Dealing with JSON

Use the inbuilt extensions to System.Text.Json for handling F# specific types

Bolero uses System.Text.Json and FSharp.SystemTextJson for JSON handling. You are free to use any other JSON library of your choosing if you so wish.


For hosted Blazor WebAssembly & Blazor server apps, Bolero is already configured to use JsonFSharpConverter for remoting by default.

For standalone Blazor WebAssembly app, use either JsonSerializerOptions with JsonFSharpConverter or add JsonFSharpConverterAttribute.

Learn more about the Blazor hosting models.

Using options

Add JsonFSharpConverter to the converters in JsonSerializerOptions, and the format will be applied to all F# types.

open System.Text.Json
open System.Text.Json.Serialization

let options = JsonSerializerOptions()
JsonSerializer.Serialize({| x = "Bolero" |}, options)

Using attributes

Add JsonFSharpConverterAttribute to the type that needs to be serialized.

open System.Text.Json
open System.Text.Json.Serialization

type Example = { x: string }
JsonSerializer.Serialize({x = "Bolero" })

Advantages and disadvantages

The options way is generally recommended because it applies the format to all F# types. In addition to your defined types, this also includes:

  • Types defined in referenced libraries that you can't modify to add an attribute. This includes standard library types such as option and Result.
  • Anonymous records.

The attribute way cannot handle the above cases.

The advantage of the attribute way is that it allows calling Serialize and Deserialize without having to pass options every time. This is particularly useful if you are passing your own data to a library that calls these functions itself and doesn't take options.

Named records

Named record fields are serialized in the order in which they were declared in the type declaration.

type Example = { x: string; y: string }

JsonSerializer.Serialize({ x = "Hello"; y = "world!" })
// --> {"x":"Hello","y":"world!"}
// --> { x = "Hello"; y = "world!" }

Anonymous records

Anonymous record fields are serialized in alphabetical order.

JsonSerializer.Serialize({| x = "Hello"; y = "world!" |}, options)
// --> {"x":"Hello","y":"world!"}

JsonSerializer.Deserialize<{| x : string ; y : string |}>("""{"x":"Hello","y":"World!"}""", options)
// --> {| x = "Hello"; y = "world!" |}

More info

For examples using standard library types such as unions, tuples, option, Map, list & Set, more information is on the FSharp.SystemTextJson project site.