Question

I wish to write a windows app which does something when I become disconnected from the internet. I was thinking of writing a very simple C#/Delphi app which simply polls every 20 seconds to see if I’m still connected.

If I have to poll I’d really like a solution other than trying to download a web page from the net. I can’t assume that a download attempt failing means ‘not online’ since there may be other apps eating up the internet bandwidth. Plus I’m sure constantly connecting/downloading from a particular site is going to get my IP blocked.

I’m sure there’s a way to tell if you’re online without downloading/connecting to a remote server but I’m not sure how.

Answer 1

Beware that connected to the Internet does not really mean anything: what if you are connected to your ISP, but the backbone is down, or all the sites you want to access are in a country that went off the grid like recently? Having a connection does not mean you can do what you want.
Anyway, as stated before you can use the InternetGetConnectedState API to test that you have a valid Internet connection configured.
As an example, the following routine told me correctly I had a LAN Connection, but failed to detect that I had my ZoneAlarm firewall set to block ‘All Internet Activity’, which means that you effectively lost all Internet connectivity.

Delphi routine:

procedure IsConnected; var   dwFlags: DWORD; begin   if InternetGetConnectedState(@dwFlags, 0) then   begin     if (dwFlags and INTERNET_CONNECTION_MODEM) = INTERNET_CONNECTION_MODEM  then       ShowMessage('Modem Connection')     else     if (dwFlags and INTERNET_CONNECTION_LAN) = INTERNET_CONNECTION_LAN then       ShowMessage('LAN Connection')     else     if (dwFlags and INTERNET_CONNECTION_PROXY) = INTERNET_CONNECTION_PROXY then       ShowMessage('Connection thru Proxy')     else     if (dwFlags and INTERNET_CONNECTION_OFFLINE) = INTERNET_CONNECTION_OFFLINE then       ShowMessage('Local system in offline mode')     else     if (dwFlags and INTERNET_CONNECTION_CONFIGURED) = INTERNET_CONNECTION_CONFIGURED then       ShowMessage('Valid connection exists, but might or might not be connected')   end   else     ShowMessage('Not Connected. Try to connect and risk of being prompted to dial into another Internet Service Provider.'); end;