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); }