asp.net导出数据为csv格式有乱码

public static ResultBoolString WriteCSVFile(StringBuilder objBuilder, string strFileName)
    {
        ResultBoolString objResult = new ResultBoolString();
        objResult.IsSign = true;
        StreamWriter rw = new StreamWriter(strFileName,false,Encoding.Default);    //Encoding.Default，这个选项选择Default即可，其它编码都会出问题
        FileInfo objInfo = new FileInfo(strFileName);
        if (!objInfo.Exists)
        {
            objInfo.CreateText();
        }
        try
        {
            rw.Write(objBuilder.ToString());
            rw.Flush();
        }
        catch (Exception e)
        {
            ErrorLayer.LogError(e);
            objResult.IsSign = false;
            objResult.StrSign = e.Message;
        }
        finally
        {
            rw.Close();
        }
        return objResult;
    }