visualstudio2010-C#cast-输入字符串的格式不正确

在尝试运行以下程序代码时，出现了此错误；

"输入字符串的格式不正确。">

该项目在以下点中断。

int bytesSentSpeed = (int)(interfaceStats.BytesSent - double.Parse(lblBytesSent.Text)) / 1024;

代码；

using System;
using System.Net.NetworkInformation;
using System.Windows.Forms;
namespace InterfaceTrafficWatch
{
    /// <summary>
    /// Network Interface Traffic Watch
    /// by Mohamed Mansour
    /// 
    /// Free to use under GPL open source license!
    /// </summary>
    public partial class MainForm : Form
    {
        /// <summary>
        /// Timer Update (every 1 sec)
        /// </summary>
        private const double timerUpdate = 1000;
        /// <summary>
        /// Interface Storage
        /// </summary>
        private NetworkInterface[] nicArr;
        /// <summary>
        /// Main Timer Object 
        /// (we could use something more efficient such 
        /// as interop calls to HighPerformanceTimers)
        /// </summary>
        private Timer timer;
        /// <summary>
        /// Constructor
        /// </summary>
        public MainForm()
        {
            InitializeComponent();
            InitializeNetworkInterface();
            InitializeTimer();
        }
        /// <summary>
        /// Initialize all network interfaces on this computer
        /// </summary>
        private void InitializeNetworkInterface()
        {
            // Grab all local interfaces to this computer
            nicArr = NetworkInterface.GetAllNetworkInterfaces();
            // Add each interface name to the combo box
            for (int i = 0; i < nicArr.Length; i++)
                cmbInterface.Items.Add(nicArr[i].Name);
            // Change the initial selection to the first interface
            cmbInterface.SelectedIndex = 0;
        }
        /// <summary>
        /// Initialize the Timer
        /// </summary>
        private void InitializeTimer()
        {
            timer = new Timer();
            timer.Interval = (int)timerUpdate;
            timer.Tick += new EventHandler(timer_Tick);
            timer.Start();
        }
        /// <summary>
        /// Update GUI components for the network interfaces
        /// </summary>
        private void UpdateNetworkInterface()
        {
            // Grab NetworkInterface object that describes the current interface
            NetworkInterface nic = nicArr[cmbInterface.SelectedIndex];
            // Grab the stats for that interface
            IPv4InterfaceStatistics interfaceStats = nic.GetIPv4Statistics();
            // Calculate the speed of bytes going in and out
            // NOTE: we could use something faster and more reliable than Windows Forms Tiemr
            //       such as HighPerformanceTimer http://www.m0interactive.com/archives/2006/12/21/high_resolution_timer_in_net_2_0.html
            int bytesSentSpeed = (int)(interfaceStats.BytesSent - double.Parse(lblBytesSent.Text)) / 1024;
            int bytesReceivedSpeed = (int)(interfaceStats.BytesReceived - double.Parse(lblBytesReceived.Text)) / 1024;
            // Update the labels
            lblSpeed.Text = nic.Speed.ToString();
            lblInterfaceType.Text = nic.NetworkInterfaceType.ToString();
            lblSpeed.Text = nic.Speed.ToString();
            lblBytesReceived.Text = interfaceStats.BytesReceived.ToString();
            lblBytesSent.Text = interfaceStats.BytesSent.ToString();
            lblUpload.Text = bytesSentSpeed.ToString() + " KB/s";
            lblDownload.Text = bytesReceivedSpeed.ToString() + " KB/s";
        }
        /// <summary>
        /// The Timer event for each Tick (second) to update the UI
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void timer_Tick(object sender, EventArgs e)
        {
            UpdateNetworkInterface();
        }
    }
}

double bytesSent = 0.0;
if(double.TryParse(lblBytesSent.Text, out bytesSent))
{
    int bytesSentSpeed = (int)(interfaceStats.BytesSent - bytesSent) / 1024;
}