多线程和锁定

Question

I have a question in multithreading subject as describe

    private List<Email> emails = new List<Email>();

    private void AddEmail(Email email)
    {
        lock (this.emails)
        {
            this.emails.Add(email);
        }
    }

    private void ReadEmails()
    {
        foreach (Email email in this.emails)
        {
            Print(email);
        }
    }

Assume the ReadEmails method lasts in 10s and at that time the AddEmail is called. So does any error occur?

Answer 1

Your ReadEmails method is not thread-safe. You have a couple of options.

Option #1:

You can place a lock around the entire foreach loop.

private void ReadEmails()
{
  lock (emails)
  {
    foreach (Email email in this.emails)
    {
      Print(email);
    }
  }
}

Option #2:

You can make a copy of the list inside a lock . Then you can iterate the copy lock-free.

private void ReadEmails()
{
  List<Email> copy;
  lock (emails)
  {
    copy = new List<Email>(this.emails);
  }
  foreach (Email email in copy)
  {
    Print(email);
  }
}

Answer 2

You should definitely add the Lock method in the Read method. If you do not do this, there is a possibility that the email is added when the Read process hasn't been completed yet. Since the foreach method requires its collection to be unchangeable during the iteration, the exception will be raised.

 private void ReadEmails()
    {
        lock(emails) {
             foreach (Email email in this.emails) {
                Print(email);
             }
        }
    }