关于.net FileUpLoad控件的问题

代码如下:

<%@ Page Language="C#" %>
<%@ Import Namespace="System.IO" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

    protected void btnAdd_Click(object sender, EventArgs e)
    {
        if (upImage.HasFile)
        {
            if (CheckFileType(upImage.FileName))
            {
                String filePath = "~/UploadImages/" + upImage.FileName;
                upImage.SaveAs(MapPath(filePath));
            }
        }
    }

    bool CheckFileType(string fileName)
    {
        string ext = Path.GetExtension(fileName);
        switch (ext.ToLower())
        {
            case ".gif":
                return true;
            case ".png":
                return true;
            case ".jpg":
                return true;
            case ".jpeg":
                return true;
            default:
                return false;
        }
    }

    void Page_PreRender()
    {
        string upFolder = MapPath("~/UploadImages/");
        DirectoryInfo dir = new DirectoryInfo(upFolder);
        dlstImages.DataSource = dir.GetFiles();
        dlstImages.DataBind();
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>FileUpload File</title>
</head>
<body>
    <form runat="server">
    <div>

    <asp:Label

        Text="Image File:"
        AssociatedControl
        Runat="server" />

    <asp:FileUpload

        Runat="server" />

    <hr />

    <asp:DataList

        RepeatColumns="3"
        runat="server">
        <ItemTemplate>
        <asp:Image
            ImageUrl='<%# Eval("Name", "~/UploadImages/{0}") %>'

            Runat="server" />
        <br />
        <%# Eval("Name") %>
        </ItemTemplate>
    </asp:DataList>

    </div>
    </form>
</body>
</html>