Add OnSerialize callbacks to POCOs

[steveharter]

Resolves#54528

Does not measurable affect performance; the is IJsonSerializable etc C# calls appear to be the same performance as a standard "if (bool)" check after a few runs.

This does not include the work necessary for source-gen.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

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Resolves #54528

Does not measurable affect performance; the is IJsonSerializable etc C# calls appear to be the same performance as a standard "if (bool)" check after a few runs.

Author:	steveharter
Assignees:	steveharter
Labels:	area-System.Text.Json
Milestone:	6.0.0

[eiriktsarpalis]

What prevents us from supporting callbacks in collection types?

[steveharter]

Nothing (same for value types) although there are very limited scenarios for both since custom converters would normally be used which implicitly have support for before\after.

Also, a richer interaction model would be better for collections - in order to provide support for "Add()" as well as holding onto any temporary collections needed for things like Arrays, Stacks and immutable collections.

[layomia]

nit: is this more clear?

Suggested change

	/// Types that have a custom converter or represent collections or values do not support this behavior.
	/// Types that have a custom converter or represent collections or primitive values do not support this behavior.

[steveharter]

Test failures in mono arm64 Debug appear unrelated:

Work item System.Buffers.Tests
Work item System.Collections.Tests