System.Windows.Forms.Timer和MessageBox

Question

can someone please let me know why the System.Windows.Forms.Timer continues to show multiple message boxes? I thought that it is on GUI thread ... and therefore after the first messagebox the GUI thread should block. But this is not the case

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();


    }

    int nValue = 0;

    void tmr_Tick(object sender, EventArgs e)
    {
        nValue++;
        MessageBox.Show(nValue.ToString());
    }

    System.Windows.Forms.Timer tmr = new System.Windows.Forms.Timer();

    private void btnStartTimer_Click(object sender, EventArgs e)
    {            
        tmr.Interval = 500;
        tmr.Enabled = true;
        tmr.Tick += new EventHandler(tmr_Tick);
    }
}

Answer 1

The MessageBox.Show() method includes (as all modal dialogs do) a message loop that continues to pump window messages.

Window messages are what allow a window to interact with the user (update itself, accept input, etc.), as well as what allows the Forms.Timer class to work.

If you want your Forms.Timer to stop ticking when the dialog is shown, you need to set the timer's Enabled property to false before you show the dialog.

Answer 2

In your Tick event stop the timer and then start again after MessageBox.Show like:

void tmr_Tick(object sender, EventArgs e)
{
    tmr.Enabled = false;
    nValue++;
    MessageBox.Show(nValue.ToString());
    tmr.Enabled = true; 
}

The reason you are getting repeated MessgeBoxes is because your timer is continuing after showing the first MessageBox.

Answer 3

A message box does not block the GUI-Thread. It's as simple as that. You can interact with the message box, after all :)

Also: The internal workings of the timer are not clear, but I would guess that it runs on another thread and just returns on the GUI-Thread.