public class Car
{
    [DataMember(Order = 0)]
    public string CarId { get; set; }
    [DataMember(Order = 1)]
    [Required]
    public string IsIncluded { get; set; }
}

[HttpPatch]
public HttpResponseMessage PatchCar(Car car)
{
    // check if submitted body is valid
    if (!ModelState.IsValid)
    {
        // Something is bad!
    }
}

public class Car
{
    [DataMember(Order = 0)]
    public string CarId { get; set; }
    [DataMember(Order = 1)]
    [Required(Patch = True, Put = False]
    public string IsIncluded { get; set; }
}