在 .net 核心 3.0 BackgroundService 應用程序中，為什么我的配置 object 作為服務運行時為空，而不是作為控制台應用程序運行？

Question

我有一個 .net Core 3.0 BackgroundService 應用程序，它在控制台模式下運行時工作正常，但是一旦我將配置 object 部署為服務，應該從 appsettings.Z466DEEC76ECDF5FCA6D38571F6324D4 加載的配置為空。 是什么賦予了？

程序.cs

public class Program
{
    public static async System.Threading.Tasks.Task Main(string[] args)
    {
        var hostbuilder = new HostBuilder()
            .UseWindowsService()
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config
                .SetBasePath(Path.GetDirectoryName(System.Reflection.Assembly.GetEntryAssembly().Location))
                .AddJsonFile("appsettings.json");
            })
            .ConfigureLogging(
                options => options.AddFilter<EventLogLoggerProvider>(level => level >= LogLevel.Information))
            .ConfigureServices((hostContext, services) =>
            {
                services.AddHostedService<Importer>().Configure<EventLogSettings>(config =>
                {
                    config.LogName = "Application";
                    config.SourceName = "Importer";
                });
            });
#if (DEBUG)
        await hostbuilder.RunConsoleAsync();
#else
        await hostbuilder.RunAsServiceAsync();
#endif
    }
}

IhostBuilder運行服務的擴展方法

public static class ServiceBaseLifetimeHostExtensions
{
    public static IHostBuilder UseServiceBaseLifetime(this IHostBuilder hostBuilder)
    {
        return hostBuilder.ConfigureServices((hostContext, services) => services.AddSingleton<IHostLifetime, ServiceBaseLifetime>());
    }

    public static Task RunAsServiceAsync(this IHostBuilder hostBuilder, CancellationToken cancellationToken = default)
    {
        return hostBuilder.UseServiceBaseLifetime().Build().RunAsync(cancellationToken);
    }
}

ServiceBaseLifetime class 處理服務生命周期

public class ServiceBaseLifetime : ServiceBase, IHostLifetime
{
    private readonly TaskCompletionSource<object> _delayStart = new TaskCompletionSource<object>();

    public ServiceBaseLifetime(IHostApplicationLifetime applicationLifetime)
    {
        ApplicationLifetime = applicationLifetime ?? throw new ArgumentNullException(nameof(applicationLifetime));
    }

    private IHostApplicationLifetime ApplicationLifetime { get; }

    public Task WaitForStartAsync(CancellationToken cancellationToken)
    {
        cancellationToken.Register(() => _delayStart.TrySetCanceled());
        ApplicationLifetime.ApplicationStopping.Register(Stop);

        new Thread(Run).Start(); // Otherwise this would block and prevent IHost.StartAsync from finishing.
        return _delayStart.Task;
    }

    private void Run()
    {
        try
        {
            Run(this); // This blocks until the service is stopped.
            _delayStart.TrySetException(new InvalidOperationException("Stopped without starting"));
        }
        catch (Exception ex)
        {
            _delayStart.TrySetException(ex);
        }
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        Stop();
        return Task.CompletedTask;
    }

    // Called by base.Run when the service is ready to start.
    protected override void OnStart(string[] args)
    {
        _delayStart.TrySetResult(null);
        base.OnStart(args);
    }

    // Called by base.Stop. This may be called multiple times by service Stop, ApplicationStopping, and StopAsync.
    // That's OK because StopApplication uses a CancellationTokenSource and prevents any recursion.
    protected override void OnStop()
    {
        ApplicationLifetime.StopApplication();
        base.OnStop();
    }
}

服務的實際實現與構造函數無關，構造函數通過 DI 獲取記錄器和配置。

private readonly ILogger<Importer> _logger;
private readonly IConfiguration _configuration;

public Importer(IConfiguration configuration, ILogger<Importer> logger)
{
    _logger = logger;
    _configuration = configuration;
}

protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
    _logger.LogInformation($"Why is {_configuration["Key1"]} empty?");
}

appsettings.json

{
    "Key1":"some value"
}

當我運行調試時，控制台應用程序啟動並運行並記錄並從 appsettings 加載配置。 當我部署為服務時，配置 object 為空。

注意：正在讀取 appsettings 文件，我可以通過更改它的名稱來判斷這一點，它會拋出找不到文件的異常。 appsettings 文件也不為空。

Answer 1

這似乎是一個XY 問題，值得重構

創建一個強類型 model 來保存所需的設置

public class ImporterSettings {
    public string Key1 { get; set; }
}

重構托管服務以依賴於設置，因為在我看來，將服務與IConfiguration緊密耦合是一種代碼味道

private readonly ILogger<Importer> _logger;
private readonly ImporterSettnigs settings;

public Importer(ImporterSettnigs settings, ILogger<Importer> logger) {
    _logger = logger;
    this.settings = settings;
}

protected override async Task ExecuteAsync(CancellationToken stoppingToken) {
    _logger.LogInformation($"This is Key1: {settings.Key1}");
}

現在正確配置啟動以使用提供的配置

public class Program {
    public static async System.Threading.Tasks.Task Main(string[] args) {
        var hostbuilder = new HostBuilder()
            .UseWindowsService()
            .ConfigureAppConfiguration((hostingContext, config) => {
                var path = Path.GetDirectoryName(System.Reflection.Assembly.GetEntryAssembly().Location);
                config
                    .SetBasePath(path)
                    .AddJsonFile("appsettings.json");
            })
            .ConfigureLogging(
                options => options.AddFilter<EventLogLoggerProvider>(level => 
                    level >= LogLevel.Information)
            )
            .ConfigureServices((hostContext, services) => {
                //get settings from app configuration.
                ImporterSettings settings = hostContext.Configuration.Get<ImporterSettings>();

                services
                    .AddSingleton(settings) //add to service collection
                    .AddHostedService<Importer>()
                    .Configure<EventLogSettings>(config => {
                        config.LogName = "Application";
                        config.SourceName = "Importer";
                    });
            });
#if (DEBUG)
        await hostbuilder.RunConsoleAsync();
#else
        await hostbuilder..Build().RunAsync();
#endif
    }
}

Answer 2

我的問題似乎是某種異步競爭條件問題（我猜，不是肯定的）。 第一次通過ExecuteAsync勾選配置未加載，但第二次通過它。 如果它遇到那個異常，我的服務就會死掉，所以我再也沒有讓它第二次打勾。