当我们把网线插到计算机上时，WINDOWS任务栏的托盘图标都会更改相应的网络图标，拔掉也会有相应的处理。一直都对这个机制感兴趣，却不知道如何做，而且公司的某个产品也需要这么一个功能。昨天在家测试某个程序的时候，发现了其中一个线程的栈中有一个叫wininet!CheckForNetworkChange的函数，IDA分析了WINNET.DLL后，有了本文。

微软在WINDOWS VISTA之后提供了一个叫NLA(Network List Manager API)的接口，用于获取网络状态变化通知的一个接口。以COM技术实现。
主要导出的COM接口如下：

  • IEnumNetworkConnections
  • IEnumNetworks
  • INetwork
  • INetworkConnection
  • INetworkConnectionEvents
  • INetworkEvents
  • INetworkListManager
  • INetworkListManagerEvents

其中INetworkListManager是一个根对象，可以获取计算机是否连接到因特网(INetworkListManager->get_IsConnectedToInternet)。还可以查询有哪些可用的网络和连接.更关键的是INetworkListManagerEvents和INetworkEvents两个类。这两个类在MSDN文档里的描述如下：

is a message sink interface that a client implements to get overall machine state related events.

也就是说我们要自己实现这两个类。而回调的方式是通过COM技术中特有的机制IConnectionPoint来搞定。实现方式如下：

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class CNetworkListManagerEvent : public INetworkListManagerEvents
{
public:
	CNetworkListManagerEvent() : m_ref(1)
	{

	}

	~CNetworkListManagerEvent()
	{

	}

	HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppvObject)
	{
		HRESULT Result = S_OK;
		if (IsEqualIID(riid, IID_IUnknown))
		{
			*ppvObject = (IUnknown*)this;
		}
		else if (IsEqualIID(riid, IID_INetworkListManagerEvents))
		{
			*ppvObject = (INetworkListManagerEvents*)this;
		}
		else
		{
			Result = E_NOINTERFACE;
		}

		return Result;
	}

	ULONG STDMETHODCALLTYPE AddRef()
	{
		return (ULONG)InterlockedIncrement(&m_ref);
	}

	ULONG STDMETHODCALLTYPE Release()
	{
		LONG Result = InterlockedDecrement(&m_ref);
		if (Result == 0)
			delete this;
		return (ULONG)Result;
	}

	virtual HRESULT STDMETHODCALLTYPE ConnectivityChanged(
		/* [in] */ NLM_CONNECTIVITY newConnectivity)
	{
		return S_OK;
	}

private:
	LONG m_ref;
};

int _tmain(int argc, TCHAR** argv, TCHAR** Env)
{
	CoInitialize(NULL);

	//
	//  通过NLA接口获取网络状态
	//
	IUnknown* pUnknown = NULL;

	HRESULT Result = CoCreateInstance(CLSID_NetworkListManager, NULL, CLSCTX_ALL, IID_IUnknown, (void**)&pUnknown);
	if (SUCCEEDED(Result))
	{
		INetworkListManager* pNetworkListManager = NULL;
		Result = pUnknown->QueryInterface(IID_INetworkListManager, (void**)&pNetworkListManager);
		if (SUCCEEDED(Result))
		{
			VARIANT_BOOL IsConnect = VARIANT_FALSE;
			Result = pNetworkListManager->get_IsConnectedToInternet(&IsConnect);
			if (SUCCEEDED(Result))
			{
				printf("IsConnect Result %s\n", IsConnect == VARIANT_TRUE ? "TRUE" : "FALSE");
			}

			IConnectionPointContainer* pCPContainer = NULL;
			Result = pNetworkListManager->QueryInterface(IID_IConnectionPointContainer, (void**)&pCPContainer);
			if (SUCCEEDED(Result))
			{
				IConnectionPoint* pConnectPoint = NULL;
				Result = pCPContainer->FindConnectionPoint(IID_INetworkListManagerEvents, &pConnectPoint);
				if (SUCCEEDED(Result))
				{
					DWORD Cookie = NULL;
					CNetworkListManagerEvent* NetEvent = new CNetworkListManagerEvent;
					Result = pConnectPoint->Advise((IUnknown*)NetEvent, &Cookie);
					if (SUCCEEDED(Result))
					{
						printf("Loop Message\n");
						MSG msg;
						while (GetMessage(&msg, NULL, 0, 0))
						{
							TranslateMessage(&msg);
							DispatchMessage(&msg);

							if (msg.message == WM_QUIT)
							{
								break;
							}
						}

						pConnectPoint->Unadvise(Cookie);

						pConnectPoint->Release();
					}
				}

				pCPContainer->Release();
			}

			pNetworkListManager->Release();
		}

		pUnknown->Release();
	}

	CoUninitialize();
	return 1;
}

因为NLA API是WINDOWS VISTA之后才有的，对于WINDOWS XP是不兼容的，但是WINDOWS XP下有一个方法可以达到同样的效果，可以参考文章Network Awareness in Windows XP，这个文章里的代码是C#的，其中关键的代码转换成 C++:

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void WaitForNetworkChnages()
{
	WSAQUERYSET querySet = { 0 };
	querySet.dwSize = sizeof(WSAQUERYSET);
	querySet.dwNameSpace = NS_NLA;

	HANDLE LookupHandle = NULL;
	WSALookupServiceBegin(&querySet, LUP_RETURN_ALL, &LookupHandle);
	DWORD BytesReturned = 0;
	WSANSPIoctl(LookupHandle, SIO_NSP_NOTIFY_CHANGE, NULL, 0, NULL, 0, &BytesReturned, NULL);
	WSALookupServiceEnd(LookupHandle);
}

void Test()
{
	WSAData data = { 0 };
	WSAStartup(MAKEWORD(2, 0), &data);
	int i = 0;
	while (1)
	{
		printf("BeginWait %d\n", i++);
		WaitForNetworkChnages();
		printf("EndWait\n");
	}

	WSACleanup();
}