#region Custom Column
    public class DataGridViewComboBoxWithNullColumn : DataGridViewComboBoxColumn
    {
        public DataGridViewComboBoxWithNullColumn(): base()
        {
            base.CellTemplate = new DataGridViewComboboxWithNullCell();
        }

        public override DataGridViewCell CellTemplate
        {
            get
            {
                return base.CellTemplate;
            }
            set
            {
                if (value != null && !value.GetType().IsAssignableFrom(typeof(DataGridViewComboboxWithNullCell)))
                {
                    throw new InvalidCastException("Must be a DataGridViewComboboxWithNullCell");
                }
                base.CellTemplate = value;
            }
        }


    }
    #endregion

    #region Custom Cell
    public class DataGridViewComboboxWithNullCell : DataGridViewComboBoxCell
    {
        protected override bool SetValue(int rowIndex, object value)
        {
            if (value == null | Convert.ToString(value) == " " | Convert.ToString(value) == "0")
            {
                value = DBNull.Value;
            }

            return base.SetValue(rowIndex, value);
        }
    }
    #endregion