Write GIF is diffirent from Read GIF,Why?

Question

i want read a GIF file to stream and then write stream to another place,but i find that my GIF file is diffirent from the old one, it is ugly then the old one, why this is the code System.Drawing.Bitmap TargetBitmap = new System.Drawing.Bitmap(bmp,bmp.Width,bmp.Height); System.Drawing.Graphics bmpGraphics; bmpGraphics = System.Drawing.Graphics.FromImage(TargetBitmap); System.Drawing.Rectangle rect = new System.Drawing.Rectangle(0, 0, bmp.Width,bmp.Height); bmpGraphics.DrawImage(bmp,rect,rect,System.Drawing.GraphicsUnit.Pixel); TargetBitmap.Save(mstream,System.Drawing.Imaging.ImageFormat.Gif); but when i read data from mstream, it is diffirent from the old one, it is not clearly then the old one, why?