using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Windows;
using System.Windows.Input;

namespace WpfApplication16
{
    public partial class MainWindow : Window
    {
        private readonly Dictionary<Key, DateTime> _pressed;
        public MainWindow()
        {
            InitializeComponent();
            KeyDown += MainWindow_KeyDown;
            KeyUp += MainWindow_KeyUp;
            _pressed = new Dictionary<Key, DateTime>();
        }

        void MainWindow_KeyUp(object sender, KeyEventArgs e)
        {
            if (_pressed.ContainsKey(e.Key))
            {
                Debug.Print("Key '{0}' has been pressed about {1} seconds", 
                    e.Key,
                    (DateTime.Now - _pressed[e.Key]).TotalSeconds);
                _pressed.Remove(e.Key);
            }
        }

        void MainWindow_KeyDown(object sender, KeyEventArgs e)
        {
            if (!_pressed.ContainsKey(e.Key))
                _pressed[e.Key] = DateTime.Now;
        }
    }
}