public static dynamic getObjectDataRequest(string ControllerName,string ActionName,dynamic returnObj,object Params)
    {
        string URL = getAPIURL();
        string RequestURL = URL + "/" + ControllerName + "/" + ActionName + "?";
        string p = Params.ToString().Replace("{","").Replace("}","").Replace(",", "&");
            RequestURL +=p ;
        RequestURL = RequestURL.Replace(" ", string.Empty);
        HttpWebRequest Request = (HttpWebRequest)WebRequest.Create(RequestURL);


        using (WebResponse Response = Request.GetResponse())
        {
            using (StreamReader Reader = new StreamReader(Response.GetResponseStream()))
            {
                string soapResult = Reader.ReadToEnd();
                JavaScriptSerializer js = new JavaScriptSerializer();
                returnObj = js.Deserialize<dynamic>(soapResult);
            }

        }
        return returnObj;
    }

var obj = new {
            LoginName=UserName,
            Password= Password,
            ApiLoginName=WebAPIUserName,
            ApiPassword= WebAPIPassword,
            ExpiryDays=ExpiryDays
        };
UserDS oUser = new UserDS();
oUser = WebAPIHelper.getObjectDataRequest("User", "LoginUserState",oUser,obj);

Cannot implicitly convert type 'System.Collections.Generic.Dictionary<string,object>' to 'UserDS'