[assembly: FunctionsStartup(typeof(Startup))]
namespace AzureFunctionsDiSample
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            var services = builder.Services;
            var provider = services.BuildServiceProvider();
            var configuration = provider.GetRequiredService<IConfiguration>();

            // 設定ファイルの内容をバインドする
            services.Configure<AppSettings>(configuration.GetSection("AppSettings"));

            // サービスのDI設定を行う
            services.AddTransient<IHelloService, HelloService>();
            services.AddTransient<IApplication, Application>();
        }
    }
}

public EntryPoint(IApplication application)
{
    _application = application;
}

[FunctionName("FunctionsDiSample")]
public IActionResult Run(
    [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
    ILogger log)
{
    log.LogInformation("Function Run");

    _application.Run();

    return new OkObjectResult("Hello Functions DI Sample");
}

private readonly AppSettings _appSettings;
private readonly IHelloService _service;

public Application(ILogger<IApplication> logger, IOptions<AppSettings> optionsAccessor, IHelloService service) : base(logger)
{
    _appSettings = optionsAccessor.Value;
    _service = service;
}