Disallow null if not option-wrapped everywhere, not just for record fields

[cmeeren]

The document Customizing the serialization format says:

By default, FSharp.SystemTextJson throws an exception when the following conditions are met:

• it is deserializing a record or a union;
• a field's JSON value is null or the field is unspecified;
• that field's type isn't an explicitly nullable F# type (like option, voption and Skippable).

What is the reason for the first requirement (emphasis mine)?

I ask because I was recently bitten by a bug in production. I am deserializing a JSON document to the type MyRootRecord, but when the JSON body itself is null, then the returned MyRootRecord is also null (even though it is a record and F# does not allow null values of it), causing some NullReferenceExecption elsewhere in the code.

I would expect that if I deserialize a top-level null value to a record/union (not wrapped in option or similar), then I get a deserialization exception similar to if it had been a record field.

In other words, the "throw on null" behavior should be governed by the target type, not by whether or not it's a property. (This could therefore also apply to array items, which I haven't tested.)

[Tarmil]

Good point, when the value itself is a record or union, we should be able to handle it. I know that by default System.Text.Json just returns null without calling converters when it encounters a null during deserialization, but it looks like we can override HandleNull in FSharp.STJ's converters to change this.

[cmeeren]

Happy that this is fixed! When will it be released?

[cmeeren]

Hi again, just wondering if it would be possible to create a new release with this change. 🙂 Otherwise I may have to work around it in other ways, which seems unnecessary since this is already implemented here, just not released yet.

[Tarmil]

Hi, sorry I'll have a release out soon 🙂

[Tarmil]

Published in v1.2.

[cmeeren]

Thanks a lot!