private void cmbClass_EditValueChanged(object sender, EventArgs e)
{
var dc = new ClassSRMDataContext(Config.connection);
string date = txtDate.Text.Substring(0, 7);
int count = (cmbClass.Properties.DataSource as IList).Count;
if (count > 0)
{
gridControl1.DataSource = dc.SelectTopMonth((int)cmbClass.EditValue, date + "/01", date + "/31");
}
}