[System.Runtime.InteropServices.DllImport("wininet")]

        private extern static bool InternetGetConnectedState(out int connectionDescription, int reservedValue);

       

        private void button1_Click(object sender, EventArgs e)

        {

            int i = 0;

            if (InternetGetConnectedState(out i, 0))

                MessageBox.Show("联网状态");

            else

                MessageBox.Show("你网线掉了啦");

修改下 

       [System.Runtime.InteropServices.DllImport("wininet")]

        private extern static bool InternetGetConnectedState(out int connectionDescription, int reservedValue);

        //判断网络是连接到互联网

        public static bool IsNetWorkConnect()

        {

            int i = 0;

          return InternetGetConnectedState(out i, 0) ? true:false;

        }

          

            if ( IsNetWorkConnect() )

                MessageBox.Show("已连接到互联网");

            else