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ASP.NET Core Optional Json Fields

Also featured in this article is enum to string converter.

ASP.NET Backend

builder.Services.AddControllersWithViews(options => {
            options.Conventions.Add(new RouteTokenTransformerConvention(
                                         new SlugifyParameterTransformer()));

        }).AddJsonOptions(options => {
            options.JsonSerializerOptions.NumberHandling = JsonNumberHandling.AllowReadingFromString;
            options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;               // JSON response is in  camelCase
            options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;  // null not serialized
            options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());                        // enums to string
            //  options.JsonSerializerOptions.IncludeFields = true;  // if you want to [de]serialize fields (i.e. get or set are omitted in the class)

Client Side

internal class Api {
    // Insert variables below here
    protected static HttpClient _client;
    protected static string BASE_URL = "";
    // protected static string BASE_URL = "";
    private static readonly ILogger _logger = typeof(Api).Log();
    private static JsonSerializerOptions _serializerOptions;

    static Api() {
        _client = new HttpClient();
        _serializerOptions = new() {
            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
        _serializerOptions.Converters.Add(new JsonStringEnumConverter());

    public string serializePayload<T>(T value) {
        return JsonSerializer.Serialize(value, _serializerOptions);

    public T deserializePayload<T>(string payload) {
        return JsonSerializer.Deserialize<T>(payload, _serializerOptions);



Since System.Text.Json is only

AlwaysProperty is always ignored.
NeverProperty is always serialized and deserialized, regardless of IgnoreNullValues configuration.
WhenWritingDefaultProperty is ignored only if it equals the default value for its type.
WhenWritingNullProperty is ignored if its value is null. This is applied only to reference-type properties and fields.

For readers in a decade or so, see JsonIgnoreCondition

using System.Text.Json.Serialization;

namespace MyApi.Models;

public enum LoginErrors {

public class StartLoginResponse {
    // this right here:
    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
    public LoginErrors? Error { get; set; }