public override void OnException(ExceptionContext context)
{
    var exception = context.Exception;
    if (exception == null)// should never happen
    {
        return;
    }
    //todo: log the exception.
    context.Result = new ObjectResult(new { Message =         exception.Message });
    if (exception is KeyNotFoundException)
    {
        context.HttpContext.Response.StatusCode = 404;//Not Found
    }
    else if (exception is ArgumentException || exception is ArgumentNullException)
    {
        context.HttpContext.Response.StatusCode = 400;//Bad Request
    }
    else if (exception is NotImplementedException)
    {
        context.HttpContext.Response.StatusCode = 501; //Not Implemented
        context.Result = new ObjectResult(new { Message = "Not implemented." });
    }
    else if (exception is InvalidOperationException)
    {
        context.HttpContext.Response.StatusCode = 500;
    }
    else if (exception is MemberAccessException)
    {
        context.HttpContext.Response.StatusCode = 401;//Unauthorized
    }
    else// Other exception types you want to handle ... 
    {
context.HttpContext.Response.StatusCode = 500;
context.Result = new ObjectResult(new { Message = "An error has occured." });//Exceptions whose message should not be shown to users.
}
}