ASP.NET Core – Create custom model validation

There are two ways to do custom model validation in ASP.NET Core:

• A custom attribute subclassed from ValidationAttribute. This is useful when you want to apply custom business logic to a particular model property with an attribute.
• Implementing IValidatableObject for class-level validation. Use this instead when you need to do validation on an entire model at once.

The documentation has examples of both. In your case, IValidatableObject would probably be the best approach.