More extensible object and collection converters

[steveharter]

This is the new issue to add a public object\POCO converter. The previous issue was #1562 which was re-purposed for a refactoring effort and subsequently closed.

API proposal to come soon. It is expected to address several issues to make authoring converters for POCOs much easier and also be leveraged by a code-gen (AOT) effort that is in progress.

See 5.0 serializer goals for high-level information on the code-gen effort.

Work

• System.Text.Json serialization order #728
• Support OnXXX methods/attributes for System.Text.Json #39896
• Implement a concept of "Required" properties #29861. Note that OnSerialize callbacks can also do this.
• Equivalent of DefaultContractResolver.CreateProperties override in System.Text.Json  #60518
• System.Text.Json: How to serialize only certain properties without using JsonIgnoreAttribute #593
• Open up metadata infrastructure of System.Text.Json  #34456
• System.Text.Json support to System.Runtime.Serialization #29975
• [API Proposal]: Add more extensive control over ignoring properties #55781
• ReferenceHandler.IgnoreCycles doesn't work with Custom Converters #51715

[steveharter]

Moving to future.

There are three aspects to this issue that overlap, so we want a single, consistent model to cover these:

• Extensible model for object and collections, as an alternative to the existing "value-type" converter model:
  • Leverage the existing object- and collection- converters that were exposed in Serializer support for easier object and collection converters #1562
  • Support before- and after- notifications for (de)serialize.
  • Hook into the existing logic for calling constructors
• Expose metadata (Open up metadata infrastructure of System.Text.Json  #34456).
  • Ability to inspect existing metadata (useful for external tooling such as JSON validation).
  • Ability to change or specify metadata.
  • Ability to specify delegate for setting and getting a property.
  • Metadata includes:
    • For an "object" includes its properties, serialization attributes, converter, and constructor delegate.
    • Metadata for an object "property" includes serialization attributes, converter and delegates to the setter\getter.
    • Metadata for a "collection" includes serialization attributes, converter, element converter and constructor delegate.
• Code-gen effort for performance (startup time, memory) Developers apps using JSON serialization start up and run faster #1568
  • Internally this uses the two items above.