C#讀取Excel檔案時提示：無法指出的錯誤

在儲存excel檔案時總提示對此目錄沒有操作許可權，於是我又在根目錄的webconfig裡加了一句　<identity   impersonate="true"   />   .
做了這樣的修改後，產生的檔案可以順利儲存了，但讀取外部excel檔案時總在xlsconn.open（）這句出錯；
後來將根目錄webconfig裡的    <identity   impersonate="true"   />刪除，加到子目錄excelfile目錄的webconfig裡就一切ＯＫ了。

public static DataSet ImportXlsToData(string fileName)
    {
      try
        {
            if (fileName == string.Empty)
            {
                throw new ArgumentNullException("上傳檔案失敗！");
            }
            //
            string oleDBConnString = String.Empty;
            oleDBConnString = "Provider=Microsoft.Jet.OLEDB.4.0;";
            oleDBConnString += "Data Source=";
            oleDBConnString += fileName;
            oleDBConnString += ";Extended Properties=Excel 8.0;";
            //
            OleDbConnection oleDBConn = null;
            OleDbDataAdapter oleAdMaster = null;
            System.Data.DataTable m_tableName = new System.Data.DataTable();
            DataSet ds = new DataSet();

            try
            {
                oleDBConn = new OleDbConnection(oleDBConnString);
                oleDBConn.Open();
                m_tableName = oleDBConn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);

                if (m_tableName != null && m_tableName.Rows.Count > 0)
                {
                    m_tableName.TableName = m_tableName.Rows[0]["TABLE_NAME"].ToString();
                }
                string sqlMaster;
                sqlMaster = " SELECT *  FROM [" + m_tableName.TableName + "]";
                oleAdMaster = new OleDbDataAdapter(sqlMaster, oleDBConn);
                oleAdMaster.Fill(ds, "m_tableName");
                oleAdMaster.Dispose();
                oleDBConn.Close();
                oleDBConn.Dispose();
            }
            catch (Exception ex)
            {
                ErrorLog.AddLog(ex);
                return null;
            }

            return ds;
            //測試是否提取資料
            //this.Datagrid1.DataSource =ds.Tables["m_tableName"];
            //this.Datagrid1.DataBind();
            //將Dataset中資料匯入SQL
            //AddDatasetToSQL(ds);

        }
        catch (Exception ex)
        {
            return null;
        }
}
比較簡單的做法是根目錄的Web.Config檔案不加<identity impersonate="true"/>
對有下載檔案的頁面指定許可權：
<location path="forecast/SupplierPlan.aspx">
    <system.web>
      <identity impersonate="true"/>
    </system.web>
  </location>