The
const
keyword is there to explicitly say "this will always have the value I give it now: it cannot be changed by anyone, ever" and the system will enforce that. But looking at your code, why would you need it at all? Change to this:
foreach (string cWeatherID in listBox2.Items)
{
string lURL = cURL + "?" + cWeatherID + "&" + cUnitID + "&format=" + cFormat;
...
}
Or better (if you use a recent version of VS)
foreach (string cWeatherID in listBox2.Items)
{
string lURL = $"{cURL}?{cWeatherID}&{cUnitID}&format={cFormat}";
...
}
And it should start to work.