In my asp.net application i hava a problem in dropdownlist

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In my asp.net application i hava a problem in dropdownlist
I want to change selected value of dropdown list at the time of editing

my code is

DataSet _DsDetail = new DataSet();
_DsDetail = objTranDetail.GetLedgerTransactionDetail(_LedgerTransID);
_AccountLedgerID = Convert.ToInt32(_DsDetail.Tables[0].Rows[0]["RelatedAccountLedgerID"].ToString());
ddlDebitAccount.SelectedValue = _AccountLedgerID.ToString();



here _AccountLedgerIDget currecteed value but in
ddlDebitAccount.SelectedValue = _AccountLedgerID.ToString();

selectedvalue is not change it contain older value


ex:
if _AccountLedgerID is 8 then ddlDebitAccount.SelectedValue not get 8
pls help me............................