac.Extensions.DependencyInjection;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
/* need usings for all the object above */
using MyCompany.ProofOfConcepts.AutofacStrategyPatternExample.Domain;
using NLog;
using NLog.Extensions.Logging;
public class Program
{
private static Logger programStaticLoggerThatNeedsToBeInitiatedInMainMethod = null;
public static int Main(string[] args)
{
Logger loggerFromNLogLogManagerGetCurrentClassLogger = NLog.LogManager.GetCurrentClassLogger(); /* if this is made a private-static, it does not log the entries */
programStaticLoggerThatNeedsToBeInitiatedInMainMethod = loggerFromNLogLogManagerGetCurrentClassLogger;
programStaticLoggerThatNeedsToBeInitiatedInMainMethod.Info("programStaticLoggerThatNeedsToBeInitiatedInMainMethod: Main.Start");
try
{
bool useCodeButNotAutofacJson = true; /* true will "code up" the DI in c# code, false will kick in the autofac.json */
string autoFacFileName = useCodeButNotAutofacJson ? "autofac.Empty.json" : "autofac.json"; /* use "empty" to prove the DI is not coming from non-empty json */
programStaticLoggerThatNeedsToBeInitiatedInMainMethod.Info(string.Format("programStaticLoggerThatNeedsToBeInitiatedInMainMethod: autoFacFileName={0}", autoFacFileName));
IConfiguration config = new ConfigurationBuilder()
.SetBasePath(System.IO.Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
.AddJsonFile(autoFacFileName)
.Build();
IServiceProvider servicesProvider = BuildDi(config, useCodeButNotAutofacJson);
using (servicesProvider as IDisposable)
{
IOrderProcessor processor = servicesProvider.GetRequiredService<IOrderProcessor>();
processor.ProcessOrder(FedExShipper.FriendlyName, new Order());
Microsoft.Extensions.Logging.ILogger loggerFromIoc = servicesProvider.GetService<ILoggerFactory>()
.CreateLogger<Program>();
loggerFromIoc.LogInformation("loggerFromIoc:Starting application");
loggerFromIoc.LogInformation("loggerFromIoc:All done!");
Console.WriteLine("Press ANY key to exit");
Console.ReadLine();
}
}
catch (Exception ex)
{
Console.WriteLine(GenerateFullFlatMessage(ex));
//// NLog: catch any exception and log it.
programStaticLoggerThatNeedsToBeInitiatedInMainMethod.Error(ex, "programStaticLoggerThatNeedsToBeInitiatedInMainMethod : Stopped program because of exception");
throw;
}
finally
{
// Ensure to flush and stop internal timers/threads before application-exit (Avoid segmentation fault on Linux)
LogManager.Shutdown();
}
Console.WriteLine("Returning 0 and exiting.");
return 0;
}
private static IServiceProvider BuildDi(IConfiguration config, bool useCodeButNotAutofacJson)
{
NLog.Extensions.Logging.NLogProviderOptions nlpopts = new NLog.Extensions.Logging.NLogProviderOptions
{
IgnoreEmptyEventId = true,
CaptureMessageTemplates = true,
CaptureMessageProperties = true,
ParseMessageTemplates = true,
IncludeScopes = true,
ShutdownOnDispose = true
};
IServiceCollection sc = new ServiceCollection()
////.AddLogging(loggingBuilder =>
////{
//// // configure Logging with NLog
//// loggingBuilder.ClearProviders();
//// loggingBuilder.SetMinimumLevel(Microsoft.Extensions.Logging.LogLevel.Trace);
//// loggingBuilder.AddNLog(config);
////})
.AddLogging(loggingBuilder =>
{
////use nlog
loggingBuilder.AddNLog(nlpopts);
NLog.LogManager.LoadConfiguration("nlog.config");
})
.AddSingleton<IConfiguration>(config);
//// // /* before autofac */ return sc.BuildServiceProvider();
//// Create a container-builder and register dependencies
Autofac.ContainerBuilder builder = new Autofac.ContainerBuilder();
// Populate the service-descriptors added to `IServiceCollection`
// BEFORE you add things to Autofac so that the Autofac
// registrations can override stuff in the `IServiceCollection`
// as needed
builder.Populate(sc);
if (useCodeButNotAutofacJson)
{
