Accessing ASP.NET Core DI Container From Static Factory Class
I know my answer is late, but I wanted to share how I did it.
First of all: It's Antipattern to use ServiceLocator so try not to use it as you can. In my case I needed it to call MediatR inside of my DomainModel to implement the DomainEvents logic.
However, I had to find a way to call a static class in my DomainModel to get an instance of some registered service from DI.
So I've decided to use the HttpContext to access the IServiceProvider but I needed to access it from a static method without mention it in my domain model.
Let's do it:
1- I've created an interface to wrap the IServiceProvider
public interface IServiceProviderProxy
{
T GetService<T>();
IEnumerable<T> GetServices<T>();
object GetService(Type type);
IEnumerable<object> GetServices(Type type);
}
2- Then I've created a static class to be my ServiceLocator access point
public static class ServiceLocator
{
private static IServiceProviderProxy diProxy;
public static IServiceProviderProxy ServiceProvider => diProxy ?? throw new Exception("You should Initialize the ServiceProvider before using it.");
public static void Initialize(IServiceProviderProxy proxy)
{
diProxy = proxy;
}
}
3- I've created an implementation for the IServiceProviderProxy which use internally the IHttpContextAccessor
public class HttpContextServiceProviderProxy : IServiceProviderProxy
{
private readonly IHttpContextAccessor contextAccessor;
public HttpContextServiceProviderProxy(IHttpContextAccessor contextAccessor)
{
this.contextAccessor = contextAccessor;
}
public T GetService<T>()
{
return contextAccessor.HttpContext.RequestServices.GetService<T>();
}
public IEnumerable<T> GetServices<T>()
{
return contextAccessor.HttpContext.RequestServices.GetServices<T>();
}
public object GetService(Type type)
{
return contextAccessor.HttpContext.RequestServices.GetService(type);
}
public IEnumerable<object> GetServices(Type type)
{
return contextAccessor.HttpContext.RequestServices.GetServices(type);
}
}
4- I should register the IServiceProviderProxy in the DI like this
public void ConfigureServices(IServiceCollection services)
{
services.AddHttpContextAccessor();
services.AddSingleton<IServiceProviderProxy, HttpContextServiceProviderProxy>();
.......
}
5- Final step is to initialize the ServiceLocator with an instance of IServiceProviderProxy at the Application startup
public void Configure(IApplicationBuilder app, IHostingEnvironment env,IServiceProvider sp)
{
ServiceLocator.Initialize(sp.GetService<IServiceProviderProxy>());
}
As a result now you can call the ServiceLocator in your DomainModel classes "Or and needed place" and resolve the dependencies that you need.
public class FakeModel
{
public FakeModel(Guid id, string value)
{
Id = id;
Value = value;
}
public Guid Id { get; }
public string Value { get; private set; }
public async Task UpdateAsync(string value)
{
Value = value;
var mediator = ServiceLocator.ServiceProvider.GetService<IMediator>();
await mediator.Send(new FakeModelUpdated(this));
}
}
Here is my opinion about your case:
If possible i would send resolved service as a parameter
public static IMessageProcessor Create(string messageType, IIpSetService ipService)
{
//
}
Otherwise service lifetime would be important.
If service is singleton i would just set dependency on configure method:
// configure method
public IApplicationBuilder Configure(IApplicationBuilder app)
{
var ipService = app.ApplicationServices.GetService<IIpSetService>();
MessageHandlerFactory.IIpSetService = ipService;
}
// static class
public static IIpSetService IpSetService;
public static IMessageProcessor Create(string messageType)
{
// use IpSetService
}
If service lifetime is scoped i would use HttpContextAccessor:
//Startup.cs
public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
}
public IApplicationBuilder Configure(IApplicationBuilder app)
{
var httpContextAccessor= app.ApplicationServices.GetService<IHttpContextAccessor>();
MessageHandlerFactory.HttpContextAccessor = httpContextAccessor;
}
// static class
public static IHttpContextAccessor HttpContextAccessor;
public static IMessageProcessor Create(string messageType)
{
var ipSetService = HttpContextAccessor.HttpContext.RequestServices.GetService<IIpSetService>();
// use it
}
Even though using Dependency Injection is a better solution, but in some cases you have to use static methods (like in Extension Methods).
For those cases you can add a static property to your static class and initialize it in your ConfigureServices method.
For example:
public static class EnumExtentions
{
static public IStringLocalizerFactory StringLocalizerFactory { set; get; }
public static string GetDisplayName(this Enum e)
{
var resourceManager = StringLocalizerFactory.Create(e.GetType());
var key = e.ToString();
var resourceDisplayName = resourceManager.GetString(key);
return resourceDisplayName;
}
}
and in your ConfigureServices:
EnumExtentions.StringLocalizerFactory = services.BuildServiceProvider().GetService<IStringLocalizerFactory>();
You can avoid the static classes and use Dependency Injection all the way through combined with:
- The use of
IApplicationLifetimeto start/stop the listener whenever the application starts/stops. - The use of
IServiceProviderto create instances of the message processors.
