c# - 无法从单例使用范围服务

我正在尝试在托管服务中使用 DBContext，但出现此错误。
我试图遵循 this 接受的答案，但不知何故它不起作用，我不确定我做错了什么。

我是 .net 新手，请指导我走向正确的方向。

public class TokenService : IHostedService
{
    public IConfiguration _Configuration { get; }
    protected IMemoryCache _cache;
    private Timer _timer;
    public IHttpClientFactory _clientFactory;
    public DBContext _DBcontext;

    private readonly IServiceScopeFactory _scopeFactory;

    public TokenService(IConfiguration configuration, IMemoryCache memoryCache, IHttpClientFactory clientFactory, DBContext DBcontext, IServiceScopeFactory scopeFactory)
    {
        _Configuration = configuration;
        _cache = memoryCache;
        _clientFactory = clientFactory;
        _scopeFactory = scopeFactory;
        _DBcontext = _scopeFactory.CreateScope().ServiceProvider.GetRequiredService<DBcontext>();
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _timer = new Timer(getOrg, null, 0, 1000);
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        //Timer does not have a stop.
        _timer?.Change(Timeout.Infinite, 0);
        return Task.CompletedTask;
    }

    public async Task getOrg()
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "organizations");
        var response = await _client_NP.SendAsync(request);
        var json = await response.Content.ReadAsStringAsync();
        OrganizationsClass.OrgsRootObject model = JsonConvert.DeserializeObject<OrganizationsClass.OrgsRootObject>(json);

        foreach (var item in model.resources)
        {
            var g = Guid.Parse(item.guid);
            var x = _DBcontext.Organizations.FirstOrDefault(o => o.OrgGuid == g);
            if (x == null)
            {
                _DBcontext.Organizations.Add(new Organizations
                {
                    OrgGuid = g,
                    Name = item.name,
                    CreatedAt = item.created_at,
                    UpdatedAt = item.updated_at,
                    Timestamp = DateTime.Now,
                    Foundation = 3
                });
            }
            else if (x.UpdatedAt != item.updated_at)
            {
                x.CreatedAt = item.created_at;
                x.UpdatedAt = item.updated_at;
                x.Timestamp = DateTime.Now;
            }
        }

        await _DBcontext.SaveChangesAsync();
    }
}

IServiceScopeFactory scopeFactory)

However, you're not quite following the recommendation for dealing with DbContext's in a singleton service. Instead of creating a single instance of DBContext in the constructor and storing it as a field, create a scope and a corresponding DBContext whenever you need it. In your case, that's in the getOrg method.

Follow these steps to achieve that:

1. Remove the _DBcontext field from your TokenService class:

2. Remove the associated assignment from the TokenService constructor:

3. In getOrg, create a scope, resolve an instance of DBContext and, lastly, dispose of the scope:

   public async Task getOrg()
   {
       var request = new HttpRequestMessage(HttpMethod.Get, "organizations");
       var response = await _client_NP.SendAsync(request);
       var json = await response.Content.ReadAsStringAsync();
       OrganizationsClass.OrgsRootObject model = JsonConvert.DeserializeObject<OrganizationsClass.OrgsRootObject>(json);
   
       using (var scope = _scopeFactory.CreateScope())
       {
           var dbContext = scope.ServiceProvider.GetRequiredService<DBcontext>();
   
           foreach (var item in model.resources)
           {
               var g = Guid.Parse(item.guid);
               var x = dbContext.Organizations.FirstOrDefault(o => o.OrgGuid == g);
               if (x == null)
               {
                   dbContext.Organizations.Add(new Organizations
                   {
                       OrgGuid = g,
                       Name = item.name,
                       CreatedAt = item.created_at,
                       UpdatedAt = item.updated_at,
                       Timestamp = DateTime.Now,
                       Foundation = 3
                   });
               }
               else if (x.UpdatedAt != item.updated_at)
               {
                   x.CreatedAt = item.created_at;
                   x.UpdatedAt = item.updated_at;
                   x.Timestamp = DateTime.Now;
               }
           }
   
           await dbContext.SaveChangesAsync();
       }
   }
   


上面的代码没有使用 _DBContext 字段，而是创建了一个本地的、范围适当的变量 dbContext 并使用它。

关于c# - 无法从单例使用范围服务，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/57580937/