c#事件的执行是线程安全的

我读了很多关于事件和线程的讨论，但所有这些都集中在"如果我取消订阅事件并稍后尝试调用它会发生什么"上。我的问题不一样……如果线程a中有一个进程在毫秒1内触发事件"I finish"，并且线程B中也有一个进程在毫秒2内触发事件"I finish"，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次用于线程A中触发的事件，1次用于线程b中触发的事件。

会发生什么??当"来自线程A的第一个事件"开始执行处理该事件的方法时，c#是否锁定该方法，并在完成执行时解锁该方法，从而允许其他等待的"事件"执行方法内容?

或者从线程A触发的事件将开始执行处理该事件的方法，并且1毫秒后从线程B触发的事件也将开始执行同一方法，而不会注意到当前该方法正在被其他"进程"执行?

我问这个，因为我想在捕获事件的方法中做一些文件写入，但如果该方法可以同时执行(取决于事件何时被触发)，我想我不能在这里这样做，因为文件上的信息将是两个进程同时写入同一文件之间的混合(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，这将不会编译，只是一个示例，以显示我在做什么:

 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); 
            }
        }
}