public ImmutableMessage(Type mappedType, object content, ImmutableAttribute[] attributes, int index)
 {
     MappedType = mappedType;
     Index = index;
     Content = content;
     _attributes = attributes;
 }
 public ImmutableEnvelope(string envelopeId, ImmutableAttribute[] attributes, ImmutableMessage[] items,
     DateTime deliverOnUtc, DateTime createdOnUtc)
 {
     EnvelopeId = envelopeId;
     DeliverOnUtc = deliverOnUtc;
     _attributes = attributes;
     Items = items;
     CreatedOnUtc = createdOnUtc;
 }