C# 缩略图（有问题）请大家帮忙，谢谢，我没有分给

希望帮我看看，为什么我以上传图片，生成缩略图，这个图片就被IIS占用了，改怎么修改。求

//aspx.cs文件

Boolean fileOK = false;
String fileExtension = System.IO.Path.GetExtension(FileUpload_img.FileName).ToLower();
String[] allowedExtensions =  { ".jpg" };
for (int i = 0; i < allowedExtensions.Length; i++)
{
    if (fileExtension == allowedExtensions[i])
    {
        fileOK = true;
    }
}
if (fileOK)
{
    try
    {

        string path_BigPic = md5code.UploadFilePic(this.FileUpload_img, ref Path_SmlPic);
        return path_BigPic;
    }
    catch (Exception ex)
    {
    }
}
else
{
    this.RegisterStartupScript("jpg", "<script>alert('格式不正确！暂只支持 jpg 格式')</script>");
    return "-1";
}

********************************

//调用的cs文件

public string UploadFilePic(FileUpload fu, ref string path_xiao)
{

    if (fu == null)
    {
        return "-1";
    }

    string pathdatu = HttpContext.Current.Server.MapPath("../UploadFiles/BigPic/");

 

    //获得客户端的文件物理路径
    string clientFileName = fu.PostedFile.FileName;

    //获得文件的后缀名称
    string extension = System.IO.Path.GetExtension(clientFileName);

 

    //得到一个   年月日+4位随机数   的字符串 作为 图片文件名
    string fileName = DateTime.Now.ToString("yyyyMMddhhmmss") + new Random().Next(1000, 9999).ToString();

    //判断文件夹是否存在
    if (!System.IO.Directory.Exists(pathdatu))
    {
        //创建文件夹
        System.IO.Directory.CreateDirectory(pathdatu);
    }

    if (!System.IO.Directory.Exists(pathdatu))
    {
        //系统异常，如：硬盘空间不够
        //写日志

        return "0";
    }

    string path = pathdatu + fileName + extension;
    fu.PostedFile.SaveAs(path);
   
    System.Drawing.Image image_da = System.Drawing.Image.FromFile(path);

    System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image_da);

    int oWidth = image_da.Width; //原图宽度
    int oHeight = image_da.Height; //原图高度

    int tWidth = 152; //设置缩略图初始宽度
    int tHeight = 152; //设置缩略图初始高度

    //按比例计算出缩略图的宽度和高度
    if (oWidth >= oHeight)
    {
        tHeight = (int)Math.Floor(Convert.ToDouble(oHeight) * (Convert.ToDouble(tWidth) / Convert.ToDouble(oWidth)));
    }
    else
    {
        tWidth = (int)Math.Floor(Convert.ToDouble(oWidth) * (Convert.ToDouble(tHeight) / Convert.ToDouble(oHeight)));
    }

    //生成缩略原图
    Bitmap tImage = new Bitmap(tWidth, tHeight);

    g = Graphics.FromImage(tImage);
    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High; //设置高质量插值法
    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;//设置高质量,低速度呈现平滑程度
    g.Clear(Color.Transparent); //清空画布并以透明背景色填充
    g.DrawImage(image_da, new Rectangle(0, 0, tWidth, tHeight), new Rectangle(0, 0, oWidth, oHeight), GraphicsUnit.Pixel);

    string tname = DateTime.Now.ToShortDateString().Replace("-", "") + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + ".jpg";

    string xiaotupath = HttpContext.Current.Server.MapPath("../UploadFiles/SmlPic/");  //小图

    string tFullName = xiaotupath + "t" + tname; //保存缩略图的物理路径

    try
    {
        //以JPG格式保存图片 ------ 保存小图文件

        tImage.Save(tFullName, System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        //释放资源

        System.GC.Collect();   //关键就在这句(如果不加,上传的图片就会被IIS进程占用)
        fu.Dispose();
        image_da.Dispose();
        g.Dispose();
        tImage.Dispose();
    }

    path_xiao = "../UploadFiles/SmlPic/t" + tname;
    return "../UploadFiles/BigPic/" + fileName + extension;
}