无法使用单例的作用域服务

Question

我正在尝试在托管服务中使用DBContext，但收到此错误。我尝试遵循此已 接受的答案，但是以某种方式无法正常工作，我不确定自己在做什么错。

我是.net的新手，请引导我朝正确的方向发展。

未处理的异常：System.InvalidOperationException：无法使用单例“ Microsoft.AspNetCore.Hosting.Internal.HostedServiceExecutor”中的作用域服务“ StatusApp.Context.DBContext”。

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();
    }
}

Answer 1

您快到这里了，但是您已经DBContext成为TokenService的构造函数中的依赖项。删除它，您将不再收到错误。

公共TokenService（
    IConfiguration配置，
    IMemoryCache memoryCache，
    IHttpClientFactory clientFactory，
    DBContext DBcontext，
    IServiceScopeFactory scopeFactory）

但是，您不太遵循有关DbContext在单例服务中处理的建议。与其DBContext在构造函数中创建单个实例并将其存储为字段，不如DBContext在需要时创建范围和相应的范围。就您而言，这就是getOrg方法。

请按照以下步骤来实现：

1. _DBcontext从您的TokenService班级中删除该字段：

   公共DBContext _DBcontext;

2. 从TokenService构造函数中删除关联的分配：

   _DBcontext = _scopeFactory.CreateScope（）。ServiceProvider.GetRequiredService <DBcontext>（）;

3. 在中getOrg，创建作用域，解析该作用域的实例DBContext，最后处置该作用域：

   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();
       }
   }
   

_DBContext上面的代码没有使用该字段，而是创建了一个适当范围的局部变量，dbContext并使用了该变量。