from : https://developercommunity.visualstudio.com/content/problem/160124/failed-to-load-package-monoandroiddesignerpackage.html
Solution by avatar image Erick J. Gonzalez · 2017年12月11日 19:00
I have the same problem here. The error occurs in VS 2017 15.5.1 as soon as I open VS and click the Toolbox. I have re-installed Xamarin using the VS Installer with no success.
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avatar imageVictor Garcia Aprea [MSFT] · 2017年12月11日 19:13 0
Hi @Erick J. Gonzalez, please don't reinstall VS, that won't help. Follow this:
1) Start VS
2) File->New->Project->Xamarin app (either iOS or Android)
3) Now click the Toolbox
4) Close VS
At this point, VS would have its toolbox cache properly created (in step #3 above) after Xamarin was properly loaded (in step #2). You only need to do this once.
Please let me know if this works for you.
thanks,
-VGA.
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