event on a property

If you just need this internally in the class, then either create a SetCardSerial method or create a property and use the setter.

If you want an event that is accessible outside the class then you will need something like this:

public class YourClass
{
    public event EventHandler CardSerialNumberChanged;

    private string cardSerialNo;

    public string CardSerialNo
    {
        get
        {
            if (cardSerialNo == null)
                return string.Empty;
            else
                return cardSerialNo;
        }
        set
        {
            if (value == string.Empty)
                value = null;
            if (cardSerialNo != value)
            {
                cardSerialNo = value;
                OnCardSerialNumberChanged(EventArgs.Empty);
            }
        }
    }

    protected virtual void OnCardSerialNumberChanged(EventArgs e)
    {
        EventHandler eh = CardSerialNumberChanged;
        if (eh != null) eh(this, e);
    }
}