.Net Core Worker Service appsettings.json 未被读取

Question

我创建了一个 .net core 辅助服务，并将 IConfiguration 注入其中，以便我可以从 appsettings.json 读取属性，但每次运行该文件时，该属性都会显示为 null。但是，如果我强制 IConfiguration 通过 ConfigurationBuilder 添加 json 文件，它就可以工作。如何使用默认设置来允许 .net core 辅助服务从 appsettings 读取？

public class ImageFileWatcher : IHostedService, IDisposable
{

    private readonly ILogger _logger;
    private readonly IConfiguration _configuration;
    FileSystemWatcher _watcher;

    public ImageFileWatcher(ILogger<ImageFileWatcher> logger, IConfiguration configuration)
    {
        _logger = logger;
        _configuration = configuration;
        // IConfigurationBuilder builder = new ConfigurationBuilder()
        // .AddJsonFile("appsettings.json")
        // .AddEnvironmentVariables();
        // this._configuration = builder.Build();
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {

        var watcherConfig = _configuration["WatcherConfig:filePath"];//this is where it comes up as null.
        _logger.LogInformation($"Image File watcher started on path{watcherConfig}");
        _watcher = new FileSystemWatcher(watcherConfig);
        _watcher.Created += OnNewImage;
        _watcher.EnableRaisingEvents = true;
        return Task.CompletedTask;
    }

这是 appsettings.json 文件

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "WatcherConfig":{

    "filePath":"/Users/###/Documents/Development/ImageDrop"
  }
  
}

这是 Program.cs 文件

public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration((hostContext, configBuilder) =>
                {


                    configBuilder
                    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                    .AddJsonFile("appsettings.Production.json", optional: true, reloadOnChange: true)
                    .AddEnvironmentVariables(prefix: "ImageService_")
                    .Build();

                })
                .ConfigureServices((hostContext, services) =>
                {

                    services.AddHostedService<ImageFileWatcher>();

                });
    }

这是 Program 类的第二次尝试：

public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)

                .ConfigureServices((hostContext, services) =>
                {
                    IConfiguration builder = new ConfigurationBuilder()
                     .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                    .AddJsonFile("appsettings.Production.json", optional: true, reloadOnChange: true)
                    .AddEnvironmentVariables(prefix: "ImageService_")
                    .Build();

                    hostContext.Configuration = builder;
                    services.AddHostedService<ImageFileWatcher>();

                });
    }

Answer 1

构建Host服务时，必须读取Program.cs中的App设置。查看 Host.CreateDefaultBuilder 的作用，如果您决定不使用默认构建器，则可能需要隐式加载设置。

如果您还有其他问题，请附上 Program.cs 文件，因为 IConfiguration 取决于其初始化而不是服务中的使用情况。