Parallel For Each 抛出 "No reference to object" 异常

Question

我有一个 Parallel.ForEach 循环执行以下操作：

ConcurrentBag<Participant> participantsList = new ConcurrentBag<Participant>() {
    new Participant() { EMail = "Test.Mail@outlook.de", FirstName = "First1", LastName = "Last1"},
    new Participant() { EMail = "Test.Mail@gmail.de", FirstName = "First2", LastName = "Last2"},
    new Participant() { EMail = "Test.Mail@yahoo.de", FirstName = "First3", LastName = "Last3"},
};

Parallel.ForEach(participantsList, (p) =>
{
    var mail = new Email("REGISTERMAIL", p.FirstName, p.LastName);
    mail.AddRecipient(p.EMail);
    mail.Send();
});

发生的情况是 ForEach-Loop 只成功处理了一个参与者。其他两个扔 "Object reference not set to an instance of an object.".

我首先得出结论，这是由于进程不是线程安全的，所以我用 ConcurrentBag 替换了 participantsList（首先是 List<> 类型）。但是还是报错。

我看不到任何其他线程可以在我的代码中共享一个集合的地方，因为如果电子邮件，每个线程都有自己的实例。

该错误的来源可能是什么？也许电子邮件中的静态属性？由于它们不会在每个实例中都被复制...

正常的 foreach 循环一切正常。

编辑：电子邮件中有一个静态字典class。我将其替换为 ConcurrentDictionary，但仍未解决问题。

解决方法：感谢 Luke Merrett，我找到了问题并解决了它：
我在多个线程上引用 HttpContext.Current。问题是当您切换线程时 HttpContext.Current 变为 null。所以我必须将当前 HttpContext.Current 传递到我启动的每个线程中：

HttpContext ctx = HttpContext.Current;
Parallel.ForEach(participantsList, (p) =>
{
    HttpContext.Current = ctx;
    var mail = new Email("REGISTERMAIL", p.FirstName, p.LastName);
    mail.AddRecipient(p.EMail);
    mail.Send();
});

更多信息

电子邮件 class 是 System.Net.Mail 的包装。构造函数接受一个创建选项，该选项运行是用户预定义并存储在静态字典中的标准电子邮件初始化。它还接收一个对象数组以在其上应用 String.Format：

public Email(string creationFlag, params object[] formatObjects)
{
    Mail = new MailMessage();
    Smtp = new SmtpClient();

    Action<Email, object[]> action;
    if (OptionsToActionsMap.TryGetValue(creationFlag.ToString(), out action))
    {
        action(this, formatObjects);
    }
}

接下来的动作是运行：

private static Action<Email, object[]> registerMail = new Action<Email, object[]>((mail, formatParams) =>
{
    mail.Smtp.Host = "smtp.sendgrid.net";

    mail.SetCredentials(WebConfigurationManager.AppSettings["mailAccount"],
                        WebConfigurationManager.AppSettings["mailPassword"]);

    mail.From = "no-reply@sender.de";

    mail.Subject = "Deine Anmeldung für die TNTC.";

    mail.AddAttachment(HttpContext.Current.Server.MapPath("~/img/TNTC-Logo.png"), "logo", new ContentType("image/png"));
    mail.AddAttachment(HttpContext.Current.Server.MapPath("~/img/Anfahrt.png"), "anfahrt", new ContentType("image/png"));

    mail.AddHtmlView(HttpContext.Current.Server.MapPath("~/EMail/MailBody.html"), formatParams);
});

现在这里有两个方法叫做 AddAttachment:

public void AddAttachment(string path, string contentId, ContentType ct)
{
    if (LinkedResources == null)
    {
        LinkedResources = new List<LinkedResource>();
    }

    var linkedResource = new LinkedResource(path, ct);
    linkedResource.ContentId = contentId;

    LinkedResources.Add(linkedResource);
}

和 AddHtmlView 调用内容类型为 "text/html":

的 AddView

public virtual void AddView(string path, string ctype, params object[] formatObjects)
{
    if (Views == null)
    {
        Views = new List<AlternateView>();
    }

    if (new ContentType(ctype) != null)
    {
        var view = AlternateView.CreateAlternateViewFromString(String.Format(File.ReadAllText(path), formatObjects), null, ctype);
        Mail.AlternateViews.Add(view);
    }
}

现在 mail.Send() 只是将链接的资源添加到每个视图并发送邮件：

public virtual void Send()
{
    if (EmailValid())
    {
        if (LinkedResources.Count() > 0)
        {
            foreach (var view in Mail.AlternateViews)
            {
                foreach (var linkedResource in LinkedResources)
                {
                    view.LinkedResources.Add(linkedResource);
                }
            }
        }

        foreach (var view in Views)
        {
            Mail.AlternateViews.Add(view);
        }

        Smtp.Send(Mail);
    }
}

Answer 1

我注意到您在操作中使用了 HttpContext，如下所示：

mail.AddAttachment(HttpContext.Current.Server.MapPath(
    "~/img/TNTC-Logo.png"), "logo", new ContentType("image/png"));

mail.AddAttachment(HttpContext.Current.Server.MapPath(
    "~/img/Anfahrt.png"), "anfahrt", new ContentType("image/png"));

mail.AddHtmlView(HttpContext.Current.Server.MapPath(
    "~/EMail/MailBody.html"), formatParams);

由于您是运行主响应线程之外的异步代码，HttpContext.Current 可能会在您的处理完成之前处理掉（即：服务器已返回响应，因此不再有任何上下文.)

要解决这个问题，您必须在启动并行线程之前传递映射路径，或者存储它们，以便它们不依赖于调用它们时存在的 HttpContext.Current。

A question along these lines was posted here.

Parallel For Each 抛出 "No reference to object" 异常

Parallel For Each throwing "No reference to object" exception

c#

parallel-processing

multithreading

parallel.foreach