programStaticLoggerThatNeedsToBeInitiatedInMainMethod.Info("Coding up Autofac DI");
/* "Keyed" is not working, do not use below */
////builder.RegisterType<FedExShipper>().Keyed<IShipper>(FedExShipper.FriendlyName);
////builder.RegisterType<UpsShipper>().Keyed<IShipper>(UpsShipper.FriendlyName);
////builder.RegisterType<UspsShipper>().Keyed<IShipper>(UspsShipper.FriendlyName);
builder.RegisterType<FedExShipper>().As<IShipper>();
builder.RegisterType<UpsShipper>().As<IShipper>();
builder.RegisterType<UspsShipper>().As<IShipper>();
builder.RegisterType<OrderProcessor>().As<IOrderProcessor>();
}
else
{
programStaticLoggerThatNeedsToBeInitiatedInMainMethod.Info("Using .json file to define Autofac DI");
// Register the ConfigurationModule with Autofac.
var module = new Autofac.Configuration.ConfigurationModule(config);
builder.RegisterModule(module);
}
Autofac.IContainer autofacContainer = builder.Build();
// this will be used as the service-provider for the application!
return new AutofacServiceProvider(autofacContainer);
}
private static string GenerateFullFlatMessage(Exception ex)
{
return GenerateFullFlatMessage(ex, false);
}
private static string GenerateFullFlatMessage(Exception ex, bool showStackTrace)
{
string returnValue;
StringBuilder sb = new StringBuilder();
Exception nestedEx = ex;
while (nestedEx != null)
{
if (!string.IsNullOrEmpty(nestedEx.Message))
{
sb.Append(nestedEx.Message + System.Environment.NewLine);
}
if (showStackTrace && !string.IsNullOrEmpty(nestedEx.StackTrace))
{
sb.Append(nestedEx.StackTrace + System.Environment.NewLine);
}
if (ex is AggregateException)
{
AggregateException ae = ex as AggregateException;
foreach (Exception flatEx in ae.Flatten().InnerExceptions)
{
if (!string.IsNullOrEmpty(flatEx.Message))
{
sb.Append(flatEx.Message + System.Environment.NewLine);
}
if (showStackTrace && !string.IsNullOrEmpty(flatEx.StackTrace))
{
sb.Append(flatEx.StackTrace + System.Environment.NewLine);
}
}
}
nestedEx = nestedEx.InnerException;
}
returnValue = sb.ToString();
return returnValue;
}
}
}
........
autofac.Empty.json (set to copy always)
{}
.......
autofac.json (set to copy always)
{
"defaultAssembly": "MyCompany.MyProject",
"components": [
{
"type": "MyCompany.MyProject.Shippers.FedExShipper",
"services": [
{
"type": "MyCompany.MyProject.Shippers.Interfaces.IShipper"
}
]
},
{
"type": "MyCompany.MyProject.Shippers.UpsShipper",
"services": [
{
"type": "MyCompany.MyProject.Shippers.Interfaces.IShipper"
}
]
},
{
"type": "MyCompany.MyProject.Shippers.UspsShipper",
"services": [
{
"type": "MyCompany.MyProject.Shippers.Interfaces.IShipper"
}
]
},
{
"type": "MyCompany.MyProject.Processors.OrderProcessor",
"services": [
{
"type": "MyCompany.MyProject.Processors.Interfaces.IOrderProcessor"
}
]
}
]
}
and csproj
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp3.1</TargetFramework>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Autofac" Version="5.1.2" />
<PackageReference Include="Autofac.Configuration" Version="5.1.0" />
<PackageReference Include="Autofac.Extensions.DependencyInjection" Version="6.0.0" />
<PackageReference Include="Microsoft.Extensions.Configuration.FileExtensions" Version="3.1.2" />
<PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="3.1.2" />
<PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.2" />
<PackageReference Include="Microsoft.Extensions.Http" Version="3.1.2" />
<PackageReference Include="Microsoft.Extensions.Logging" Version="3.1.2" />
<PackageReference Include="Microsoft.Extensions.Logging.Console" Version="3.1.2" />
<PackageReference Include="NLog.Extensions.Logging" Version="1.6.1" />
</ItemGroup>
From
https://autofaccn.readthedocs.io/en/latest/integration/netcore.html
PS
In the autofac version I had to change the Logger being injected to be a LoggerFactory.
