public string CalculateSignature(IEnumerable <KeyValuePair <string, string> > items, string appId)
{
if (!IDKeyMap.ContainsKey(appId))
{
throw new KeyNotFoundException($"ApiKey for AppId {appId} not found.");
}
var apiKey = IDKeyMap[appId];
string arguments = items.Where(x => !String.IsNullOrEmpty(x.Value))
.OrderBy(x => x.Key)
.Aggregate(new StringBuilder(),
(sb, next) => sb.Append($"&{next.Key}={next.Value}"),
result => result.Append($"&key={apiKey}").ToString().TrimStart('&'));
var hash = MD5.Create().ComputeHash(Encoding.UTF8.GetBytes(arguments));
var signature = BitConverter.ToString(hash).Replace("-", string.Empty);
return(signature);
}