C# fetching Active Ip address in LAN

Question

C# fetching Active Ip address in LAN, this works fine in console application, wen it comes on form based application i getting errors, iam new to c# pls help. getting a error object reference for non-static field

namespace WindowsFormsApplication1
{

public partial class Form1 : Form
{
    private static List<Ping> pingers = new List<Ping>();
    private static int instances = 0;

    private static object @lock = new object();

    private static int result = 0;
    private static int timeOut = 250;

    private static int ttl = 5;
    public  String IP;


    public Form1()
    {
        InitializeComponent();
    }





    private void button1_Click(object sender, EventArgs e)
            {

                IP = textBox1.Text;



                    string baseIP = "192.168.1.";

                    Console.WriteLine("Pinging 255 destinations of D-class in {0}*", baseIP);

                    CreatePingers(255);

                    PingOptions po = new PingOptions(ttl, true);
                    System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding();
                    byte[] data = enc.GetBytes("abababababababababababababababab");

                    SpinWait wait = new SpinWait();
                    int cnt = 1;

                    Stopwatch watch = Stopwatch.StartNew();

                                foreach (Ping p in pingers)
                                {
                                    lock (@lock)
                                    {
                                        instances += 1;
                                    }

                                    p.SendAsync(string.Concat(baseIP, cnt.ToString()), timeOut, data, po);
                                    cnt += 1;
                                }

                    while (instances > 0)
                    {
                        wait.SpinOnce();
                    }

                    watch.Stop();

                    DestroyPingers();

                    Console.WriteLine("Finished in {0}. Found {1} active IP-addresses.", watch.Elapsed.ToString(), result);
                    Console.ReadKey();  
            }



                        public static void Ping_completed(object s, PingCompletedEventArgs e)

                    {
                        lock (@lock)
                        {
                            instances -= 1;
                        }

                        if (e.Reply.Status == IPStatus.Success)
                        {
                            IP  = e.Reply.Address.ToString();
                            result += 1;
                        }
                        else
                        {
                            //Console.WriteLine(String.Concat("Non-active IP: ", e.Reply.Address.ToString()))
                        }
                    }


                            private static void CreatePingers(int cnt)
                            {
                                for (int i = 1; i <= cnt; i++)
                                {
                                    Ping p = new Ping();
                                    p.PingCompleted += Ping_completed;
                                    pingers.Add(p);
                                }
                            }

                                private static void DestroyPingers()
                                    {
                                    foreach (Ping p in pingers)
                                        {
                                             p.PingCompleted -= Ping_completed;
                                             p.Dispose();
                                        }

                                        pingers.Clear();

                                     }

}

}

Answer 1

You use the IP "variable" as field in your class.

public String IP;

But you cannot access fields in static methods. As far as I can see there is no need for your 3 methods to be static. I guess this is the result of copy/pasting from the console application, where you coded your methods to be static :)

So you need either to:

• Make your methods non-static by removing the static keyword, or
• Make all your fields (that are accessed within static methods) static by adding the static keyword

I'd suggest you to make all members non-static, since they "belong" to the form.

Answer 2

之所以会出现此错误，是因为IP被声明为非静态字段并且正在以静态方法使用。

Answer 3

Finally this code works.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net.NetworkInformation;
using System.Threading;
using System.Diagnostics;
using System.Net;


namespace WindowsFormsApplication3
{
public partial class Form1 : Form
{
    private static List<Ping> pingers = new List<Ping>();
    private static int instances = 0;

    private static object @lock = new object();

    private static int result = 0;
    private static int timeOut = 250;

    private static int ttl = 5;
    private static string[] IPs=new string[100];

    public Form1()
    {
        InitializeComponent();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        string baseIP = "192.168.1.";

        IP.Items.Add("Pinging 255 destinations of D-class in " + baseIP+"xxx");

        CreatePingers(255);


        PingOptions po = new PingOptions(ttl, true);
        System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding();
        byte[] data = enc.GetBytes("abababababababababababababababab");

        SpinWait wait = new SpinWait();
        int cnt = 1;

        Stopwatch watch = Stopwatch.StartNew();

        foreach (Ping p in pingers)
        {
            lock (@lock)
            {
                instances += 1;
            }

            p.SendAsync(string.Concat(baseIP, cnt.ToString()), timeOut, data, po);
            cnt += 1;

        }


    }

    public static void Ping_completed(object s, PingCompletedEventArgs e)
    {
        lock (@lock)
        {
            instances -= 1;
        }

        if (e.Reply.Status == IPStatus.Success)
        {

            Console.WriteLine(string.Concat("Active IP: ", e.Reply.Address.ToString()));

            IPs[result] = e.Reply.Address.ToString();
            result += 1;
        }
        else
        {
            //Console.WriteLine(String.Concat("Non-active IP: ", e.Reply.Address.ToString()))
        }
    }


    private static void CreatePingers(int cnt)
    {
        for (int i = 1; i <= cnt; i++)
        {
            Ping p = new Ping();
            p.PingCompleted += Ping_completed;
            pingers.Add(p);
        }
    }

    private static void DestroyPingers()
    {
        foreach (Ping p in pingers)
        {
            p.PingCompleted -= Ping_completed;
            p.Dispose();
        }

        pingers.Clear();

    }