Threading Method Question

Question

I'm using the following method to show a modeless Message Box.

public void ShowMessageBox(string Message)
{
  var thread = new Thread(
    () =>
    {
      MessageBox.Show(Message);
    });
  thread.Start();
}

The "() => {...}" is something I've never seen before. What is the name for this code pattern?

Also, thread.Start starts the thread, and it automatically closes once the "()=>{...}" method completes (when the Message Box is OK'ed), right? If so, can you please point me to some official documentation saying that the thread closes automatically?

Thanks!

Answer 1

This is called a Lambda Expression. You can read more here.

Answer 2

That is a Lambda. In this case, you're using it to create a new anonymous method that will be run when the new Thread is started.

It's the (near) equivalent of:

public void ShowMessageBox(string Message)
{
    var thread = new Thread(ShowBox);
    thread.Start(Message);
}

public void ShowBox(object message)
{
    MessageBox.Show(message.ToString());
}

Answer 3

Its a statement lambda.

Yes, thread is active as long as this anonymous method is running. Since after MessageBox.Show() there is no other statements, thread will exit, and this must be true... if you are in doubt add this before start:

thread.Name = "LALALA";

And then debug your app. When the message box apear, pause execution, go to Threads View and you will see LALALA running. Click OK and pause again, there should be no "LALALA"... =)

Answer 4

Lambda expression, C# version 3 feature.

Don't use this code. A message box needs a parent window, something it can make sure to be on top of. It can normally find a parent by itself by iterating the windows that were created on the same thread. Not in this case though, there are no other windows, it has to pick the desktop window as the parent.

That will go wrong badly when the user is working in an app window or switches focus to another app, the message box disappears behind the foreground window. There is no obvious way for the user to tell that it is there, she'll just loses sight of it. It could be hours, if not days, before she finds it back. That thread is meanwhile consuming resources badly, you would probably never consider it if you knew that this message box requires a megabyte of memory. In extreme cases, you'll crash the program with OOM.

The common alternative in Windows UI programming is a balloon tooltip provided by a NotifyIcon. Or your own form with the TopMost property set to True so it cannot easily get lost. Also allows you to control the position, important for "non-modal" notifications that should not get in the way. Set that form's ShowWithoutActivation property to true in the form constructor so it doesn't steal the focus.