v3.1.2: Provide guidance on null in XML. #4612
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There really isn't a native `null` type in XML, as both elements and attributes that are empty have an empty string value. We also need to leave the behavior implementation-defined for compatibility. However, the `xsi:nil` attribute is the closest thing to a `null` element. Attributes are harder, and the best I can come up with is letting `null` behave the same as an omitted attribute for the purpose of serialization.
handrews
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xml
clarification
ralfhandl
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May 19, 2025
ralfhandl
approved these changes
May 19, 2025
lornajane
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May 22, 2025
lornajane
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May 22, 2025
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@handrews Will you create the same change for 3.2? Can we link the PRs together when you do? |
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@lornajane removing JSON examples was just 3.2, although I would not object to removing them in 3.1.2 if folks want. I was thinking of adding this to PR #4592 as otherwise it will be a mess of git conflicts. Alternatively, we could wait for #4592 to merge and then I'll port this (and other XML-related 3.1.2 changes). I was going to do one big 3.1.2->3.2 sync PR with individual commits (like I did last time around for the final sync PRs), but I can do this one separately if needed. |
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Fixes:
There really isn't a native
nulltype in XML, as both elements and attributes that are empty have an empty string value.We also need to leave the behavior implementation-defined for compatibility.
However, attaching the
xsi:nil=trueattribute to an empty element is the closest thing to anullelement.Attributes are harder, and the best I can come up with is letting
nullbehave the same as an omitted attribute for the purpose of serialization. This means that for round-tripping, whether a missing attribute is parsed into anullproperty or as an omitted property depends on whether the schema allowsnull.This attribute behavior is a bit weird, but I can't come up with anything else that has a hope of round-tripping propery, and we can't outright forbid
nullin a patch release. If PR #4592 is accepted, we could forbidtype: [..., "null"]ortype: "null"withxml: {nodeType: "attribute"}and leave the behavior with the deprecatedxml: {attribute: true}unchanged.So I guess the question here is whether the attribute guidance is more confusing than helpful. The fact that we got asked about it suggests that we ought to say something, though.