The type or namespace name 'DataAnnotations' does not exist in the namespace 'System.ComponentModel' (are you missing and assembly reference?)

Fix this with this NuGet command:

Install-Package System.ComponentModel.Annotations

 
 

If you are isolating your POCOs in the core project of an Onion Architecture implementation which will feed an Entity Framework Code First approach in the infrastructure project, you will need to explicitly loop in the annotations for the keys at the .NET Core project for core logic.