.net 按比例显示图片的缩略图

后台：

foreach (DataListItem item in dlContent.Items)
{
    System.Web.UI.WebControls.Image img = (System.Web.UI.WebControls.Image)item.FindControl("userphoto");

    string fileName = Request.PhysicalApplicationPath + img.ImageUrl.Replace("/", @"\");
    System.Drawing.Bitmap objPic = new System.Drawing.Bitmap(fileName);
    int height = objPic.Height;
    int width = objPic.Width;
    if (height >= width)
        img.Attributes["height"] = "130";
    else
    {
        img.Attributes["width"] = "100";
    }
}
前台：

<asp:Image ID="userphoto" ImageUrl='<%# Eval("filename") %>' runat="server" CssClass="image"/>