C#修改图片尺寸以及不改变原有图片比例

public static void image()
        {
            System.Drawing.Image img = System.Drawing.Image.FromFile("图片路径");
            Bitmap bt = new Bitmap(img);
            //获取图片位置颜色
            Color cl = bt.GetPixel(10, 10);
            int Width = img.Width;
            int Height = img.Height;
            int marginx =  Height / 2;
            float dpiX = img.HorizontalResolution;
            float dpiY = img.VerticalResolution;
            //设置新图的大小
            Bitmap bitmap= new Bitmap(Width, Width, PixelFormat.Format24bppRgb);
            //设置位图文件的水平和垂直分辨率，与Img一致
            bitmap.SetResolution(dpiX, dpiY);
            //在位图文件上填充一个新图
            Graphics graphics = Graphics.FromImage(bitmap);
            System.Drawing.Rectangle Rec = new System.Drawing.Rectangle(0, 0, Width, Width);
            //定义颜色
            SolidBrush mySolidBrush = new SolidBrush(cl);
            //将新图填充为获取原图位置的颜色
            graphics.FillRectangle(mySolidBrush, Rec);
            //向新图中填充Img
            graphics.DrawImage(img, 0, marginx, Rec, GraphicsUnit.Pixel);
            graphics.Dispose();
            GC.Collect();
            bitmap.Save("保存图片路径", System.Drawing.Imaging.ImageFormat.Jpeg);
        }