Is there a way to hide properties with null values for JsonProvider?

[nikoudel]

This piece of code:

open FSharp.Data

type ColorProvider = JsonProvider<"""
[
  {
    "color": "Red",
    "code": 15
  },
  {
    "color": "Green"
  }
]
""", SampleIsList=true>

let value = ColorProvider.Root(color = "Blue", code = None)

printf "%A" value

Produces this JSON:

{
  "color": "Blue",
  "code": null
}

Can I somehow configure the provider to skip the code property in this case?

[JeffreyPalmer]

Any thoughts on how to address this or work around it? This issues is causing my generated JSON to be rejected by the server I'm integrating with. Thanks!

[github-actions]

🤖 This is an automated response from Repo Assist.

Currently there is no built-in option to omit null fields when serializing optional properties. When a JsonProvider-generated constructor receives None for an optional field, it outputs "field": null in the JSON rather than omitting the field entirely.

Workaround: construct the JSON value manually using JsonValue.Record to include only the fields you want:

open FSharp.Data

let buildColor (color: string) (code: int option) =
    let fields =
        [| yield "color", JsonValue.String color
           match code with
           | Some c -> yield "code", JsonValue.Number(decimal c)
           | None -> () |]
    JsonValue.Record fields

// Usage:
let json = buildColor "Blue" None
printfn "%s" (json.ToString())
// Output: {"color": "Blue"}

This gives you full control over which fields are included. It's more verbose than the generated constructor, but it's the reliable path when you need null-free output for a strict server API.

The underlying limitation is that JsonProvider's generated constructors don't have a "skip null fields" mode. Adding a static parameter like OmitNullFields=true would be a reasonable enhancement — if anyone wants to implement it, contributions are welcome!

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[dsyme]

Repo Assist - implement a PR to add OmitNullFields. Don't forget to update TypeProviderInstantiation tests

[github-actions]

🤖 This is an automated response from Repo Assist.

I've implemented this feature! A draft PR has been created on branch repo-assist/fix-issue-1245-omit-null-fields with the OmitNullFields static parameter for JsonProvider.

How it works:

// Default (backward-compatible): optional fields are serialized as null
type ColorProvider = JsonProvider<"""[{"color": "Red", "code": 15}, {"color": "Green"}]""", SampleIsList = true>
let v1 = ColorProvider.Root(color = "Blue", code = None)
// v1.ToString() → {"color": "Blue", "code": null}

// With OmitNullFields=true: None fields are omitted entirely
type ColorProviderOmit = JsonProvider<"""[{"color": "Red", "code": 15}, {"color": "Green"}]""", SampleIsList = true, OmitNullFields = true>
let v2 = ColorProviderOmit.Root(color = "Blue", code = None)
// v2.ToString() → {"color": "Blue"}

The default remains false for full backward compatibility. All 262 tests pass (3 new tests added specifically for this feature).

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