You can fill combo box in two ways.. either by using the datasource property or by using the for loop to fill.
1st Way - Data source Property
SqlDataAdapter da = new SqlDataAdapter("SELECT ItemName FROM TBItemList", oSqlConn);
DataTable dt = new DataTable();
da.Fill(dt);
cbxparameter.DataSource=dt;
cbxparameter.DisplayMember="ItemName";
cbxparameter.ValueMember="ItemID";
2nd Way - For Loop
SqlDataAdapter da = new SqlDataAdapter("SELECT ItemName FROM TBItemList", oSqlConn);
DataTable dt = new DataTable();
da.Fill(dt);
for(int i=0;i<dt.rows.count;i++)
{
cbxparameter.Items.Add(dt.rows[i]["ItemName"].toString());
}