[DllImport("wininet.dll", EntryPoint = "InternetGetConnectedState")]

public extern static bool InternetGetConnectedState(out int conState, int reder);

//参数说明 constate 连接说明 ，reder保留值

public bool IsConnectedToInternet()

{

    int Desc=0;

    return InternetGetConnectedState(out  Desc, 0);

}

private void button2_Click(object sender, EventArgs e)

{

    if (IsConnectedToInternet())

        MessageBox.Show("已连接在网上!","提示");

    else

        MessageBox.Show("未连接在网上!!","提示"); 

}