Create a snake-case HTTP response in ASP.NET

HttpResponseExtensions.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace Hylaine.Shared.Extensions
{
    public static class HttpResponseExtensions
    {
        public static async Task<HttpResponseMessage> PostAsJsonAsync<TModel>(this HttpClient client, string requestUrl, TModel model)
        {
            var json = JsonConvert.SerializeObject(model);
            var stringContent = new StringContent(json, Encoding.UTF8, "application/json");
            return await client.PostAsync(requestUrl, stringContent);
        }

        public static string ToSnakeCase(this string input)
        {
            if (string.IsNullOrEmpty(input))
                return input;

            var buffer = "";

            for (var i = 0; i < input.Length; i++)
            {
                var isLast = (i == input.Length - 1);
                var isSecondFromLast = (i == input.Length - 2);

                var curr = input[i];
                var next = !isLast ? input[i + 1] : '\0';
                var afterNext = !isSecondFromLast && !isLast ? input[i + 2] : '\0';

                buffer += char.ToLower(curr);

                if (!char.IsDigit(curr) && char.IsUpper(next))
                {
                    if (char.IsUpper(curr))
                    {
                        if (!isLast && !isSecondFromLast && !char.IsUpper(afterNext))
                            buffer += "_";
                    }
                    else
                        buffer += "_";
                }

                if (!char.IsDigit(curr) && char.IsDigit(next))
                    buffer += "_";
                if (char.IsDigit(curr) && !char.IsDigit(next) && !isLast)
                    buffer += "_";
            }

            return buffer;
        }

        public static async Task<T> ReadSnakeCaseAsAsync<T>(this HttpContent content)
        {
            var settings = new JsonSerializerSettings
            {
                ContractResolver = new SnakeCaseContractResolver(),
                DateFormatHandling = DateFormatHandling.IsoDateFormat
            };
            var json = await content.ReadAsStringAsync();
            var obj = JsonConvert.DeserializeObject<T>(json, settings);
            return obj;
        }
    }

    public class SnakeCaseContractResolver : DefaultContractResolver
    {
        protected override string ResolvePropertyName(string propertyName)
        {
            return propertyName.ToSnakeCase();
        }
    }
}