First thing, let's move the configuration to its own class that can be populated from the appsettings.json:
public class RabbitOptions
{
public string HostName { get; set; }
public string UserName { get; set; }
public string Password { get; set; }
public int Port { get; set; }
}
// In appsettings.json:
{
"Rabbit": {
"hostName": "192.168.99.100",
"username": "guest",
"password": "guest",
"port": 5672
}
}
Next, convert MessageHandlerFactory into a non-static class that receives an IServiceProvider as a dependency. It will use the service provider to resolve the message processor instances:
public class MessageHandlerFactory
{
private readonly IServiceProvider services;
public MessageHandlerFactory(IServiceProvider services)
{
this.services = services;
}
public IMessageProcessor Create(string messageType)
{
switch (messageType.ToLower())
{
case "ipset":
return services.GetService<IpSetMessageProcessor>();
case "endpoint":
return services.GetService<EndpointMessageProcessor>();
default:
throw new Exception("Unknown message type");
}
}
}
This way your message processor classes can receive in the constructor any dependencies they need (as long as you configure them in Startup.ConfigureServices). For example, I am injecting an ILogger into one of my sample processors:
public class IpSetMessageProcessor : IMessageProcessor
{
private ILogger<IpSetMessageProcessor> logger;
public IpSetMessageProcessor(ILogger<IpSetMessageProcessor> logger)
{
this.logger = logger;
}
public void Process(string message)
{
logger.LogInformation("Received message: {0}", message);
}
}
Now convert MessageListener into a non-static class that depends on IOptions<RabbitOptions> and MessageHandlerFactory.It's very similar to your original one, I just replaced the parameters of the Start methods with the options dependency and the handler factory is now a dependency instead of a static class:
public class MessageListener
{
private readonly RabbitOptions opts;
private readonly MessageHandlerFactory handlerFactory;
private IConnection _connection;
private IModel _channel;
public MessageListener(IOptions<RabbitOptions> opts, MessageHandlerFactory handlerFactory)
{
this.opts = opts.Value;
this.handlerFactory = handlerFactory;
}
public void Start()
{
var factory = new ConnectionFactory
{
HostName = opts.HostName,
Port = opts.Port,
UserName = opts.UserName,
Password = opts.Password,
VirtualHost = "/",
AutomaticRecoveryEnabled = true,
NetworkRecoveryInterval = TimeSpan.FromSeconds(15)
};
_connection = factory.CreateConnection();
_channel = _connection.CreateModel();
_channel.ExchangeDeclare(exchange: "myExchange", type: "direct", durable: true);
var queueName = "myQueue";
QueueDeclareOk ok = _channel.QueueDeclare(queueName, true, false, false, null);
_channel.QueueBind(queue: queueName, exchange: "myExchange", routingKey: "myRoutingKey");
var consumer = new EventingBasicConsumer(_channel);
consumer.Received += ConsumerOnReceived;
_channel.BasicConsume(queue: queueName, noAck: false, consumer: consumer);
}
public void Stop()
{
_channel.Close(200, "Goodbye");
_connection.Close();
}
private void ConsumerOnReceived(object sender, BasicDeliverEventArgs ea)
{
// get the details from the event
var body = ea.Body;
var message = Encoding.UTF8.GetString(body);
var messageType = "endpoint"; // hardcoding the message type while we dev...
//var messageType = Encoding.UTF8.GetString(ea.BasicProperties.Headers["message-type"] as byte[]);
// instantiate the appropriate handler based on the message type
IMessageProcessor processor = handlerFactory.Create(messageType);
processor.Process(message);
// Ack the event on the queue
IBasicConsumer consumer = (IBasicConsumer)sender;
consumer.Model.BasicAck(ea.DeliveryTag, false);
}
}
Almost there, you will need to update the Startup.ConfigureServices method so it knows about your services and options (You can create interfaces for the listener and handler factory if you want):
public void ConfigureServices(IServiceCollection services)
{
// ...
// Add RabbitMQ services
services.Configure<RabbitOptions>(Configuration.GetSection("rabbit"));
services.AddTransient<MessageListener>();
services.AddTransient<MessageHandlerFactory>();
services.AddTransient<IpSetMessageProcessor>();
services.AddTransient<EndpointMessageProcessor>();
}
Finally, update the Startup.Configure method to take an extra IApplicationLifetime parameter and start/stop the message listener in the ApplicationStarted/ApplicationStopped events (Although I noticed a while ago some issues with the ApplicationStopping event using IISExpress, as in this question):
public MessageListener MessageListener { get; private set; }
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, IApplicationLifetime appLifetime)
{
appLifetime.ApplicationStarted.Register(() =>
{
MessageListener = app.ApplicationServices.GetService<MessageListener>();
MessageListener.Start();
});
appLifetime.ApplicationStopping.Register(() =>
{
MessageListener.Stop();
});
// ...
}