در زمان افزودن reference جدید به xamarin با خطا مواجه شدیم

باید از روش زیر عمل کنید.مثلا زمان افزودن cardview

It sounds like you might have a corrupt download. Please try deleting the zips folder inside C:UsersusernameAppDataLocalXamarin.

Then, delete the obj/ folder from your solution's project folders, restart Xamarin Studio, and rebuild.

error that can happen ...

Error   11  Please install package: 'Android Support Library' available in SDK installer. Android resource directory C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3embedded./ doesn't exist.    MyProject.Droid
Error   5   Please install package: 'Android Support Library' available in SDK installer. Java library file C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3embeddedclasses.jar doesn't exist.    MyProject.Droid
Error   8   Please install package: 'Android Support Library' available in SDK installer. Java library file C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3embeddedlibs/internal_impl-21.0.3.jar doesn't exist.  MyProject.Droid
Error   4   Reason: C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3android_m2repository_r10.zip is not a valid zip file   MyProject.Droid
Error   7   Reason: C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3android_m2repository_r10.zip is not a valid zip file   MyProject.Droid
Error   10  Reason: C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3android_m2repository_r10.zip is not a valid zip file   MyProject.Droid
Error   3   Unzipping failed. Please download https://dl-ssl.google.com/android/repository/android_m2repository_r10.zip and extract it to the C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3content directory.   MyProject.Droid
Error   6   Unzipping failed. Please download https://dl-ssl.google.com/android/repository/android_m2repository_r10.zip and extract it to the C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3content directory.   MyProject.Droid
Error   9   Unzipping failed. Please download https://dl-ssl.google.com/android/repository/android_m2repository_r10.zip and extract it to the C:UsersdevAppDataLocalXamarinAndroid.Support.v421.0.3content directory.   MyProject.Droid