C# 生成缩略图的代码

private void GetThumbnailImage(string originalPath, string destPath, int thumbnailWidth, int thumbnailHeight, string mode, int left, int top)
{
    System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalPath);

    int originalWidth = originalImage.Width;
    int originalHeight = originalImage.Height;

    switch(mode)
    {
        case "WH"://指定宽高缩放（可能变形）
            break;
        case "W"://指定宽，高按比例
            thumbnailHeight = Convert.ToInt32(originalHeight*((double)thumbnailWidth/originalWidth));
            break;
        case "H"://指定高，宽按比例
            thumbnailWidth = Convert.ToInt32(originalWidth*((double)thumbnailHeight/originalHeight));
            break;
        case "Cut"://指定宽高剪切，不变形
            originalWidth = thumbnailWidth;
            originalHeight = thumbnailHeight;
            break;
    }

    //新建一个bmp图片
    System.Drawing.Bitmap bitmapX = new Bitmap(thumbnailWidth, thumbnailHeight);

    //新建一个画板
    System.Drawing.Graphics graphicsX = System.Drawing.Graphics.FromImage(bitmapX);

    //设置高质量插值法
    graphicsX.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

    //设置高质量，低速度呈现平滑程度
    graphicsX.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

    //清空画布并以透明背景色填充
    graphicsX.Clear(System.Drawing.Color.Transparent);

    //在指定位置并且按指定大小绘制原图片的指定部分
    graphicsX.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, thumbnailWidth, thumbnailHeight), new System.Drawing.Rectangle(left, top, originalWidth, originalHeight), System.Drawing.GraphicsUnit.Pixel);

    try
    {
        bitmapX.Save(destPath,System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    catch(Exception ex)
    {
        throw ex;
    }
    finally
    {
        originalImage.Dispose();
        bitmapX.Dispose();
        graphicsX.Dispose();
    }
}

要得到Image不必保存到文件
uploadFile.PostedFile.SaveAs(path);
System.Drawing.Image imageX = System.Drawing.Image.FromFile(path);
只要这样
imageX = Image.FromStream(uploadFile.PostedFile.InputStream);

要得到Image同样不必保存到文件
bitmapX.Save(destPath, ImageFormat.Jpeg);
imageX = System.Drawing.Image.FromFile(destPath);
只要这样
imageX = (Image)bitmapX;
Bitmap是继承自Image的