NewtonSoft add JSONIGNORE at runTime

Based on @Underscore post above, I created a list of properties to exclude on serialization.

public class DynamicContractResolver : DefaultContractResolver {
    private readonly string[] props;

    public DynamicContractResolver(params string[] prop) {
        this.props = prop;
    }

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization) {
        IList<JsonProperty> retval = base.CreateProperties(type, memberSerialization);

        // return all the properties which are not in the ignore list
        retval = retval.Where(p => !this.props.Contains(p.PropertyName)).ToList();

        return retval;
    }
}

Use:

string json = JsonConvert.SerializeObject(car, Formatting.Indented, 
    new JsonSerializerSettings { ContractResolver = new DynamicContractResolver("ID", "CreatedAt", "LastModified") });

No need to do the complicated stuff explained in the other answer.

NewtonSoft JSON has a built-in feature for that:

public bool ShouldSerializeINSERT_YOUR_PROPERTY_NAME_HERE()
{
    if(someCondition){
        return true;
    }else{
        return false;
    }
}

It is called "conditional property serialization" and the documentation can be found here.

Warning: first of all, it is important to get rid of [JsonIgnore] above your {get;set;} property. Otherwise it will overwrite the ShouldSerializeXYZ behavior.

I think it would be best to use a custom IContractResolver to achieve this:

public class DynamicContractResolver : DefaultContractResolver
{
    private readonly string _propertyNameToExclude;

    public DynamicContractResolver(string propertyNameToExclude)
    {
        _propertyNameToExclude = propertyNameToExclude;
    }

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        IList<JsonProperty> properties = base.CreateProperties(type, memberSerialization);

        // only serializer properties that are not named after the specified property.
        properties =
            properties.Where(p => string.Compare(p.PropertyName, _propertyNameToExclude, true) != 0).ToList();

        return properties;
    }
}

The LINQ may not be correct, I haven't had a chance to test this. You can then use it as follows:

string json = JsonConvert.SerializeObject(car, Formatting.Indented,
   new JsonSerializerSettings { ContractResolver = new DynamicContractResolver("LastModified") });

Refer to the documentation for more information.