multithreading - c# events execution are thread safe?

Question

I read a lot of event and threads discussion, but all of then focus in "what happen" if I unsuscribe from an event and try to call it later. My question is different...what will happen if I have a process in thread A that fires the event "I finish" in millisecond 1, and also have a process in thread B that fires the event "I finish" in millisecond 2.

Both processes are suscribed to the same method to listen and handle the event. So, C# has to execute the method that handles the event 2 times: 1 time for the event fired in thread A, and 1 time for the event fired from thread B.

What will happen?? Does C# locks the method when the "first event coming from thread A" starts execution of the method that handles the event, and unlock the method when it finish execution, thus allowing other waiting "events" to execute the method content??

Or the event fired from thread A will start execution of the method that handles the event, and 1 millisecond later the event that was fired from thread B will also start execution on the same method wihtout notice that currently the method is being executed by other "process"???

Im asking this, because I want to do some file writing in the method that catch the event, but if the method can be executed simultaneously (depending on when the event is fired), I guess I cannot do it here, since the info on the file will be a mix between 2 processes writing to the same file at the same time (not valid information on the file).

My code looks like this (a little bit long, sorry). please note this will not compile, is just a sample to show what Im doing:

 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# will not doing any locking for you. If the events can be raised by multiple threads simultaneously, you must write code to handle that (if necessary).

You can use a lock statement to prevent multiple threads from executing it:

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

Where lockingObject is a field inside your class declared like:

private readonly object lockingObject = new object();

You also have to be careful about threading in a method that raises an event.

Suppose you have an event in your class called MyEvent. You should not do this:

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

If another thread can detach from MyEvent, then it's possible that it could detach between line {1} and line {2}. If that happens, line {2} will throw a null reference exception because MyEvent will have suddenly become null!

The correct way to do this is:

private void RaiseMyEvent()
{
    var handler = MyEvent;

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

Now the null reference exception can't happen.

However, note that when using multiple threads it's possible for an event handler to get called after the thread has detached it!