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    static class MacAddressProvider

    {

        const int PING_TIMEOUT = 1000;

        [DllImport("iphlpapi.dll", ExactSpelling = true)]

        static extern int SendARP(int DestIP, int SrcIP, byte[] pMacAddr, ref uint PhyAddrLen);

        // *********************************************************************

        /// <summary>

        /// Gets the MAC address from ARP table in colon (:) separated format.

        /// </summary>

        /// <param name="hostNameOrAddress">Host name or IP address of the

        /// remote host for which MAC address is desired.</param>

        /// <returns>A string containing MAC address; null if MAC address could

        /// not be found.</returns>

        public  static string GetMACAddressFromARP(string hostNameOrAddress)

        {

            if (!IsHostAccessible(hostNameOrAddress))

                return null;

            IPHostEntry hostEntry = Dns.GetHostEntry(hostNameOrAddress);

            if (hostEntry.AddressList.Length == 0)

                return null;   

            byte[] macAddr = new byte[6];

            uint macAddrLen = (uint)macAddr.Length;

            if (SendARP((int)hostEntry.AddressList[0].Address, 0, macAddr, ref macAddrLen) != 0)

                return null;

            StringBuilder macAddressString = new StringBuilder();

            for (int i = 0; i < macAddr.Length; i++)

            {

                if (macAddressString.Length > 0)

                    macAddressString.Append(":");

                macAddressString.AppendFormat("{0:x2}", macAddr[i]);

            }

            return macAddressString.ToString();

        }

        // *********************************************************************

        /// <summary>

        /// Checks to see if the host specified by

        /// <paramref name="hostNameOrAddress"/> is currently accessible.

        /// </summary>

        /// <param name="hostNameOrAddress">Host name or IP address of the

        /// remote host for which MAC address is desired.</param>

        /// <returns><see langword="true" /> if the host is currently accessible;

        /// <see langword="false"/> otherwise.</returns>

        private static bool IsHostAccessible(string hostNameOrAddress)

        {

            Ping ping = new Ping();

            PingReply reply = ping.Send(hostNameOrAddress, PING_TIMEOUT);

            return reply.Status == IPStatus.Success;

        }

    }

 const int WM_COPYDATA = 0x4A;

        public struct COPYDATASTRUCT

        {

            public IntPtr dwData;

            public int cbData;

            [MarshalAs(UnmanagedType.LPStr)]

            public string lpData;

        }

        protected override void WndProc(ref Message m)

        {

            try

            {

                switch (m.Msg)

                {

                    case WM_COPYDATA:

                        COPYDATASTRUCT cds = (COPYDATASTRUCT)m.GetLParam(typeof(COPYDATASTRUCT));

                        MessageBox.Show(cds.lpData);

                        break;

                    default:

                        base.WndProc(ref m);

                        break;

                }

            }

            catch (Exception ex)

            {

                MessageBox.Show(ex.Message);

            }

        }

const int WM_COPYDATA = 0x4A;

        [DllImport("user32.dll", CharSet = CharSet.Auto)]

        public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, uint wParam, ref COPYDATASTRUCT lParam);

        public struct COPYDATASTRUCT

        {

            public IntPtr dwData;

            public int cbData;

            [MarshalAs(UnmanagedType.LPStr)]

            public string lpData;

        }

        private void OnButtonSendClick(object sender, EventArgs e)

        {

            string msg = this.tbMsg.Text.Trim();

            if (string.IsNullOrEmpty(msg))

            {

                MessageBox.Show("메세지를 입력해주세요");

                return;

            }

           Process []pro  =  Process.GetProcessesByName("HowToPostMessage");

           if(pro.Length > 0)

            {

                byte[] buff = System.Text.Encoding.Default.GetBytes(msg);

                COPYDATASTRUCT cds = new COPYDATASTRUCT();

                cds.dwData = IntPtr.Zero;

                cds.cbData = buff.Length+1;

                cds.lpData = msg;

                SendMessage(pro[0].MainWindowHandle, WM_COPYDATA, 0, ref cds);

            }

        }

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