string connStr = "";
            string fileType = System.IO.Path.GetExtension(fileName);
            if (fileType == ".xls")
            {
                connStr = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=" + filePath + ";" + ";Extended Properties=\"Excel 8.0;HDR=YES;IMEX=1\"";
            }
            else
            {
                //connStr = "Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=" + filePath + ";" + ";Extended Properties=\"Excel 12.0;HDR=YES;IMEX=1\"";
                connStr = "Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=" + filePath + ";" + ";Extended Properties='Excel 12.0; HDR=Yes; IMEX=1'";
            }
            string sql_F = "Select * FROM [{0}]";
            OleDbConnection conn = null;
            OleDbDataAdapter da = null;
            DataTable dtSheetName = null;
            DataSet ds = new DataSet();
                conn = new OleDbConnection(connStr);
                conn.Open();
                string sheetName = "";
           dtSheetName= conn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, new object[] { null, null, null, "TABLE" });
           da = new OleDbDataAdapter();
                for (int i = 0; i < dtSheetName.Rows.Count; i++)
            {
                  sheetName = (string)dtSheetName.Rows[i]["TABLE_NAME"];
                    if (sheetName .Contains("$") && !sheetName .Replace("'", "").EndsWith("$"))
                    {
                        continue;
                    }
                    da.SelectCommand=new OleDbCommand(string.Format(sql_F,sheetName),conn);
                    da.Fill(ds, string.Format("ds{0}", i));
                   //  DataSet dsItem = new DataSet();
                   // da.Fill(dsItem);
                   
                   //ds.Tables.Add(dsItem.Tables[0].Copy());

读取Excel的时候，报错，提示是：外部表不是预期的格式，但是我如果打开了该excel文档后，又能正常读取了，这个是什么原因？该怎么解决？