c# - 如何访问用AddSingleton<T>注入(inject)的后台服务的成员

Question

在asp.netcore web api项目中，有一个名为TimedHostedService的后台服务是通过AddSingleton<>注入(inject)的，当 Controller 访问该服务的成员[ExecutionCount]时，该值总是等于0,当成员[ExecutionCount]设置为static时，值发生变化，不知道为什么？属性 [ExecutionCount] 必须是静态的吗？

TimedHostedService:

public class TimedHostedService : IHostedService, IDisposable
{
    private int executionCount = 0;
    private readonly ILogger<TimedHostedService> _logger;
    private Timer _timer;

    public int ExecutionCount
    {
        get { return executionCount; }
        set { executionCount = value; }
    }

    public TimedHostedService(ILogger<TimedHostedService> logger)
    {
        _logger = logger;
    }

    public Task StartAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Timed Hosted Service running.");

        _timer = new Timer(DoWork, null, TimeSpan.Zero,
            TimeSpan.FromSeconds(1));

        return Task.CompletedTask;
    }

    private void DoWork(object state)
    {
        var count = Interlocked.Increment(ref executionCount);

        _logger.LogInformation(
            "Timed Hosted Service is working. Count: {Count}", count);
    }

    public Task StopAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Timed Hosted Service is stopping.");

        _timer?.Change(Timeout.Infinite, 0);

        return Task.CompletedTask;
    }

    public void Dispose()
    {
        _timer?.Dispose();
    }
}

通过AddSingleton<>注入(inject):

 public void ConfigureServices(IServiceCollection services)
    {
        services.AddSingleton<TimedHostedService>();
        services.AddControllers();
    }

 public static IHostBuilder CreateHostBuilder(string[] args)
    {
        var hostBuilder = Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            })
            .ConfigureServices(services =>
            {
                services.AddHostedService<TimedHostedService>();
            })
            .UseContentRoot(Directory.GetCurrentDirectory())

        return hostBuilder;
    }

从 WeatherForecastController 访问属性 [ExecutionCount]:

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    private static readonly string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    private readonly ILogger<WeatherForecastController> _logger;

    private readonly TimedHostedService _timedHostedService;

    public WeatherForecastController(ILogger<WeatherForecastController> logger,TimedHostedService timedHostedService)
    {
        _logger = logger;
        _timedHostedService = timedHostedService;
    }

    [HttpGet]
    public IEnumerable<WeatherForecast> Get()
    {
        int timeValue = _timedHostedService.ExecutionCount;
        _logger.LogInformation("ExecutionCount:{0}", timeValue);

        var rng = new Random();
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = rng.Next(-20, 55),
            Summary = Summaries[rng.Next(Summaries.Length)]
        })
        .ToArray();
    }
}

Answer 1

您将服务的两个实例添加到服务集合中。线路

services.AddSingleton<TimedHostedService>();

添加您的 Controller 通过依赖注入(inject)和行获取的实例

services.AddHostedService<TimedHostedService>();

添加作为托管服务运行的实例。

尝试以下操作:

services.AddHostedService((sp) => sp.GetRequiredService<TimedHostedService>());
services.AddSingleton<TimedHostedService>();

保持这个顺序很重要。这会将托管服务添加到服务集合(作为单例)，并为实际托管服务使用相同的实例。