There shouldn't be any issue with the
txtclass.SelectedValue = (objDataSet.Tables.Rows["Class"]).ToString();
, assuming that txtclass is actually a dropdownlist. Can you check that there are no rogue white spaces at the beginning or end of the list? The values should read <asp:ListItem>Anglo Indian</asp:ListItem>.
Any item you try to select with '.SelectedValue = ' must already be in the list and must be an exact match of the string between the tags