New Post: What happens with Copyopacity in version 7

I am using Magick.net to make some changes on user pictures. To Cut out a specific part of the picture I used composite with the compositoperator.Copyopacity it have been working perfectly. Then I upgrade to version 7 and now I have to user other compositoperators I found that the DstIn is something like copyOpacity and is doing my job. But the problem is that when I apply that command the pixels that are in the mask picture (the auxiliary picture which is black with some transparent area, the transparent are the area that I wanted to cut out) and are not 100% black transfers to the final picture so I have a gray and ghostly lines on the final picture. I thought maybe I have to edit the mask picture and remove all the pixels that are not 100 percent black but the problem is that I have to resize the mask in relation to the user uploaded files.
my questions :
How can cut out a part of the user picture and have a picture as my result that has the cut out part and the rest is transparent ?
the technology and framework that I am using: .Net Mvc c# and Magick.net version 7