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

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

//參數說明 constate 連接說明 ，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("已連接在網上!","提示");

    else

        MessageBox.Show("未連接在網上!!","提示"); 

}