How to disallow empty string when deserializing record types with newtonsoft in F#?

Question

Is there any way to force parsing of only non-empty string fields of a record type in F# using Newtonsoft.Json?

#r """Newtonsoft.Json.dll"""

open Newtonsoft.Json

type Customer = {
    Name:           string
    Email:          string
    ContactPhoneNo: string
}

// one or more fields can be empty
let customer = {
    Name =           ""
    Email =          "[email protected]"
    ContactPhoneNo = "+123456789"
}

let serializedCustomer =
    JsonConvert.SerializeObject(customer)

// this parses correctly with the Name field set as ""
// But as the name field is empty, it should not parse it
let deserializedCustomer =
    JsonConvert.DeserializeObject<Customer>(serializedCustomer)

Answer 1

You might want to consider using Newtonsoft's schema support for this, which is in a separate package called Newtonsoft.Json.Schema. You can specify many different kinds of constraints using annotations. For example, to disallow blank names, you can use MinLength:

open System.ComponentModel.DataAnnotations

type Customer = {
    [<MinLength(1)>]
    Name:           string
    Email:          string
    ContactPhoneNo: string
}

Once you've annotated your type, you can generate a schema:

let generator = JSchemaGenerator()
let schema = generator.Generate(typeof<Customer>)

Then use it to validate the serialized JSON:

let jsonCustomer = JObject.Parse(serializedCustomer)
let isValid = jsonCustomer.IsValid(schema)

If you want to skip the overhead of first loading JSON into a JObject in order to validate it, you can use a JSchemaValidatingReader instead:

use strReader = new System.IO.StringReader(serializedCustomer)
use txtReader = new JsonTextReader(strReader)
use vldReader = new JSchemaValidatingReader(txtReader, Schema = schema)
let messages = ResizeArray()
vldReader.ValidationEventHandler.Add(fun args -> messages.Add(args.Message))
let serializer = JsonSerializer()
let deserializedCustomer = serializer.Deserialize<Customer>(vldReader)
printfn "%A" deserializedCustomer
let isValid = (messages.Count = 0)
printfn "%A" isValid

See this documentation for details.