C# 限制大小 压缩图片的方法

一： 限制大小并压缩图片（压缩图片会不会失真的问题不太清楚，我试过2个图片，肉眼看着都一样，好像没失真，如果有人知道请留言告诉我）

/// <summary>
    /// 生成缩略图
    /// </summary>
    /// <param name="originalImagePath">源图路径（物理路径）</param>
    /// <param name="thumbnailPath">缩略图路径（物理路径）</param>
    /// <param name="width">缩略图宽度</param>
    /// <param name="height">缩略图高度</param>
    /// <param name="mode">生成缩略图的方式</param> 
    public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode, string type)
    {
        System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalImagePath);
        int towidth = width;
        int toheight = height;
        int x = 0;
        int y = 0;
        int ow = originalImage.Width;
        int oh = originalImage.Height;
        switch (mode)
        {
            case "HW"://指定高宽缩放（可能变形） 
                break;
            case "W"://指定宽，高按比例 
                toheight = originalImage.Height * width / originalImage.Width;
                break;
            case "H"://指定高，宽按比例
                towidth = originalImage.Width * height / originalImage.Height;
                break;
            case "Cut"://指定高宽裁减（不变形） 
                if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                {
                    oh = originalImage.Height;
                    ow = originalImage.Height * towidth / toheight;
                    y = 0;
                    x = (originalImage.Width - ow) / 2;
                }
                else
                {
                    ow = originalImage.Width;
                    oh = originalImage.Width * height / towidth;
                    x = 0;
                    y = (originalImage.Height - oh) / 2;
                }
                break;
            case "DB"://等比缩放（不变形，如果高大按高，宽大按宽缩放） 
                if ((double)originalImage.Width / (double)towidth < (double)originalImage.Height / (double)toheight)
                {
                    toheight = height;
                    towidth = originalImage.Width * height / originalImage.Height;
                }
                else
                {
                    towidth = width;
                    toheight = originalImage.Height * width / originalImage.Width;
                }
                break;
            default:
                break;
        }
        //新建一个bmp图片
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
        //新建一个画板
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
        //设置高质量插值法
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
        //设置高质量,低速度呈现平滑程度
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        //清空画布并以透明背景色填充
        g.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
        g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),
        new System.Drawing.Rectangle(x, y, ow, oh),
        System.Drawing.GraphicsUnit.Pixel);
        try
        {
            //保存缩略图
            if (type == "JPG")
            {
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
            if (type == "BMP")
            {
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp);
            }
            if (type == "GIF")
            {
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif);
            }
            if (type == "PNG")
            {
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png);
            }
        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }

二：限制图片大小

  public void SmallPicWidth(string strOldPic, string strNewPic, double intWidth)
    {
        System.Drawing.Bitmap objPic, objNewPic;
        try
        {
            objPic = new System.Drawing.Bitmap(strOldPic);
            double intHeight = (intWidth / Convert.ToDouble(objPic.Width)) * objPic.Height;
            objNewPic = new System.Drawing.Bitmap(objPic, Convert.ToInt32(intWidth), Convert.ToInt32(intHeight));
            objNewPic.Save(strNewPic);
        }
        catch (Exception exp) { throw exp; }
        finally
        {
            objPic = null;
            objNewPic = null;
        }
    }

三：压缩图片（会失真）,可以根据ept = new EncoderParameter(ecd, 50L);中 50L来控制图片质量，越大质量越好。

public void ystp(string filePath, string filePath_ystp) 
    {
        //Bitmap
        Bitmap bmp = null;
        //ImageCoderInfo  
        ImageCodecInfo ici = null;
        //Encoder
        Encoder ecd = null;
        //EncoderParameter
        EncoderParameter ept = null;
        //EncoderParameters
        EncoderParameters eptS = null;
        try
        {
            bmp = new Bitmap(filePath);
            ici = this.getImageCoderInfo("image/jpeg");
            ecd = Encoder.Quality;
            eptS = new EncoderParameters(1);
            ept = new EncoderParameter(ecd, 50L);
            eptS.Param[0] = ept;
            bmp.Save(filePath_ystp, ici, eptS);
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
        finally
        {
            bmp.Dispose();
            ept.Dispose();
            eptS.Dispose();
        }
    }
    private ImageCodecInfo getImageCoderInfo(string coderType)// 获取图片编码类型信息
    {
        ImageCodecInfo[] iciS = ImageCodecInfo.GetImageEncoders();

        ImageCodecInfo retIci = null;

        foreach (ImageCodecInfo ici in iciS)
        {
            if (ici.MimeType.Equals(coderType))
                retIci = ici;
        }

        return retIci;
    }