JsonSerializationExtensions.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using Utils.Helpers;

namespace YourNamespace
{
    public static class JsonSerializationExtensions
    {
        private static readonly SnakeCaseNamingStrategy _snakeCaseNamingStrategy
            = new SnakeCaseNamingStrategy();

        private static readonly JsonSerializerSettings _snakeCaseSettings = new JsonSerializerSettings
        {
            ContractResolver = new DefaultContractResolver
            {
                NamingStrategy = _snakeCaseNamingStrategy
            }
        };

        public static string ToSnakeCase<T>(this T instance)
        {
            if (instance == null)
            {
                throw new ArgumentNullException(paramName: nameof(instance));
            }

            return JsonConvert.SerializeObject(instance, _snakeCaseSettings);
        }

        public static string ToSnakeCase(this string @string)
        {
            if (@string == null)
            {
                throw new ArgumentNullException(paramName: nameof(@string));
            }

            return _snakeCaseNamingStrategy.GetPropertyName(@string, false);
        }
    }
}