Multithreading means executing more than one code block at a time .Usually it is used to create more responsive user interface(User interface that is not freezed up) ,although it can be used for other tasks as well like performing some action while still waiting for a response from web service.

WPF supports a Single Threaded Apartment Model(STA) like the windows forms which introduces following constraints in a WPF application.

1. The thread that creates a WPF UI element owns the elements and other threads can not interact with the UI elements directly,this is known as thread affinity.
 

2. WPF objects that have thread affinity derive from DispatcherObject at some point
   in their class hierarchy

        private void Update()
        {
            // Simulate some work taking place
            Thread.Sleep(TimeSpan.FromSeconds(5));
            txtName.Text = "Here is some new text.";
        }
    }




We can use a dispatcher to update the textbox from a new thread without throwing an exception.

private void Update()
        {
            // Simulate some work taking place
            Thread.Sleep(TimeSpan.FromSeconds(5));
            this.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
                        (ThreadStart)delegate()
                        {
                            txtName.Text = "Here is some new text.";
                        }
                          );
           
        }

The Dispatcher.BeginInvoke() method takes two parameters. The first indicates the priority of the task.The second BeginInvoke() parameter is a delegate that points to the method with the code you want to execute