Dependency Injection On The Fly in ASP.net Core

Question

Is it possible to get a dependency on the fly by somehow getting a reference to IServiceProvider, or some class that can resolve dependencies? For example, when handling an exception with UseExceptionHandler to output something meaningful to the client, I would also like to do some custom logging to note some stuff about the exception that was thrown.

For example, lets say I have this code in the Configure method in the Startup class of my ASP.net Core project:

app.UseExceptionHandler(
  builder =>
    {
      builder.Run(
        async context =>
          {
            context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
            context.Response.ContentType = "text/html";

            var error = context.Features.Get<IExceptionHandlerFeature>();
            if (error != null)
            {
              // TODO Log Exception.  Would like to do something like this:
              // var logger = ServiceProvider.Resolve<ILogger>();
              // logger.LogCritical("Unhandled Error :"error.Error.ToString());
              await context.Response.WriteAsync($"<h1>Error: {error.Error.Message}</h1>").ConfigureAwait(false);
            }
          });
    });

How can I get an instance of ILogger when I don't have a constructor to pass in ILogger?

Answer 1

You can access the ServiceProvider in builder.ApplicationServices. From there you optain an instance of an ILoggerFactory and then you create the logger for your exception handler.

app.UseExceptionHandler(
            builder =>
            {
                builder.Run(
                    async context =>
                    {
                        ...
                        var lf = builder.ApplicationServices.GetService<ILoggerFactory>();
                        var logger = lf.CreateLogger("myExceptionHandlerLogger");
                        logger.LogDebug("I am a debug message");
                        ...
                    });
            });