无法通过 MediatR 使用作用域服务

Question

我有一个 ASP.NET 核心应用程序运行 .NET 5 和 C# 9。它还在后台运行一个 Discord 机器人。我在 Startup.cs 中的 ConfigureServices() 方法如下所示。

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews();
    
    var client = new DiscordSocketClient(new DiscordSocketConfig
    {
        AlwaysDownloadUsers = true,
        MessageCacheSize = 10000,
            
        GatewayIntents = GatewayIntents.Guilds | GatewayIntents.GuildMessages |
                         GatewayIntents.GuildMessageReactions | GatewayIntents.GuildPresences,
            
        LogLevel = LogSeverity.Info
    });
        
    var commandService = new CommandService(new CommandServiceConfig
    {
        LogLevel = LogSeverity.Debug,
        DefaultRunMode = RunMode.Sync,
        CaseSensitiveCommands = false,
        IgnoreExtraArgs = false,
    });
    
    services
        .AddMediatR(Assembly.GetEntryAssembly())
        .AddHostedService<StartupService>()
        .AddHostedService<DiscordListener>()
        .AddScoped<ITestService, TestService>()
        .AddSingleton(client)
        .AddSingleton(provider =>
        {
            commandService.AddModulesAsync(Assembly.GetEntryAssembly(), provider);
            return commandService;
        })
        .AddSingleton(Configuration);
}

如您所见，我已将 ITestService 和 TestService 添加为作用域服务。

public class TestService : ITestService
{
    public async Task<string> GetString()
    {
        await Task.Delay(1);
        return "hey";
    }
}

public interface ITestService
{
    Task<string> GetString();
}

然后我将此服务注入我的命令模块。

public class TestModule : ModuleBase<SocketCommandContext>
{
    private readonly ITestService _testService;

    public TestModule(ITestService testService)
    {
        _testService = testService;
    }

    [Command("ping")]
    public async Task Ping()
    {
        var str = await _testService.GetString();
        await ReplyAsync(str);
    }
}

但是，应用程序不响应 ping 命令。事实上，我用于接收消息的处理程序根本没有命中（我已经通过断点进行了检查）。这是监听事件并发布相关 MediatR 通知的托管服务。

public partial class DiscordListener : IHostedService
{
    private readonly DiscordSocketClient _client;
    private readonly IServiceScopeFactory _serviceScopeFactory;

    public DiscordListener(
        DiscordSocketClient client,
        IServiceScopeFactory serviceScopeFactory)
    {
        _client = client;
        _serviceScopeFactory = serviceScopeFactory;
    }
    
    public Task StartAsync(CancellationToken cancellationToken)
    {
        _client.MessageReceived += MessageReceived;
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _client.MessageReceived -= MessageReceived;
        return Task.CompletedTask;
    }

    // Creating our own scope here
    private async Task MessageReceived(SocketMessage message)
    {
        using var scope = _serviceScopeFactory.CreateScope();
        var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();
        await mediator.Publish(new MessageReceivedNotification(message));
    }
}

这是处理通知的通知处理程序。

public class CommandListener : INotificationHandler<MessageReceivedNotification>
{
    private readonly IConfiguration _configuration;
    private readonly DiscordSocketClient _client;
    private readonly CommandService _commandService;
    private readonly IServiceProvider _serviceProvider;

    public CommandListener(
        IConfiguration configuration,
        DiscordSocketClient client, 
        CommandService commandService, 
        IServiceProvider serviceProvider)
    {
        _configuration = configuration;
        _client = client;
        _commandService = commandService;
        _serviceProvider = serviceProvider;
    }
    
    public async Task Handle(MessageReceivedNotification notification, CancellationToken cancellationToken)
    {
        if (!(notification.Message is SocketUserMessage message)
            || !(message.Author is IGuildUser user)
            || user.IsBot)
        {
            return;
        }
        
        var argPos = 0;
        var prefix = _configuration["Prefix"];
        
        if (message.HasStringPrefix(prefix, ref argPos))
        {
            var context = new SocketCommandContext(_client, message);
            using var scope = _serviceProvider.CreateScope();
            await _commandService.ExecuteAsync(context, argPos, scope.ServiceProvider);
        }
    }
}

澄清一下，_client.MessageReceoved += ... 处的断点未命中。如果我将 ITestService 和 TestService 实现更改为 Singleton，则会命中处理程序并且命令按预期工作。知道我做错了什么吗？

如果您想查看完整代码，

Here 是项目的 GitHub 回购。不算太大。

Answer 1

这是混合单例和作用域服务时的典型问题。如果您最终遇到的情况是单例正在解析范围内的服务，则这是不允许的。

来自此处的文档 https://docs.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-3.1

不要从单例中解析作用域服务。它可能导致服务在处理后续请求时具有不正确的状态。没关系：

从作用域服务或瞬态服务中解析单例服务。从另一个作用域或瞬态服务解析作用域服务。默认情况下，在开发环境中，从另一个生命周期更长的服务解析一个服务会抛出异常。有关详细信息，请参阅范围验证。

还有关于 https://dotnetcoretutorials.com/2018/03/20/cannot-consume-scoped-service-from-singleton-a-lesson-in-asp-net-core-di-scopes/amp/

的更多讨论

无法通过 MediatR 使用作用域服务

Unable to Consume Scoped Service via MediatR

c#

asp.net-mvc

mediatr

asp.net-core

discord.net