[DllImport("wininet.dll")]

private extern static bool getInterState(out int conn, int val);

public Form1()  {
InitializeComponent(); 
getNet();
}

//判斷當前是否連接入Internet網絡
private void getNet()
{
int Out;
if (getInterState(out Out, 0) == true)
{
label1.Text = " 網絡已連接！";
}else{
label1.Text = " 網絡未成功連接！";
}
}
}