[英].Using a Factory Pattern with NET Core 3.1 Dependency Injection
我有一个 .NET Core 3.1 API 可能正在使用 RabbitMq 或 Azure 服务总线。 选择将通过配置参数确定。 由于要使用的配置是运行时决定的,我希望将工厂模式与 .NET Core 的依赖注入一起使用。 我在https://medium.com/@mailbox.viksharma/factory-pattern-using-built-in-dependency-injection-of-asp.net-core-f91bd3b58665找到了一篇文章,但无法让工厂工作。 任何帮助将不胜感激。
由于 IServiceProvider,问题发生在工厂 class 内。 我在尝试 GetService 时收到错误System.NullReferenceException: Object 引用未设置为 object 的实例。
工厂 class
public class MessageServiceFactory
{
readonly IServiceProvider serviceProvider;
public MessageServiceFactory(IServiceProvider serviceProvider)
{
this.serviceProvider = serviceProvider;
}
public IMessagingService GetMessagingService()
{
var messageProvider = ConfigProvider.GetConfig("MessageService", "Messaging_Service");
switch(messageProvider)
{
case "AzureServiceBus": return (IMessagingService)serviceProvider.GetService(typeof(MassTransitAzureServiceBusMessagingService));
case "RabbitMq": return (IMessagingService)serviceProvider.GetService(typeof(MassTransitRabbitMqMessagingService));
default: throw new ArgumentException("Invalid message service");
};
}
}
服务接口
public interface IMessagingService
{
Task Publish(object payload);
}
RabbitMq 具体实现
public class MassTransitRabbitMqMessagingService : IMessagingService
{
readonly IMassTransitRabbitMqTransport massTransitTransport;
public MassTransitRabbitMqMessagingService(IMassTransitRabbitMqTransport massTransitTransport)
{
//transport bus config already happens in massTransitTransport constructor
this.massTransitTransport = massTransitTransport;
}
public async Task Publish(object payload)
{
....
}
}
在 Startup.cs 中配置服务
public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton(Configuration);
services.AddScoped<IMassTransitRabbitMqTransport, MassTransitRabbitMqTransport>();
services.AddScoped<IMassTransitAzureServiceBusTransport, MassTransitAzureServiceBusTransport>();
services.AddScoped<MessageServiceFactory>();
services.AddScoped<IMessagingService, MassTransitAzureServiceBusMessagingService>(s => s.GetService<MassTransitAzureServiceBusMessagingService>());
services.AddScoped<IMessagingService, MassTransitRabbitMqMessagingService>(s => s.GetService<MassTransitRabbitMqMessagingService>());
services.AddControllers();
}
Controller
[ApiController]
[Route("api/[controller]")]
public class ListenerController : ControllerBase
{
readonly ILogger<ListenerController> logger;
readonly MessageServiceFactory messageFactory;
public ListenerController(
ILogger<ListenerController> logger,
MessageServiceFactory messageFactory)
{
this.logger = logger;
this.messageFactory = messageFactory;
}
[HttpPost]
public async Task<IActionResult> Post()
{
var payload = new
{
...
};
await messageFactory.GetMessagingService().Publish(payload);
return Ok(
new GDMSResponse()
{
ProcessedDate = DateTime.Now,
SuccessFlag = true
}
);
}
}
这需要在更改配置后重新启动,但我认为这样做没有问题。
public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton(Configuration);
services.AddScoped<IMassTransitRabbitMqTransport, MassTransitRabbitMqTransport>();
services.AddScoped<IMassTransitAzureServiceBusTransport, MassTransitAzureServiceBusTransport>();
var messageProvider = Configuration.GetConfig("MessageService", "Messaging_Service");
switch(messageProvider)
{
case "AzureServiceBus":
services.AddScoped<IMessagingService, MassTransitAzureServiceBusMessagingService>();
break;
case "RabbitMq":
services.AddScoped<IMessagingService, MassTransitRabbitMqMessagingService>();
break;
default:
throw new ArgumentException("Invalid message service");
};
services.AddControllers();
}
其他说明
我注意到您同时提供了具体类型和工厂:
services.AddScoped<IMessagingService, MassTransitAzureServiceBusMessagingService>(s => s.GetService<MassTransitAzureServiceBusMessagingService>());
我认为应该是:
services.AddScoped<IMessagingService>(s => s.GetService<MassTransitAzureServiceBusMessagingService>());
不确定它是否有所作为。
2021 年 1 月更新最近我不得不自己做这件事并想出了这个解决方案:
public static IServiceCollection ConfigureEventBus(this IServiceCollection services, IConfiguration configuration)
{
var queueSettings = new QueueSettings();
configuration.GetSection("QueueSettings").Bind(queueSettings);
if (configuration.GetValue<bool>("AzureServiceBusEnabled"))
{
services.AddMassTransit(x =>
{
x.UsingAzureServiceBus((context, cfg) =>
{
cfg.Host(queueSettings.HostName);
});
});
}
else
{
services.AddMassTransit(x =>
{
x.UsingRabbitMq((context, cfg) =>
{
cfg.ConfigureEndpoints(context);
cfg.Host(queueSettings.HostName, queueSettings.VirtualHost, h =>
{
h.Username(queueSettings.UserName);
h.Password(queueSettings.Password);
});
});
});
}
services.AddMassTransitHostedService();
services.AddSingleton<IEventBus, MassTransitEventBus>();
return services;
}
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