C# 检测网络链接

private const long INTERNET_CONNECTION_MODEM = 1;//Local system uses a modem to connect to the Internet.
private const long INTERNET_CONNECTION_LAN = 2; //Local system uses a local area network to connect to the Internet.
private const long INTERNET_CONNECTION_PROXY = 4;//Local system uses a proxy server to connect to the Internet.
private const long INTERNET_CONNECTION_MODEM_BUSY = 8;   //No longer used.
private const long INTERNET_CONNECTION_CONFIGURED = 64; //Local system has a valid connection to the Internet, but it might or might not be currently connected.
private const long INTERNET_CONNECTION_OFFLINE = 32; // Local system is in offline mode.
private const long INTERNET_RAS_INSTALLED = 16; //Local system has RAS installed.
protected override void OnStartup(StartupEventArgs e)
{
    long lfag;
    string strConnectionDev = "";
    if (InternetGetConnectedState(out lfag, 0))
        strConnectionDev = "网络连接正常!";
    else
        strConnectionDev = "网络连接不可用!";
    if ((lfag & INTERNET_CONNECTION_OFFLINE) > 0)
        strConnectionDev += "OFFLINE 本地系统处于离线模式。";
    if ((lfag & INTERNET_CONNECTION_MODEM) > 0)
        strConnectionDev += "Modem 本地系统使用调制解调器连接到互联网。";
    if ((lfag & INTERNET_CONNECTION_LAN) > 0)
        strConnectionDev += "LAN 本地系统使用的局域网连接到互联网。";
    if ((lfag & INTERNET_CONNECTION_PROXY) > 0)
        strConnectionDev += "a   Proxy";
    if ((lfag & INTERNET_CONNECTION_MODEM_BUSY) > 0)
        strConnectionDev += "Modem   but   modem   is   busy";
    MessageBox.Show(strConnectionDev); 
    base.OnStartup(e);
}

//定义（引用）API函数
[DllImport("wininet.dll")]
public static extern bool InternetGetConnectedState(out long lpdwFlags, long dwReserved);