c# 事件执行是线程安全的吗？

Question

我阅读了很多关于事件和线程的讨论，但所有这些讨论都集中在“如果我取消订阅事件并稍后尝试调用它会发生什么”。 我的问题是不同的......如果我在线程 A 中有一个进程在 1 毫秒内触发事件“我完成”，并且在线程 B 中有一个进程在毫秒 2 内触发事件“我完成”，将会发生什么。

两个进程都使用相同的方法来监听和处理事件。 因此，C# 必须执行处理事件的方法 2 次:1 次是在线程 A 中触发的事件，1 次是从线程 B 触发的事件。

会发生什么？？ C#是否在“第一个来自线程A的事件”开始执行处理该事件的方法时锁定方法，并在执行完成时解锁方法，从而允许其他等待的“事件”执行该方法内容？？

或者从线程 A 触发的事件将开始执行处理该事件的方法，1 毫秒后从线程 B 触发的事件也将开始执行相同的方法，而不会注意到当前正在执行该方法通过其他“过程”？？？

我问这个，因为我想在捕获事件的方法中写一些文件，但是如果该方法可以同时执行(取决于触发事件的时间)，我想我不能在这里做，因为文件上的信息将是同时写入同一文件的 2 个进程的混合(文件上的无效信息)。

我的代码看起来像这样(有点长，抱歉)。 请注意，这不会编译，只是展示我在做什么的示例:

 public partial class MainForm : Form
{
    FTPClientManager client = null;

    public MainForm()
    {
        InitializeComponent();
    }

    private void btnConnect_Click(object sender, EventArgs e)
    {
        Connect(this.tbFTPServer.Text.Trim());
        this.lstLog.Items.Add("Connected"); //This lstLog is a list box that will have a list of downloaded files from all threads.
    }

    void Connect(string urlStr)
    {
        try {
            client = new FTPClientManager();
            //subscribe to the event
            client.FileDownloadCompleted += new EventHandler<FileDownloadCompletedEventArgs>(client_FileDownloadCompleted);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

    void client_FileDownloadCompleted(object sender, FileDownloadCompletedEventArgs e)
    {
        this.Invoke(new EventHandler<FileDownloadCompletedEventArgs>(
             client_FileDownloadCompletedHandler), sender, e);
    }

    void client_FileDownloadCompletedHandler(object sender, FileDownloadCompletedEventArgs e)
    {
        string log = string.Format("{0} Instance {5} Download from {1} to {2} is completed. Length: {3} Time: {4}. ",
            DateTime.Now, e.ServerPath, e.LocalFile.FullName, e.LocalFile.Length, e.DownloadTime, e.ftpInstance);

        this.lstLog.Items.Add(log);
    }

    private void btnDownload_Click(object sender, EventArgs e)
    {
        client.DownloadFiles();
    }
}   

public class FTPClientManager {
    FTPDownloadClient[] arrayDownloadClient = new FTPDownloadClient[2];        
    public event EventHandler<FileDownloadCompletedEventArgs> FileDownloadCompleted;

    public void DownloadFiles()
    {
        for (int i = 0; i < 2; i++)
        {
            arrayDownloadClient[i] = new FTPDownloadClient();
            //subscribe to the event. each instance of FTPDownloadClient will suscribe to the same event.
            arrayDownloadClient[i].FileDownloadCompleted += new EventHandler<FileDownloadCompletedEventArgs>(downloadClient_FileDownloadCompleted);
        }

        //download one set of files in thread A
        arrayDownloadClient[0].DownloadFiles(list_of_files_to_download);

        //download another set of files in thread B
        arrayDownloadClient[1].DownloadFiles(another_list_of_files_to_download);
    }

