JsonFieldExtractor.cs

using System.Text.Json;

namespace TxFormFieldMapper;

internal static class JsonFieldExtractor
{
    public static IReadOnlyList<string> GetFieldNames(string jsonPath)
    {
        using var stream = File.OpenRead(jsonPath);
        using var document = JsonDocument.Parse(stream);

        var names = new SortedSet<string>(StringComparer.OrdinalIgnoreCase);
        CollectObjectPropertyNames(document.RootElement, parentPath: null, names);

        return names.ToList();
    }

    private static void CollectObjectPropertyNames(JsonElement element, string? parentPath, ISet<string> names)
    {
        switch (element.ValueKind)
        {
            case JsonValueKind.Object:
                foreach (var property in element.EnumerateObject())
                {
                    var path = string.IsNullOrWhiteSpace(parentPath)
                        ? property.Name
                        : $"{parentPath}.{property.Name}";

                    names.Add(path);
                    names.Add(property.Name);
                    CollectObjectPropertyNames(property.Value, path, names);
                }

                break;

            case JsonValueKind.Array:
                foreach (var item in element.EnumerateArray())
                {
                    CollectObjectPropertyNames(item, parentPath, names);
                }

                break;
        }
    }
}