Also, there is a refresh option in the function (according to MSDN). It worked well for me (you should set it before any user agent change). Then the question code could be changed like this:
[DllImport("urlmon.dll", CharSet = CharSet.Ansi)]
private static extern int UrlMkSetSessionOption(
int dwOption, string pBuffer, int dwBufferLength, int dwReserved);
const int URLMON_OPTION_USERAGENT = 0x10000001;
const int URLMON_OPTION_USERAGENT_REFRESH = 0x10000002;
public void ChangeUserAgent()
{
string ua = "Googlebot/2.1 (+http://www.google.com/bot.html)";
UrlMkSetSessionOption(URLMON_OPTION_USERAGENT_REFRESH, null, 0, 0);
UrlMkSetSessionOption(URLMON_OPTION_USERAGENT, ua, ua.Length, 0);
}
http://stackoverflow.com/questions/937573/changing-the-user-agent-of-the-webbrowser-control
