I'm developing a middleware which I would like to have an optional dependency on a internal logging library. In another words, if MyLoggingService is registered, great!, else, life goes on and ill log to console.
But by declaring public async Task Invoke(HttpContext httpContext, MyLoggingService logger), I get a runtime error saying that it was not registred. I tried setting a default value to null but that didn't work. Also, because its a middleware, I can't overload the Invoke method.
Is there a solution other than requesting the service collection and resolving the dependency myself?
Instead of making dependencies optional, consider:
IMyLoggingServiceFor instance:
public class CustomMiddleware1 : IMiddleware
{
private readonly IMyLoggingService logger;
public CustomMiddleware1(IMyLoggingService logger) => this.logger = logger;
public async Task InvokeAsync(HttpContext context, RequestDelegate next)
{
this.logger.Log("Before");
await next(context);
this.logger.Log("After");
}
}
Null Object implementation:
public sealed class NullMyLoggingService : IMyLoggingService
{
public void Log(LogEntry e) { }
}
Registrations:
services.AddSingleton<IMyLoggingService>(new NullMyLoggingService());
app.Use<CustomMiddleware1>();
The call to AddSingleton<IMyLoggingService>(new NullMyLoggingService()) ensures a registration for IMyLoggingService always exists. This prevents complexity in consumers, who would otherwise have to add conditional logic for the case that the logger isn't present.
This null implementation can be replaced by simply adding a second IMyLoggingService after the first:
services.AddScoped<IMyLoggingService, DbMyLoggingService>();
app.Use<CustomMiddleware1>();
The answer is incredibly simple:
public async Task Invoke(HttpContext httpContext, MyLoggingService logger = null)