c# – 具有实体框架和多线程的NullReferenceException

1-安全吗？
这似乎就像我在这些帖子中看到的那样：

Entity Framework + Multiple Threads + Lazy Load

Is it safe to use one Entity Framework Context per thread? … yes? how?

2 – 在创建我的上下文时有一个例外,但只有一次在3中,我无法找到原因.

这是我创建上下文的代码：

public partial class Entities
{
    private static Entities mfgEntities = new Entities();
    private static readonly Dictionary<int,Entities>  ThreadContexts = new Dictionary<int,Entities>();

    public static Entities Context
    {
        get
        {
            if (HttpContext.Current != null)
            {
                string objectContextKey = HttpContext.Current.GetHashCode().ToString("x");
                if (!HttpContext.Current.Items.Contains(objectContextKey))
                {
                    HttpContext.Current.Items.Add(objectContextKey,new Entities());
                }
                return HttpContext.Current.Items[objectContextKey] as Entities;
            }
            else
            {
                int threadId = Thread.CurrentThread.ManagedThreadId;
                if (!ThreadContexts.ContainsKey(threadId))
                {
                    try
                    {
                        ThreadContexts.Add(threadId,new Entities());
                    }
                    catch (Exception ex)
                    {
                        throw new Exception("Erreur lors de la création de l'entity context");
                    }
                }
                return ThreadContexts[threadId];
            }
            return mfgEntities;
        }
    }
}

它在行上抛出NullReferenceException：

ThreadContexts.Add(threadId,new Entities());

而ThreadContexts,threadId和新实体不为空.

谢谢你的帮助.