Is there a way to ignore get-only properties in Json.NET without using JsonIgnore attributes?

You can do this by implementing a custom IContractResolver and using that during serialization. If you subclass the DefaultContractResolver, this becomes very easy to do:

class WritablePropertiesOnlyResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        IList<JsonProperty> props = base.CreateProperties(type, memberSerialization);
        return props.Where(p => p.Writable).ToList();
    }
}

Here is a test program demonstrating how to use it:

using System;
using System.Collections.Generic;
using System.Linq;
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;

class Program
{
    static void Main(string[] args)
    {
        Widget w = new Widget { Id = 2, Name = "Joe Schmoe" };

        JsonSerializerSettings settings = new JsonSerializerSettings
        {
            ContractResolver = new WritablePropertiesOnlyResolver()
        };

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

        Console.WriteLine(json);
    }
}

class Widget
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string LowerCaseName
    {
        get { return (Name != null ? Name.ToLower() : null); }
    }
}

Here is the output of the above. Notice that the read-only property LowerCaseName is not included in the output.

{"Id":2,"Name":"Joe Schmoe"}