CODE EXAMPLE FOR CSHARP

c# return error status code based on exception

public IHttpActionResult GetUser(int userId, DateTime lastModifiedAtClient)
{
    var user = new DataEntities().Users.First(p => p.Id == userId);
    if (user.LastModified <= lastModifiedAtClient)
    {
        return StatusCode(HttpStatusCode.NotModified);
    }
    return Ok(user);
}