从 ASP.NET Core WebAPI 中的 appsettings.json 获取一些值

Question

I am creating a scheduled Task in ASP.NET Core WebAPI. I am trying to get 5 URLs from appsettings.json which are not in a section, just in the root of appsettings.json. If I hardcode the strings in the Final class (mentioned in the last of this question), it works fine, but

This is my Program.cs:

    public static void Main(string[] args)
    {
        var configuration = GetConfiguration();
        var host = BuildWebHost(configuration, args);
        host.Run();
        //CreateWebHostBuilder(configuration,args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(IConfiguration _configuration,string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .UseConfiguration(_configuration);

    private static IConfiguration GetConfiguration()
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
            .AddEnvironmentVariables();

        return builder.Build();
    }

Following is my Startup.cs:

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.Configure<ScheduledTaskSettings>(Configuration);
        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        services.AddSingleton<IOrderSchedulerTask, OrderSchedulerTask>();
        services.AddScheduler((sender, args) =>
        {
            Console.Write(args.Exception.Message);
            args.SetObserved();
        });
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
            app.UseHsts();
        }

        app.UseHttpsRedirection();
        app.UseMvc();
    }

I have also created ScheduledTaskSettings.cs with the following class. The string names are identical to the names in appsettings.json.

public class ScheduledTaskSettings
{
    public string ConnectionString { get; set; }

    public string CustomersApiEndpointUrl { get; set; }

    public string SubscriptionsApiEndpointUrl { get; set; }

    public string ProductsApiEndpointUrl { get; set; }

    public string SubscribersApiEndpointUrl { get; set; }

    public string CronScheduleTime { get; set; }
}

I have an interface IOrderSchedulerTask.cs:

public interface IOrderSchedulerTask
{
    string Schedule { get; }
    Task ExecuteAsync(CancellationToken cancellationToken);
}

And the final class OrderSchedulerTask.cs

public class OrderSchedulerTask : IOrderSchedulerTask
{

    // Implementing IOrderScheduler 
    // Scheduled for every one minute
    public string Schedule => "*/1 * * * *";

    public async Task ExecuteAsync(CancellationToken cancellationToken)
    {
        var httpClient = new HttpClient();
        OrderScheduler.GenerateOrders();
    }
}

public class OrderScheduler
{
    public static ScheduledTaskSettings _scheduledTaskSettngs;
    public OrderScheduler(ScheduledTaskSettings scheduledTaskSettings)
    {
        _scheduledTaskSettngs = scheduledTaskSettings;
    }

private static string CustomerApiUrl = _scheduledTaskSettngs.CustomersApiEndpointUrl;
    private static string SubscriptionApiUrl = _scheduledTaskSettngs.SubscriptionsApiEndpointUrl;
    private static string CatalogApiUrl = _scheduledTaskSettngs.ProductsApiEndpointUrl;
    private static string SubscriberUrl = _scheduledTaskSettngs.SubscribersApiEndpointUrl;
    private static string _connectionString = _scheduledTaskSettngs.ConnectionString;

    private static List<Customer> customers = null;
    private static List<Subscription> subscriptions = null;
    private static List<CatalogItem> products = null;
    private static List<Subscriber> subscribers = null;
    private static List<Order> orders = new List<Order>();

    /// <summary>
    /// Generate the orders for next day
    /// </summary>
    public static async void GenerateOrders()
    {
        // Get Customers Data
        GetCustomers();

        //Get Subscription Data
        GetSubscriptions();

        //Get Products Data
        GetProducts();

        //Get Subscribers Data
        GetSubscribers();

        var order = new Order();
        using (var connection = new SqlConnection(_connectionString))
        {
            connection.Open();
            foreach (var item in subscriptions)
            {
                //
            }
        }

    }
    private static async void GetCustomers()
    {
        //Uses CustomerApiUrl String
    }
    private static async void GetSubscriptions()
    {
         //Uses SubscriptionApiUrl String
    }
    private static async void GetProducts()
    {
        //Uses CatalogApiUrl String
    }

    private static async void GetSubscribers()
    {
    //Uses SubscriberUrl String
    }

}

If I hardcode the strings, it works fine, but it throws the following exception when I use the configuration method:

Unhandled Exception:
Unhandled Exception: 
Unhandled Exception:
Unhandled Exception: System.TypeInitializationException: The type initializer for 
'Ordering.ScheduledTask.Tasks.OrderScheduler' threw an exception. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Ordering.ScheduledTask.Tasks.OrderScheduler..cctor() in ... OrderSchedulerTask.cs  Line 22

Can someone please guide me through about what should I be doing to correct it.

Answer 1

There are a few issues I see with your code:

1. Why are you creating a custom ConfigurationBuilder in your Program.cs ? The default builder that you create with WebHost.CreateDefaultBuilder will already set up a configuration that uses both appsettings.json and appsettings.<Environment>.json , as well as other sources like the environment variables or command line arguments.

   So there is no need to create a custom configuration and I would advise you against creating one unless you actually have a good reason to do so.

2. services.Configure<ScheduledTaskSettings>(Configuration) is a method of the Options framework . You configure an IOptions<ScheduledTaskSettings> that way. So in order to consume that, you will have to inject IOptions<ScheduledTaskSettings> into your OrderScheduler ; not just ScheduledTaskSettings .

3. In OrderScheduler , you inject the task settings (again, this should be options), but you use that to statically set _scheduledTaskSettngs which would be shared between all instances. That is generally a bad idea in applications that make use of DI: You explicitly want this to stay an instance member so that it belongs to the lifetime of the object.

   What's ultimately causing your errors is the following part:

    private static string CustomerApiUrl = _scheduledTaskSettngs.CustomersApiEndpointUrl; private static string SubscriptionApiUrl = _scheduledTaskSettngs.SubscriptionsApiEndpointUrl; private static string CatalogApiUrl = _scheduledTaskSettngs.ProductsApiEndpointUrl; private static string SubscriberUrl = _scheduledTaskSettngs.SubscribersApiEndpointUrl; private static string _connectionString = _scheduledTaskSettngs.ConnectionString;

   Since these are all static fields, they will be initialized statically when the type is first used. At that point, no instance will have been created yet, so the static _scheduledTaskSettngs will not be initialized yet. So it's null , explaining the NullReferenceException s you get.

   This approach also prevents you from reacting to changes of that configuration since the values are only read once and then stored statically.

You should make your OrderScheduler a singleton and properly inject it into other things, so that you can use it as an instance that has a well-defined lifetime:

public class OrderScheduler
{
    public readonly ScheduledTaskSettings _scheduledTaskSettings;
    public OrderScheduler(IOptions<ScheduledTaskSettings> scheduledTaskSettings)
    {
        _scheduledTaskSettings = scheduledTaskSettings.Value;
    }

    // using read-only properties instead
    private string CustomerApiUrl => _scheduledTaskSettngs.CustomersApiEndpointUrl;
    private string SubscriptionApiUrl => _scheduledTaskSettngs.SubscriptionsApiEndpointUrl;
    private string CatalogApiUrl => _scheduledTaskSettngs.ProductsApiEndpointUrl;
    private string SubscriberUrl => _scheduledTaskSettngs.SubscribersApiEndpointUrl;
    private string _connectionString => _scheduledTaskSettngs.ConnectionString;

    // …
    // instance members only, no “static”
}