private void btnExport_Click(object sender, EventArgs e)
        {
            frmNhapKho f = new frmNhapKho();

            f.MdiParent = this.MdiParent;
            f.Show();
            this.Dispose();
        }
        private void btnSave_Click(object sender, EventArgs e)
        {
            //kiêm tra form trước khi save
            string strError = string.Empty;

            if (!isValidateForm(out strError))
            {
                MessageBox.Show(strError, "Cảnh báo", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return;
            }
            else
            {
                Save();
                frmNhapKho f = new frmNhapKho();
                f.MdiParent = this.MdiParent;
                f.Show();
                this.Dispose();
            }
        }