如何在Blazor后台服务中注入DbContextFactory？

我正在开发第一个Blazor Server应用，这也是我的第一个Entity Framework Core项目。需求是搭建一个后台服务，每天凌晨检查数据库中昨日新增的特定类型记录，若存在则提取、格式化后发送邮件给相关人员。

手动通过页面触发报告时，EF查询、内容格式化及邮件发送的代码都能正常运行，但现在卡在了**如何给后台服务提供DbContextFactory**来执行EF相关操作。

之前在Razor页面里都是用@inject IDbContextFactory<miniDARTSContext> DbFactory通过依赖注入获取实例，但后台服务要在Program.cs里配置，没法用Razor的注入方式。

我按照微软教程配置了名为PhaseChangeReportService的后台服务，原本它能每10秒向控制台输出运行计数，但我对其中的间接调用逻辑不太理解。

尝试修改PhaseChangeReportService的构造函数，添加IDbContextFactory<miniDARTSContext>参数后，构造函数的断点根本没触发，服务没有任何输出，也没抛出异常。

相关代码片段 #

原始PhaseChangeReportService代码 #

namespace miniDARTS.ScopedService
{
    public sealed class PhaseChangeReportService : IScopedProcessingService
    {
        private int _executionCount;
        private readonly ILogger<PhaseChangeReportService> _logger;
        
        public PhaseChangeReportService(ILogger<PhaseChangeReportService> logger)
        {
            _logger = logger;                        
        }            

        public async Task DoWorkAsync(CancellationToken stoppingToken)
        {
            while (!stoppingToken.IsCancellationRequested)
            {
                ++_executionCount;

                _logger.LogInformation("{ServiceName} working, execution count: {Count}", nameof(PhaseChangeReportService), _executionCount);

                await Task.Delay(10_000, stoppingToken);
            }
        }
    }    
}

修改后的构造函数 #

private readonly ILogger<PhaseChangeReportService> _logger;
private readonly IDbContextFactory<miniDARTSContext> _dbContextFactory;

public PhaseChangeReportService(ILogger<PhaseChangeReportService> logger, IDbContextFactory<miniDARTSContext> dbContextFactory)
{
    _logger = logger;                        
    _dbContextFactory = dbContextFactory;
}   

Program.cs相关代码 #

// Configure the database connection.
string connectionString = builder.Configuration.GetConnectionString("miniDARTSContext");
var serverVersion = new MySqlServerVersion(new Version(8, 0, 28));
builder.Services.AddDbContextFactory<miniDARTSContext>(options => options.UseMySql(connectionString, serverVersion), ServiceLifetime.Scoped);

IHost host = Host.CreateDefaultBuilder(args)
    .ConfigureServices(services =>
    {
        services.AddHostedService<ScopedBackgroundService>();
        services.AddScoped<IScopedProcessingService, PhaseChangeReportService>();        
    })
    .Build();

host.RunAsync();

IScopedProcessingService.cs #

namespace miniDARTS.ScopedService
{
    public interface IScopedProcessingService
    {
        Task DoWorkAsync(CancellationToken stoppingToken);
    }
}

ScopedBackgroundService.cs #

namespace miniDARTS.ScopedService;

public sealed class ScopedBackgroundService : BackgroundService
{
    private readonly IServiceProvider _serviceProvider;
    private readonly ILogger<ScopedBackgroundService> _logger;
    
    public ScopedBackgroundService(IServiceProvider serviceProvider, ILogger<ScopedBackgroundService> logger)
    {
        _serviceProvider = serviceProvider;
        _logger = logger;        
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation($"{nameof(ScopedBackgroundService)} is running.");

        await DoWorkAsync(stoppingToken);
    }

    private async Task DoWorkAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation($"{nameof(ScopedBackgroundService)} is working.");

        using (IServiceScope scope = _serviceProvider.CreateScope())
        {
            IScopedProcessingService scopedProcessingService = scope.ServiceProvider.GetRequiredService<IScopedProcessingService>();

            await scopedProcessingService.DoWorkAsync(stoppingToken);
        }
    }

    public override async Task StopAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation($"{nameof(ScopedBackgroundService)} is stopping.");

        await base.StopAsync(stoppingToken);
    }
}

我的疑问 #

我肯定在服务/依赖注入的基础概念上有误解，但搜了一圈没找到可行方案，希望得到帮助。

内容的提问来源于stack exchange，提问作者Neil Wehneman