Here is the OrderProcessor alternate version. You'll have do the same "Microsoft.Extensions.Logging.ILoggerFactory loggerFactory" alternative injection for all 3 concrete "Shipper"'s as well.
namespace MyCompany.MyProject.Processors
{
using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.Logging;
public class OrderProcessor : IOrderProcessor
{
////private readonly IDictionary<string, IShipper> shippers; /* :( I couldn't get IDictionary<string, IShipper> to work */
private readonly IEnumerable<IShipper> shippers;
private Microsoft.Extensions.Logging.ILogger<OrderProcessor> logger;
public OrderProcessor(Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, IEnumerable<IShipper> shprs)
{
if (null == loggerFactory)
{
throw new ArgumentOutOfRangeException("loggerFactory is null");
}
if (null == shprs)
{
throw new ArgumentOutOfRangeException("ShipperInterface(s) is null");
}
this.logger = loggerFactory.CreateLogger<OrderProcessor>();
this.shippers = shprs;
}
public void ProcessOrder(string preferredShipperAbbreviation, Order ord)
{
this.logger.LogInformation(string.Format("About to ship. ({0})", preferredShipperAbbreviation));
/* below foreach is not needed, just "proves" everything was injected */
int counter = 0;
foreach (IShipper sh in this.shippers)
{
this.logger.LogInformation(string.Format("IEnumerable:ShipperInterface. ({0} of {1}) -> ({2})", ++counter, this.shippers.Count(), sh.GetType().Name));
}
IShipper foundShipper = this.FindIShipper(preferredShipperAbbreviation);
foundShipper.ShipOrder(ord);
}
private IShipper FindIShipper(string preferredShipperAbbreviation)
{
IShipper foundShipper = this.shippers.FirstOrDefault(s => s.FriendlyNameInstance.Equals(preferredShipperAbbreviation, StringComparison.OrdinalIgnoreCase));
if (null == foundShipper)
{
throw new ArgumentNullException(
string.Format("ShipperInterface not found in shipperProviderMap. ('{0}')", preferredShipperAbbreviation));
}
return foundShipper;
}
}
}
not related to autofac
nlog.config (set to copy always)
<?xml version="1.0" encoding="utf-8" ?>
<!-- XSD manual extracted from package NLog.Schema: https://www.nuget.org/packages/NLog.Schema-->
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xsi:schemaLocation="NLog NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
autoReload="true"
internalLogFile="MyCompany.MyProject.Nlog.internalLogFile.log"
internalLogLevel="Info" >
<!-- the targets to write to -->
<targets>
<!-- write logs to file -->
<target xsi:type="File" name="target1" fileName="MyCompany.MyProject.Nlog.MyConsoleAppProgram.log"
layout="${date}|${level:uppercase=true}|${message} ${exception}|${logger}|${all-event-properties}" />
<target xsi:type="Console" name="target2"
layout="${date}|${level:uppercase=true}|${message} ${exception}|${logger}|${all-event-properties}" />
</targets>
<!-- rules to map from logger name to target -->
<rules>
<logger name="*" minlevel="Trace" writeTo="target1,target2" />
</rules>
</nlog>