ASP.NET核心AddHostedService和AddSingleton关系

Question

我同时使用 AddSingleton 和 AddHostedService 在后端有一个很长的运行服务 (BackgroundService)，而控制器访问相同的服务来获取数据。以下是我的发现：

如果同时使用AddSingleton 和AddHostedService，BackgroundService 将被初始化两次（不是Singleton）。控制器只能访问在 AddSingleton 中创建的控制器。

调试输出（构造函数调用两次，看秒数的区别）

BGService constructor service addGame:games count is 1 BGService constructor service addGame:games count is 1 ExecuteAsync 1 :games count is 1:[ "CONSTRUCTOR GAME AT 10/29/2021 10:37:25 AM(0)" ] ...... ExecuteAsync 7 :games count is 1:[ "CONSTRUCTOR GAME AT 10/29/2021 10:37:25 AM(0)" ] service addGame:games count is 2 After AddGame: [
"CONSTRUCTOR GAME AT 10/29/2021 10:37:24 AM(0)", "GAME 0(1)" ]

如果只使用 AddSingleton 就成功了。
如果仅使用 AddHostedService，则 BackgroundService 为运行，但 DI 无法在控制器中工作（异常）。

**> 失败：

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware1 An unhandled exception has occurred while executing the request. System.InvalidOperationException: Unable to resolve service for type 'CoreServiceSignalRSample.service.IBGService' while attempting to activate 'CoreServiceSignalRSample.Controllers.WeatherForecastController'.
at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, Boolean isDefaultParameterRequired)
at lambda_method(Closure , IServiceProvider , Object[] )**

我的问题是，DI 一个 BackgroundService 的正确方法是什么，为什么推荐使用 AddHostedService？

示例代码可以在 https://github.com/huangpat/CoreServiceSignalRSample 找到。

Startup.cs 配置服务

    services.AddSingleton<IBGService>(new BGService());
    services.AddHostedService<BGService>();

BGSercie.cs

private List<string> games;

public BGService()
{
    Console.WriteLine("BGService constructor");
    this.games = new List<string>();
    this.addGame("Constructor Game at " + DateTime.Now.ToString());
}

public async Task<bool> addGame(string fGame)
{
    if (this.games.Count <= 20)
    {
        this.games.Add(fGame.ToUpper() + "(" + Convert.ToString(this.games.Count) + ")");
        Console.WriteLine("service addGame:games count is " + Convert.ToString(this.games.Count));
        return true;
    }
    else
    {
        return false;
    }
}


protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
    int i = 0;
    while (true)
    {
        i++;
        if (games.Count == 0)
        {
            this.addGame("ExecuteAsync Game at " + DateTime.Now.ToString());
        }
        JsonSerializer js = new JsonSerializer();
        
        Console.WriteLine("ExecuteAsync {1} :games count is {0}:{2}", this.games.Count, i, JsonConvert.SerializeObject(this.games, Formatting.Indented));

        await Task.Delay(1000 * 2 * 5);
    }
    //throw new NotImplementedException();

    /

/ 使用线程池开始集合中的每个游戏 }

Answer 1

每个注册都会使用自己的解析规则创建自己的类型描述符（即默认情况下它将使用相应的构造函数）。您可以使用向实施工厂注册来解决它：

services.AddSingleton<BGService>();
services.AddSingleton<IBGService>(provider => provider.GetRequiredService<BGService>());
services.AddHostedService<BGService>(provider => provider.GetRequiredService<BGService>())

ASP.NET核心AddHostedService和AddSingleton关系

ASP.NET Core AddHostedService and AddSingleton relationship

.net

service

dependency-injection