using System;

using System.Reflection;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        Foo foo = new Foo

        {

            Something = "This is something",

            Nothing = null,

            Number = 5,

            Child = new Foo

            {

                Something = "Child Object",

                Nothing = null,

                Number = 6,

                Child = null

            }

        };

        JsonSerializerSettings settings = new JsonSerializerSettings

        {

            ContractResolver = new NullSubstitutionPropertyValueResolver("Nothing to see here"),

            Formatting = Formatting.Indented

        };

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

        Console.WriteLine(json);

    }

}

​

public class Foo

{

    public string Something { get; set; }

    public string Nothing { get; set; }

    public int Number { get; set; }

    public Foo Child { get; set; }

}

​

public class NullSubstitutionPropertyValueResolver : DefaultContractResolver

{

    private readonly string _substitutionValue;

    public NullSubstitutionPropertyValueResolver(string substitutionValue)

    {

        _substitutionValue = substitutionValue;

    }

    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)

    {

        JsonProperty result = base.CreateProperty(member, memberSerialization);

        PropertyInfo property = member as PropertyInfo;

        if (property.PropertyType == typeof(string))

        {

            result.ValueProvider = new StringValueProvider(property, _substitutionValue);

        }

        return result;

    }

}

​

public class StringValueProvider : IValueProvider

{

    private PropertyInfo _targetProperty;

    private string _substitutionValue;

    public StringValueProvider(PropertyInfo targetProperty, string substitutionValue)

    {

        _targetProperty = targetProperty;

        _substitutionValue = substitutionValue;

    }

    // 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, value);

    }

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

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

    // the return value is what gets written to the JSON

    public object GetValue(object target)

    {

        object value = _targetProperty.GetValue(target);

        return value == null ? _substitutionValue : value;

    }

}

​