C#读取Excel文件时提示：无法指出的错误

在保存excel文件时总提示对此目录没有操作权限，于是我又在根目录的webconfig里加了一句　<identity   impersonate="true"   />   .
做了这样的修改后，生成的文件可以顺利保存了，但读取外部excel文件时总在xlsconn.open（）这句出错；
后来将根目录webconfig里的    <identity   impersonate="true"   />删除，加到子目录excelfile目录的webconfig里就一切ＯＫ了。

public static DataSet ImportXlsToData(string fileName)
    {
      try
        {
            if (fileName == string.Empty)
            {
                throw new ArgumentNullException("上传文件失败！");
            }
            //
            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;
            //测试是否提取数据
            //this.Datagrid1.DataSource =ds.Tables["m_tableName"];
            //this.Datagrid1.DataBind();
            //将Dataset中数据导入SQL
            //AddDatasetToSQL(ds);

        }
        catch (Exception ex)
        {
            return null;
        }
}
比较简单的做法是根目录的Web.Config文件不加<identity impersonate="true"/>
对有下载文件的页面指定权限：
<location path="forecast/SupplierPlan.aspx">
    <system.web>
      <identity impersonate="true"/>
    </system.web>
  </location>