无法访问asp.net mvc核心中的已处置对象UserManager

Question

我的服务中有以下代码，应该创建UserManager实例

public class SubscriptionService : ISubscriptionService
{

    private UserManager<ApplicationUser> userManager;
    public UserManager<ApplicationUser> UserManager
    {
        get
        {
            return userManager;  

        }
        private set { userManager = value; }
    }

    public SubscriptionService(
        UserManager<ApplicationUser> userManager,   
        StripeCustomerService customerService, 
        StripeSubscriptionService subscriptionService)
    {
        this.userManager = userManager;
        this.customerService = customerService;
        this.subscriptionService = subscriptionService;
    }

    public SubscriptionService(UserManager<ApplicationUser> userManager)
    {
        this.userManager = userManager;
    }

当第一次在课堂内调用userManager时，它可以正常工作，但第二次调用会返回错误“cannot access a disposed object UserManager”

我在控制器中使用SubscriptionService类：

public ISubscriptionService SubscriptionService
{
    get
    {
        return subscriptionService ?? new SubscriptionService(_userManager);
    }
    private set
    {
        subscriptionService = value;
    }
}

有人能指出我错在哪里以及如何解决这个问题吗？

我正在使用ASP.NET Core 1.0

编辑：这是在首次执行函数

后立即检查的用户管理器