If you recently updated Xamarin to the latest stable version in Visual Studio 2015 or Visual Studio 2017, you might have encountered an annoying error when trying to build:
Unable to copy appname.dll from obj to bin because it is being used by another process
This error typically happens the second time that you try to build after you start Visual Studio. The first time everything works OK, then suddenly you cannot build anymore. Cleaning the solution doesn’t help, the only “fix” is to restart Visual Studio which is of course really annoying when you are developing.
This error is known, as shown by the Bugzilla issue #56275. There is also an old forum discussion on the Xamarin forums, which is a bit confusing because it might be a recurring error in the Xamarin updates.
Fortunately, the Bugzilla discussion also carries a workaround. This is a Visual Studio extension that you can install, which should fix the issue. I also talked to Microsoft about this and got confirmation that the error is fixed in a future release of Visual Studio. The engineer who replied to me also gave me a VSIX to fix the issue. I am not entirely sure what the difference is with the one in the Bugzilla issue, so I will just copy it here for your convenience. As far as I can tell, both VSIX files work well.