如何添加css类到html通用控件div？

我这样创建了一个div标签：

I created a div tag like this:

System.Web.UI.HtmlControls.HtmlGenericControl dynDiv = 
    new System.Web.UI.HtmlControls.HtmlGenericControl("DIV");

我在div标签中添加了样式，如下所示：

I added style to the div tag like this:

dynDiv.Style.Add(HtmlTextWriterStyle.BorderStyle, "1px solid #DBE0E4");
dynDiv.Style.Add(HtmlTextWriterStyle.Height, "auto");
dynDiv.Style.Add(HtmlTextWriterStyle.MarginTop, "5px");
dynDiv.Style.Add(HtmlTextWriterStyle.PaddingBottom, "5px");
dynDiv.Style.Add(HtmlTextWriterStyle.Width, "682px");

但我需要通过位于文件夹

But I need to control the style of the div tag through an external css file located at folder ~/css/maincss.css.

如何将css文件应用到这个div？

How can I apply the css in that file to this div?