    //In theory, the method downloadClient_FileDownloadCompleted will be executed by any instance of FTPDownloadClient
    //running in either thread A or thread B, whichever finish first downloading a file.
    //My question comes in the execution of this method.
    //Lets say the process in thread A finish downloading and fires the event.
    //Lets say the process in thread B finish downloading 1 millisecond after thread A finish, so it also fires the event.
    //how C# manage the execution of the downloadClient_FileDownloadCompleted??
    //does the event coming from thread A will lock the method downloadClient_FileDownloadCompleted, execute it, and when finish execution unlock the method 
    //and allows the event coming from thread B start locking, processing, unlock ??
    //Or the method will be executed "at the same time" (1 millisecond difference) by each event fired from thread A and thread B??
    void downloadClient_FileDownloadCompleted(object sender, FileDownloadCompletedEventArgs e)
    {
        this.OnFileDownloadCompleted(e);
    }

    protected virtual void OnFileDownloadCompleted(FileDownloadCompletedEventArgs e)
    {
        if (FileDownloadCompleted != null)
        {
            //this will fire the event, so the main form will catch it
            //again, this fire can be triggered from process in thread A or from process in thread B
            FileDownloadCompleted(this, e); 
        }
    }
}

public class FTPDownloadClient {
    public event EventHandler<FileDownloadCompletedEventArgs> 
            FileDownloadCompleted;

    public void DownloadFiles(string [] files_to_download)
    {
        ParameterizedThreadStart threadStart =
                new ParameterizedThreadStart(StartDownloadFiles);
            Thread downloadThread = new Thread(threadStart);
            downloadThread.IsBackground = true;
            downloadThread.Start(new object[] { files_to_donwload });
    }

    //This metod will download all the files in the list passed as parameter.
    //Every file downloaded will raise the event FileDownloadComplete, so a message can be added to the lstlog on the main form
    void StartDownloadFiles(object state)
        {
            var paras = state as object[];

            string [] files = paras[0] as string [];

            foreach (var file in files)
            {
                DownloadOneFile(file);
            }
        }

     void DownloadFile(string onefile)
     {
            //Donwload file done here
            var fileDownloadCompletedEventArgs = new FileDownloadCompletedEventArgs
            {
               LocalFile = new FileInfo(destPath),
               ServerPath = onefile,
               DownloadTime = fileDownloadTime.ElapsedMilliseconds.ToString()
            };

            this.OnFileDownloadCompleted(fileDownloadCompletedEventArgs);
     }

     protected virtual void OnFileDownloadCompleted(FileDownloadCompletedEventArgs e)
        {
            if (FileDownloadCompleted != null)
            {
                //the event is fired when the file being downloaded by this thread is finish.
                //so, thread A will fire this event from its current thread
                //and also thread B will fire the same event from its own thread.
                FileDownloadCompleted(this, e); 
            }
        }
}

Answer 1

C# 不会为您做任何锁定。如果多个线程可以同时引发事件，则您必须编写代码来处理该事件(如有必要)。

您可以使用 lock statement防止多个线程执行它:

private void MyEventHandler(object sender, EventArgs e)
{
    lock (lockingObject)
    {
        // Handle event here.
        // Only one thread at a time can reach this code.
    }
}

lockingObject 是类中的一个字段，声明如下:

private readonly object lockingObject = new object();

您还必须小心处理引发事件的方法中的线程。

假设您的类(class)中有一个名为 MyEvent 的事件。你不应该这样做:

private void RaiseMyEvent()
{
    if (MyEvent != null)                  // {1}
        MyEvent(this, new EventArgs());   // {2}
}

如果另一个线程可以从 MyEvent 分离，那么它可能会在行 {1} 和行 {2} 之间分离。如果发生这种情况，第 {2} 行将抛出空引用异常，因为 MyEvent 会突然变为空!

正确的做法是:

private void RaiseMyEvent()
{
    var handler = MyEvent;

    if (handler != null) 
        handler (this, new EventArgs()); 
}

现在空引用异常不会发生。

但是，请注意，当使用多个线程时，事件处理程序有可能在线程分离后 被调用!