DispatcherTimer vs a regular Timer in WPF app for a task scheduler

Question

Please, explain the difference between "DispatcherTimer" and "a regular Timer" that @Kent Boogaart meant for using in a multithreading WPF app as a task sheduler in this topic:

http://stackoverflow.com/questions/1341829/advice-needed-for-multi-threading-strategy-for-wpf-application

in the commentaries to one of the post (quote):

-If all the DispatcherTimer does is kick off another thread, what is the point of using the DispatcherTimer? ....those threads don't need to be started on the UI thread. You could just use a regular Timer and avoid interrupting the UI altogether

What is "a regular timer" that is meant? How they ("DispatcherTimer" and "a regular Timer") differ regarding their impact on UI?

(Until reading this post I thought about DispatcherTimer as a natural way of using timers in WPF. What is the cases when this is not true?)

Answer 1

DispatcherTimer is the regular timer. It fires its Tick event on the UI thread, you can do anything you want with the UI. System.Timers.Timer is an asynchronous timer, its Elapsed event runs on a thread pool thread. You have to be very careful in your event handler, you are not allowed to touch any UI component. And you'll need to use the lock statement where ever you access class members that are also used on the UI thread.

In the linked thread, the Timer class was more appropriate because the OP was trying to run code asynchronously on purpose.

Answer 2

Regular Timer's Tick event is actually fired on the thread where the Timer was created, so in the tick event, in order to access anything with the UI, you will have to go through dispatcher.begininvoke as mentioned below.

RegularTimer_Tick(object sender, EventArgs e)
{
    txtBox1.Text = "count" + i.ToString(); 
    // error can not access
    // txtBox1.Text property outside dispatcher thread...

    // instead you have to write...
    Dispatcher.BeginInvoke( (Action)delegate(){
       txtBox1.Text = "count " + i.ToString();
    });
}

In case of Dispatcher Timer, you can access UI elements without doing begin invoke or invoke as follow...

DispatcherTimer_Tick(object sender, EventArgs e)
{
    txtBox1.Text = "Count " + i.ToString();
    // no error here..
}

DispatcherTimer just provides convenience over Regular Timer to access UI objects easily.