dev combobox edit 怎么设置让选项清空

//清空客户、料号、物料编码
        private void cbCustomersShort_EditValueChanged(object sender, EventArgs e)
        {            
            if(this.cbCustomers.Text != null )
            {
                cbCustomers.EditValue = null;
                cbCustomers.Properties.Items.Clear();
            }
            if (this.cbObjectID != null)
            {
                cbObjectID.EditValue = null;
                cbObjectID.Properties.Items.Clear();
            }
            if (this.txtCoding != null)
            {
                txtCoding.EditValue = null;         
            }       
        }