Question

I would like to ask is this code thread safe? There is a problem with attachment object. It is passed by reference to the new thread where MailHelper use it and sometimes the attachment object is mixed between threads.

public static void Start() 
{
    foreach (var message in messages)
    {
        //skip code
        var fileName = httpWebResponse.GetResponseHeader("filename");
        var fileStream = httpWebResponse.GetResponseStream();
        var attachment = new Attachment(fileStream, fileName);

        var thread = new Thread(() =>
        {
            var dictionary = new ListDictionary
            {
                { "$Url$", message.Url }
            };

            MailHelper.SendMessage(dictionary,
                message.Mail.Headers.From.Address, 
                "EmailConvertSuccess.txt",
                attachment)
        });

        thread.Start();
    }
}

Answer 1

No this will probably not be working – but it’s not only the attachment (see Darins answer) but the message object you use as an iterator as well – you will have to copy it to a local instance before calling your Thread like this:

var messageCopy = message;
new Thread(a =>
        MailHelper.SendMessage(
            new ListDictionary { { "$Url$", messageCopy .Url } },
            messageCopy.Mail.Headers.From.Address, 
            "EmailConvertSuccess.txt",
            a as MailAttachment)
    ).Start(attachment);

If you really want to you could pass this as parameter – just like Darin did with it’s variant but I don’t think this is really needed)