Xamarin: Can not resolve reference: `System.Threading.Tasks.Extensions`, referenced by `MySqlConnector`.

I believe you are using the System.Threading.Tasks.Extensions in the version of 4.5.1 or 4.5.2, you can try to uninstall the 4.5.1 or 4.5.2 and install version 4.5.0 in your Xamarin. Android project.

As of now (14-Jan-2019), It is a workaround before Xamarin solves the root cause of this issue in the newer version.