private async Task<Stream> GetStreamAndWriteStartWrapperIfNecessaryAsync(XmlDictionaryWriter writer, object[] parameters, object returnValue) { Stream streamValue = GetStreamValue(parameters, returnValue); if (streamValue == null) throw DiagnosticUtility.ExceptionUtility.ThrowHelperArgumentNull(_partName); if (WrapperName != null) await writer.WriteStartElementAsync(null, WrapperName, WrapperNamespace); await writer.WriteStartElementAsync(null, PartName, PartNamespace); return streamValue; }