Definition

Action<int> example1 =
   (int x) => Console.WriteLine("Write {0}", x);
Action<int, int> example2 =
   (x, y) => Console.WriteLine("Write {0} and {1}", x, y);
Action example3 =  
   () => Console.WriteLine("Done");

using System.Windows.Threading;  // in WindowsBase.dll
 
public partial class MyWindow : Window
{
  public MyWindow()
  {
    InitializeComponent();
    new Thread (Work).Start();
  }
 
  void Work()
  {
    Thread.Sleep (5000);           // Simulate time-consuming task
    UpdateMessage ("The answer");
  }
 
  void UpdateMessage (string message)
  {
    Action action = () => txtMessage.Text = message;
    Dispatcher.Invoke (action);
  }
}

Source:

• Threading (Part 2): Rich Client Applications and Thread Affinity

• DotNetPerls: Action
• MSDN: Action of Delete
• StackOverflow: Use of Action Delete in C#
• WPF UI Update from Background Threads