startup:
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();

        //全局
        services.AddSingleton<ITenantResolver, QueryStringTenantResolver>();
        services.AddScoped<ICurrentTenant, CurrentTenant>();
        services.AddScoped<ITenantRepository, TenantRepository>();

        //dapper会用到
        services.Configure<ConnectionString>(Configuration.GetSection("ConnectionStrings"));
    }

MultiTenantMiddleware:
private readonly RequestDelegate _nextDelegate;
private readonly ITenantResolver _tenantResolver;
private readonly ITenantRepository _tenantRepository;
private ICurrentTenant _currentTenant;//不能使用readonly，因为要改变它的值

    public MultiTenantMiddleware(RequestDelegate nextDelegate
        , ITenantResolver tenantResolver
        , ITenantRepository tenantRepository
        , ICurrentTenant currentTenant)
    {
        _nextDelegate = nextDelegate;
        _tenantResolver = tenantResolver;
        _tenantRepository = tenantRepository;
        _currentTenant = currentTenant;
    }

controller:
private readonly ITenantRepository _tenantRepository;
private readonly ICurrentTenant _currentTenant;

    public HomeController(ITenantRepository tenantRepository
        , ICurrentTenant currentTenant)
    {
        _tenantRepository = tenantRepository;
        _currentTenant = currentTenant;
    }

谢谢您，这是我的部分代码。

@屌丝大叔的笔记: 使用构造(ctor)注入的始终是单例的，并且Scope是不能注册在root provider的,可以参见我写的demo。

@屌丝大叔的笔记:
我进行了一个测试，就是你这个原因，因为项目中肯定不验证Scope的，所以导致中间件中ctor注册scope服务是无效的，scope应该在Invoke中使用。

• 测试同样存在Null
  

附上我的测试代码:

1. ICurrenTenant

namespace cnblog_133057
{
    public interface ICurrentTenant
    {
        string TenantId { get; }
        string ConnectionString { get; }

        void SetTenant(string id, string conn);
    }

    public class CurrentTenant : ICurrentTenant
    {
        public string TenantId { get; private set; }

        public string ConnectionString { get; private set; }

        public void SetTenant(string id, string conn)
        {
            TenantId = id;
            ConnectionString = conn;
        }
    }
}

2. TenantMiddleware

namespace cnblog_133057
{
    public class TenantMiddleware
    {
        private readonly RequestDelegate _next;
        private readonly ICurrentTenant _currentTenant;

        public TenantMiddleware(RequestDelegate next, ICurrentTenant currentTenant)
        {
            _next = next;
            _currentTenant = currentTenant;
        }

        public async Task InvokeAsync(HttpContext httpContext)
        {
            var __tenantId = httpContext.Request.Headers["__tenantId"];
            var conn = "xxxxxx";
            _currentTenant.SetTenant(__tenantId, conn);
            await _next(httpContext);
        }
    }
}

3. 添加注册

app.UseMiddleware(typeof(TenantMiddleware));

4. 控制器测试

namespace cnblog_133057.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class WeatherForecastController : ControllerBase
    {
        private readonly ICurrentTenant _tenant;
        private static readonly string[] Summaries = new[]
        {
            "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
        };

        private readonly ILogger<WeatherForecastController> _logger;

        public WeatherForecastController(ILogger<WeatherForecastController> logger, ICurrentTenant tenant)
        {
            _logger = logger;
            _tenant = tenant;
        }

        [HttpGet]
        public IEnumerable<WeatherForecast> Get()
        {
            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)],
                Tenant = $"{_tenant.TenantId};{_tenant.ConnectionString}"
            })
            .ToArray();
        }
    }
}

上面在模板项目中运行是会报错的，下面我为了测试强制不进行报错，添加如下代码

public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseDefaultServiceProvider(configure =>
                    {
                        configure.ValidateOnBuild = false;
                        configure.ValidateScopes = false;
                    });
                    webBuilder.UseStartup<Startup>();
                });

配置不验证ValidateScopes 和在编译的时候不进行验证ValidateOnBuild

使用Invoke测试

变更一下Invoke

public async Task InvokeAsync(HttpContext httpContext, ICurrentTenant _currentTenant)
{
      var __tenantId = httpContext.Request.Headers["__tenantId"];
       var conn = "xxxxxx";
       _currentTenant.SetTenant(__tenantId, conn);
       await _next(httpContext);
}

@Jonny-Xhl: 牛逼啊，大哥。我看了你给的链接，换成以下方式就行了：
var _currentTenant = httpContext.RequestServices.GetRequiredService<ICurrentTenant>();
_currentTenant.ChangeValue(tenant);

@屌丝大叔的笔记: 中间件构造中注入慎用!!!，很容易导致内存泄漏

@Jonny-Xhl: 知道了，谢谢。