using System;

using System.Collections.Generic;

using System.Linq;

using System.Reflection;

using System.Text;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var myObject = new MyModelClass("blablabla", "<>@%#^^@!%");

        var settings = new JsonSerializerSettings

        {

            ContractResolver = new CustomResolver(),

            Formatting = Formatting.Indented

        };

        string json = JsonConvert.SerializeObject(myObject, settings);

        Console.WriteLine(json);

    }

}

​

​

public class MyModelClass

{

    [JsonProperty("first_field")]

    public string FirstField { get; set; }

​

    [UrlEncode]

    [JsonProperty("second_field")]

    public string SecondField { get; set; }

​

    public MyModelClass(string first, string second)

    {

        FirstField = first;

        SecondField = second;

    }

}

​

​

public class UrlEncodeAttribute : Attribute { }

​

public class CustomResolver : DefaultContractResolver

{

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)

    {

        IList<JsonProperty> props = base.CreateProperties(type, memberSerialization);

​

        // Find all string properties that have a [UrlEncode] attribute applied

        // and attach an UrlEncodingValueProvider instance to them

        foreach (JsonProperty prop in props.Where(p => p.PropertyType == typeof(string)))

        {

            PropertyInfo pi = type.GetProperty(prop.UnderlyingName);

            if (pi != null && pi.GetCustomAttribute(typeof(UrlEncodeAttribute), true) != null)

            {

                prop.ValueProvider = new UrlEncodingValueProvider(pi);

            }

        }

​

        return props;

    }

​

    protected class UrlEncodingValueProvider : IValueProvider

    {

        PropertyInfo targetProperty;

​

        public UrlEncodingValueProvider(PropertyInfo targetProperty)

        {

            this.targetProperty = targetProperty;

        }

​

        // SetValue gets called by Json.Net during deserialization.

        // The value parameter has the original value read from the JSON;

        // target is the object on which to set the value.

        public void SetValue(object target, object value)

        {

            targetProperty.SetValue(target, (string)value);

        }

​

        // GetValue is called by Json.Net during serialization.

        // The target parameter has the object from which to read the string;

        // the return value is the string that gets written to the JSON

        public object GetValue(object target)

        {

            string value = (string)targetProperty.GetValue(target);

            return System.Web.HttpUtility.UrlEncode(value);

        }

    }

}

​

​

​

// System.Web is not accessible in .Net Fiddle, so this is a shim to stand in for the HttpUtility class.

// This is for demonstration purposes only.  Do not use this in production code; use the real System.Web.HttpUtility class instead.

namespace System.Web

{

    public static class HttpUtility

    {

        public static string UrlEncode(string url)

        {

            StringBuilder sb = new StringBuilder();

            if (!string.IsNullOrEmpty(url))

            {

                foreach (char c in url)

                {

                    if (IsUrlSafeChar(c))

                        sb.Append(c);

                    else

                    {

                        sb.Append('%');

                        sb.Append(((int)c).ToString("X2"));

                    }

                }

            }

            return sb.ToString();

        }

        private static bool IsUrlSafeChar(char c)

        {

            char[] safePunct = new char[] { '(', ')', '*', '-', '.', '!', '_' };

            return ((c >= 'a' && c <= 'z') ||

                    (c >= 'A' && c <= 'Z') ||

                    (c >= '0' && c <= '9') ||

                    safePunct.Contains(c));

        }

    }

}