/// <summary>
/// Modifies an existing trade offer.
/// </summary>
/// <param name="partnerSid">The SteamId64 (ulong) of the person whose offer will be modified.</param>
/// <param name="tradeoffermessage">An otpional message to be sent with the trade offer. Can be null.</param>
/// <param name="serverid">Almost always 1, not quite sure what other numbers do.</param>
/// <param name="tradeofferidCountered">The TradeId of the offer to counter or modify.</param>
/// <param name="offer">A TradeOffer object containing the trade parameters. </param>
/// <param name="container">Auth Cookies MUST be passed here, the function will fail if not.</param>
/// <returns>A SendOfferResponse object.</returns>
public SendOfferResponse ModifyTradeOffer(ulong partnerSid, string tradeoffermessage,
string serverid, uint tradeofferidCountered, TradeOffer offer, CookieContainer container)
{
const string url = "https://steamcommunity.com/tradeoffer/new/send";
container.Add(new Cookie("bCompletedTradeOfferTutorial", "true")
{
Domain = "steamcommunity.com"
});
string sessionid = (from Cookie cookie in container.GetCookies(new Uri("https://steamcommunity.com"))
where cookie.Name == "sessionid"
select cookie.Value).FirstOrDefault();
var data = new Dictionary <string, string>
{
{ "sessionid", sessionid },
{ "serverid", serverid },
{ "partner", partnerSid.ToString() },
{ "tradeoffermessage", tradeoffermessage },
{ "json_tradeoffer", JsonConvert.SerializeObject(offer) },
{ "captcha", string.Empty },
{ "trade_offer_create_params", "{}" },
{ "tradeofferid_countered", tradeofferidCountered.ToString() }
};
return(_web.Fetch(url, "POST", data, container, false,
"https://steamcommunity.com/tradeoffer/" + tradeofferidCountered + "/")
.DeserializeJson <SendOfferResponse>());
}