How to calculate the total time a user spending on an application?

Question

I want to create an application that able to calculate the total time the user (i.e. myself) spent on a particular application, for example Firefox. And this application should

Answer 1

Here is almost the entire source for a WinForms app that does this. Note that it only adds time when Firefox has focus.

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace FireFoxWatch
{
  public partial class Form1 : Form
  {
    [DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll", SetLastError = true)]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

    private TimeSpan fireFoxElapsedTime = new TimeSpan();

    public Form1()
    {
      InitializeComponent();
    }

    // this handler is called each time the Timer component's interval is reached.
    private void timer1_Tick(object sender, EventArgs e)
    {
      var wnd = GetForegroundWindow();
      uint procId;
      GetWindowThreadProcessId(wnd, out procId);

      var process = Process.GetProcessById((int)procId);
      if (process.ProcessName.Equals("firefox", StringComparison.CurrentCultureIgnoreCase))
        fireFoxElapsedTime += new TimeSpan(0, 0, 0, 0, timer1.Interval);

      //TODO: If fireFoxElapsedTime > Some predetermined TimeSpan, play a sound.
      // Right now it just updates a display label.
      label1.Text = fireFoxElapsedTime.ToString();
    }

    // start the timer when the form loads.
    private void Form1_Load(object sender, EventArgs e)
    {
      timer1.Start();
    }
  }
}

The only part not shown here, is that I made a default WinForms app, then added a "Timer" component from the toolbox, which is named "timer1" in the code, and was the default name.

The above code just updates the total time on a winform label, but you could easily add code to play a sound.