Getting the error Unable to delete file obj dll warning MSB3061 when trying to build in VS2015 has driven me round the twist on a number of occasions. I generally go through all the obj and bin folders of each of my projects deleting DLLs, and eventually it seems to work itself out.

Well I discovered, a quicker way stop this issue is to jump onto Task Manager (ctrl+shift+esc) and kill any Visual Studio Debug tasks running. Simple!