Add JsonObject ordering and extension data support

[steveharter]

As a follow-up to #51025 add the features:

• Ability to have JsonObject extension properties via:
  [JsonExtensionData] public JsonObject ExtensionData {get; set;}
  • Done in a linker-friendly way so the source-gen effort will be able to trim JsonNode out.
  • Today only (I)Dictionary<string, JsonElement> is supported as an extension property. Since JsonElement is read-only, it can't be effectively used for modifying extension data, so supporting JsonObject will enable that scenario.
• Deterministic ordering semantics so subsequent Remove() and Add() do not change the ordering of existing items. New items are appended. It is possible now to add methods like "AddBefore(node)" and "AddAfter(node)" if desired.
  • Internally the items are held by a List<> but once a threshold is hit an additional Dictionary<> is created for fast lookup by property name. Once the dictionary is created, both are kept in sync. Enumeration is always done by the list for deterministic ordering.

Minor items:

• Converter refactoring of the System.Object converter to detect whether to use JsonElement or JsonNode. Previously, the JsonNode converter handled this and the System.Object converter only knew about JsonElement. That could cause issues for users that ask for the converter for System.Object.
• Additional test coverage.
• Misc feedback from original PR.

Fixes #51572
(no longer using an IDictionary field; instead Dictionary<> used).

[dotnet-issue-labeler]

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Issue Details

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As a follow-up to #51025 add the features:

• Ability to have extension properties by of type JsonObject of the form:
  [JsonExtensionData] public JsonObject ExtensionData {get; set;}
  • Done in a linker-friendly way so the source-gen effort will be able to trim JsonNode out.
• Deterministic ordering semantics so subsequent Remove() and Add() do not change the ordering of existing items. New items are appended. It is possible now to add methods like "AddBefore(node)" and "AddAfter(node)" if desired.
  • Internally the items are held by a List<> but once a threshold is hit an additional Dictionary<> is created for fast lookup by property name. Once the dictionary is created, both are kept in sync. Enumeration is always done by the list for deterministic ordering.

Minor items:

• Converter refactoring of the System.Object converter to detect whether to use JsonElement or JsonNode. Previously, the JsonNode converter handled this and the System.Object converter only knew about JsonElement. That could cause issues for users that ask for the converter for System.Object.
• Additional test coverage.
• Misc feedback from original PR.

Fixes #51572
(no longer using an IDictionary field; instead Dictionary<> used).

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

[layomia]

Doesn't seem like the dictionary can be null if this method is called.

[eiriktsarpalis]

Also, if this cannot be changed into an assert perhaps rewrite to return _dictionary?.ContainsKey(propertyName);

[steveharter]

_dictionary can be null here whenever the threshold isn't hit yet. E.g.:

JsonObject jObject = JsonNode.Parse("{}").AsObject();
bool exists = jObject.ContainsKey("FOO");