add new JsonIgnoreAttribute WhenWriting/WhenReading

[WeihanLi]

feat: add new JsonIgnoreAttribute WhenWriting/WhenReading

fixes #66490

• test cases

[dotnet-issue-labeler]

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, 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.

[dotnet-issue-labeler]

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, 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.

[stephentoub]

@eiriktsarpalis, how does someone specify multiple of these? e.g. if I have a property that I want to be ignored when reading and when writing null, how do I achieve that? Or is that not a scenario that matters?

[eiriktsarpalis]

Not possible, which shows the limits of the JsonIgnoreCondition design. These enums eventually map to the JsonPropertyInfo.ShouldSerialize and JsonPropertyInfo.Set delegates, so a user wanting to introduce their own ignore conditions can configure these via the contract customization APIs.

[eiriktsarpalis]

@WeihanLi thanks for submitting the PR, however since we're currently in the process of finishing up .NET 9 we are no longer merging PRs adding new features. We can revisit this in a couple weeks' time once the .NET 9 branch has been snapped from main.

[eiriktsarpalis]

Apologies for the delay getting back to you, we can continue work on this PR for .NET 10 now.