Server-Side Validation

Let's continue with the application that we built in the previous chapter. To perform a server-side validation, we need to do the following:

1. Add Data Annotation attributes to the ViewModels model class. The input data is validated against this metadata and the model state is updated  automatically.
2. Update the view method to display the validation message for each of the fields. The span tag helper with the asp-validation-for attribute will be used to display the validation error message.
3. Update the controller action method to verify the model state. If the model state is valid, we insert the data into the database. Otherwise, the ViewModel is updated and the view method is rendered again with the validation error message so that the user can update with valid input data and submit the form again.

Updating ViewModels with the Data Annotation Attribute

The Data Annotation attribute defines the validation rules for the properties of the Model/ViewModel. If the input data does...