json_null_deserialization.cs

#nullable enable
using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

var json = "[" +
           "{ \"name\": \"Credito Padano\", \"type\": null, \"holder_names\":[ \"James\", \"Bond\", null ] }," +
           "{ \"name\": \"Revolut\", \"holder_names\": null }" +
           "]";

JsonSerializerOptions options = new()
{
    // Ignore null values when serializing and deserializing reference types
    IgnoreNullValues = true,
};

var accounts = JsonSerializer.Deserialize<List<Account>>(json, options);

foreach (var account in accounts)
{
    // It is always safe to call account.HolderNames.Count because `null` will not be deserialized into it
    // And the default behavior would be an empty list
    Console.WriteLine($"Account {account.Name} has {account.HolderNames.Count} holder names");
    
    // It is always safe to call ToLower() on the type since null will be ignored and default values is used instead
    Console.WriteLine($"Account {account.Name} is of type: {account.Type.ToLower()}");

    foreach (var holderName in account.HolderNames)
    {
        // `null` however will still deserialized if it is an item in a collection
        // as if it is there it is by design :shrug:
        Console.WriteLine($"-> Name: [{holderName?.ToUpper()}]");
    }
}


public class Account
{
    [JsonPropertyName("name")] 
    public string? Name { get; set; }

    [JsonPropertyName("type")] 
    public string Type { get; set; } = "current";

    [JsonPropertyName("holder_names")] 
    public List<string?> HolderNames { get; set; } = new();
}
#nullable restore

Output is

Account Credito Padano has 3 holder names
Account Credito Padano is of type: current
-> Name: [JAMES]
-> Name: [BOND]
-> Name: []
Account Revolut has 0 holder names
Account Revolut is of type: current