How to default a null JSON property to an empty array during serialization with a List<T> property in JSON.NET?

You could always lazy load an empty list if its null.

OR

Use the NullValueHandling option on the JsonDeserializer.

var settings = new JsonSerializerSettings();
settings.NullValueHandling = NullValueHandling.Ignore;

return JsonConvert.DeserializeObject<T>(json, settings);

http://james.newtonking.com/json/help/index.html?topic=html/SerializationSettings.htm

I was going to suggest using a custom JsonConverter to solve this, but a converter will not get called for null values. Instead, you will need to use a custom IContractResolver in combination with a custom IValueProvider. Here is the code you would need (inspired by this answer):

class NullToEmptyListResolver : DefaultContractResolver
{
    protected override IValueProvider CreateMemberValueProvider(MemberInfo member)
    {
        IValueProvider provider = base.CreateMemberValueProvider(member);

        if (member.MemberType == MemberTypes.Property)
        {
            Type propType = ((PropertyInfo)member).PropertyType;
            if (propType.IsGenericType && 
                propType.GetGenericTypeDefinition() == typeof(List<>))
            {
                return new EmptyListValueProvider(provider, propType);
            }
        }

        return provider;
    }

    class EmptyListValueProvider : IValueProvider
    {
        private IValueProvider innerProvider;
        private object defaultValue;

        public EmptyListValueProvider(IValueProvider innerProvider, Type listType)
        {
            this.innerProvider = innerProvider;
            defaultValue = Activator.CreateInstance(listType);
        }

        public void SetValue(object target, object value)
        {
            innerProvider.SetValue(target, value ?? defaultValue);
        }

        public object GetValue(object target)
        {
            return innerProvider.GetValue(target) ?? defaultValue;
        }
    }
}

Here is a demo which shows how to use the resolver:

class Program
{
    static void Main(string[] args)
    {
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.ContractResolver = new NullToEmptyListResolver();
        settings.ObjectCreationHandling = ObjectCreationHandling.Replace;
        settings.Formatting = Formatting.Indented;

        Console.WriteLine("Serializing object with null lists...");
        Foo foo = new Foo();
        string json = JsonConvert.SerializeObject(foo, settings);
        Console.WriteLine(json);
        Console.WriteLine();

        Console.WriteLine("Deserializing JSON with null lists...");
        json = @"{ ""IntList"" : null, ""StringList"" : null }";
        foo = JsonConvert.DeserializeObject<Foo>(json, settings);
        Console.WriteLine("IntList size: " + foo.IntList.Count);
        Console.WriteLine("StringList size: " + foo.StringList.Count);
    }
}

class Foo
{
    public List<int> IntList { get; set; }
    public List<string> StringList { get; set; }
}

Output:

Serializing object with null lists...
{
  "IntList": [],
  "StringList": []
}

Deserializing JSON with null lists...
IntList size: 0
StringList size: 0

The following property will have empty collection assigned to it after deserialization instead of null in both cases: when the property is omitted in JSON or when set to null explicitly:

class A
{
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public IEnumerable<int> Prop { get; set; } = new List<int>();
}