判断计算机是否能够连接到Internet

//判断计算机是否能够连接到Internet的API
//wininet.dll是Windows应用程序网络相关模块。该文件隶属%\WINDOWS\SYSTEM32目录下动态库连接文件。该文件不可缺失，属于关键链接库。当文件丢失或者损坏时，届时将无法完成Explorer进程（即桌面以及基于该进程的IE浏览器等，可使用不基于IE浏览器进行访问网络）
public sealed class NetworkHelper
{
#region 网络状态

/// <summary>
/// 监听计时器
/// </summary>
private Timer listenTimer;

private static NetworkHelper instance;

/// <summary>
/// 监听间隔
/// </summary>
const int LISTEN_TIME_SPAN = 2000;

//IsNetworkAlive Description
const int NETWORK_ALIVE_LAN = 1;
const int NETWORK_ALIVE_WAN = 2;
const int NETWORK_ALIVE_AOL = 4;

const int FLAG_ICC_FORCE_CONNECTION = 1;

private NetworkHelper()
{

}

static NetworkHelper()
{
instance = new NetworkHelper();
}

public static NetworkHelper GetNetworkHelperInstance()
{
return instance;
}

/// <summary>
/// 检查网络是否连通，有延迟
/// </summary>
/// <param name="connectionDescription"></param>
/// <param name="reservedValue"></param>
[DllImport("wininet.dll")]
private extern static bool InternetGetConnectedState(out int connectionDescription, int reservedValue);

/// <summary>
///
/// </summary>
/// <param name="lpszUrl">连接饿地址</param>
/// <param name="dwFlags">FLAG_ICC_FORCE_CONNECTION</param>
/// <param name="dwReserved">0</param>
[DllImport("wininet.dll")]
private extern static bool InternetCheckConnection(string lpszUrl, int dwFlags, int dwReserved);

/// <summary>
/// 检查网络是否连通,需要启动服务
/// </summary>
/// <param name="connectionDescription"></param>
[DllImport("sensapi.dll")]
private extern static bool IsNetworkAlive(out int connectionDescription);

/// <summary>
/// 检查是否能建立Internet连接，VISTA不可用
/// </summary>
/// <param name="dest"></param>
/// <param name="ptr">0</param>
[DllImport("sensapi.dll")]
private extern static bool IsDestinationReachable(string dest, IntPtr ptr);

/// <summary>
/// 互联网是否可用
/// </summary>
/// <returns></returns>
public bool IsInternetAlive()
{
int status;
//检查网络是否可用
if (IsNetworkAlive(out status))
{
//如果WAN可用，检查能否建立连接
//if (status == NETWORK_ALIVE_WAN)
//{
if (InternetCheckConnection("http://www.baidu.com", FLAG_ICC_FORCE_CONNECTION, 0) ||
InternetCheckConnection("http://www.sina.com.cn", FLAG_ICC_FORCE_CONNECTION, 0) ||
InternetCheckConnection("http://www.163.com", FLAG_ICC_FORCE_CONNECTION, 0))
{
return true; //如果能建立连接返回TRUE
}
else
return false;
//}
//else
//    return false;
}
return false;
}

/// <summary>
/// 为NetworkStatusChanged事件处理程序提供数据
/// </summary>
public class NetworkChangedEventArgs : EventArgs
{
public NetworkChangedEventArgs(bool status)
{
IsNetworkAlive = status;
}

public bool IsNetworkAlive
{
get;
private set;
}
}

/// <summary>
/// 表示NetworkStatusChanged事件的方法
/// </summary>
public delegate void NetworkChangedEventHandler(object sender, NetworkChangedEventArgs e);

/// <summary>
/// 网络状态变更时触发的事件
/// </summary>
public event NetworkChangedEventHandler NetworkStatusChanged;

/// <summary>
/// 网络状态变更时触发的事件
/// </summary>
private void OnNetworkStatusChanged(NetworkChangedEventArgs e)
{
if (NetworkStatusChanged != null)
NetworkStatusChanged(this, e);
}

/// <summary>
/// 监听网络状态
/// </summary>
public void ListenNetworkStatus(SynchronizationContext context)
{
//获得当前网络状态，并通知

bool currentStatus = IsInternetAlive();
OnNetworkStatusChanged(new NetworkChangedEventArgs(currentStatus));

//启动监听网络状态，2秒钟检查一次，当状态变更时触发事件
listenTimer = new Timer(delegate
{
bool tmpStatus = IsInternetAlive();
if (currentStatus != tmpStatus)
{
currentStatus = tmpStatus;
context.Post(delegate { OnNetworkStatusChanged(new NetworkChangedEventArgs(currentStatus)); },null);

}
}
, null
, 0
, LISTEN_TIME_SPAN);

}

/// <summary>
/// 停止监听网络状态
/// </summary>
public void CloseListenNetworkStatus()
{
listenTimer.Dispose();
}