ISerializable is used to provide custom binary serialization, usually for BinaryFormatter (and perhaps for remoting purposes). Without it, it uses the fields, which can be:
- inefficient; if there are fields that are only used for efficiency at runtime, but can be removed for serialization (for example, a dictionary may look different when serialized)
- inefficient; as even for fields that are needed it needs to include a lot of additional metadata
- invalid; if there are fields that cannot be serialized (such as event delegates, although they can be marked
[NonSerialized]) - brittle; your serialization is now bound to the field names – but fields are meant to be an implementation detail; see also Obfuscation, serialization and automatically implemented properties
By implementing ISerializable you can provide your own binary serialization mechanism. Note that the xml equivalent of this is IXmlSerializable, as used by XmlSerializer etc.
For DTO purposes, BinaryFormatter should be avoided – things like xml (via XmlSerializer or DataContractSerializer) or json are good, as are cross-platform formats like protocol buffers.
For completeness, protobuf-net does include hooks for ISerializable (allowing you to use a portable binary format without writing lots of code), but BinaryFormatter wouldn’t be your first choice here anyway.