public string RequiredPostValue(string key)
{
Debug.Assert(_requestInfo.HttpMethod == HttpMethods.Post);
var values = _requestInfo.GetPostDataWhenFormEncoded().GetValues(key);
if(values == null || values.Length != 1)
throw new ApplicationException("The query " + _requestInfo.RawUrl + " should have 1 value for "+key);
return values[0];
}