Alternative for c# attribute validation

Question

 

 

Server-side validation: Using [Required] attribute it's to validate the text field, but here, checkbox value decides the text fields should exist or not. So, if attributes used for validation, it throws an error.

 

It can be accomplished in client-side. But I have to validate this from server-side.

 

I came through preprocessor directives since it is simply hardcoded, It's not helpful.

 

how to validate text field based on condition (server-side)