升级到ASP.NET Core 2.0后，无法从单例IActiveUsersService中使用作用域服务IMongoDbContext

问题描述

我今天将项目更新为ASP.NET Core 2，并且出现以下错误:

I updated a project to ASP.NET Core 2 today and I get the following error:

我有以下注册:

services.AddSingleton<IActiveUsersService, ActiveUsersService>();
services.AddScoped<IMongoDbContext, MongoDbContext>();
services.AddSingleton(option =>
{
   var client = new MongoClient(MongoConnectionString.Settings);
   return client.GetDatabase(MongoConnectionString.Database);
})



public class MongoDbContext : IMongoDbContext
{
   private readonly IMongoDatabase _database;

   public MongoDbContext(IMongoDatabase database)
   {
      _database = database;
   }

   public IMongoCollection<T> GetCollection<T>() where T : Entity, new()
   {
      return _database.GetCollection<T>(new T().CollectionName);
   }
}

public class IActiveUsersService: ActiveUsersService
{

   public IActiveUsersService(IMongoDbContext mongoDbContext)
   {
      ...
   }
}

为什么DI无法使用该服务?对于ASP.NET Core 1.1，一切正常.

Why DI can't consume the service? All works fine for ASP.NET Core 1.1.

推荐答案

您不能使用寿命较短的服务.范围服务仅按请求存在，而单例服务仅创建一次并共享实例.

You can't use a service with a smaller lifetime. Scoped services only exist per-request, while singleton services are created once and the instance is shared.

现在应用程序中仅存在一个IActiveUsersService实例.但是它想依赖MongoDbContext，它是作用域的，并且是根据请求创建的.

Now only one instance of IActiveUsersService exists in the app. But it wants to depend on MongoDbContext, which is Scoped, and is created per-request.

您将必须:

1. 使MongoDbContext为单身人士，或
2. 将IActiveUsersService设为作用域，或
3. 将MongoDbContext作为函数参数传递给用户服务

1. Make MongoDbContext a Singleton, or
2. Make IActiveUsersService Scoped, or
3. Pass MongoDbContext into the user service as a function argument

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