How to set custom validation just where Entry.state == EntityState.Added

Question

How to set custom validation that when Entry.state == EntityState.Added is true the below code would work! Custom Validation Code:

Answer 1

@Gert Arnold post is a way,and if you want use attribute,you can find the object from ValidationContext by ObjectInstance property

var user = validationContext.ObjectInstance as User;
if(user==null) return new ValidationResult("...");
using(var db=new SomeDbContext()){
    var has=db.User.Any(x=>x.Id!=user.Id && x.email==value);
    if (has) return new ValidationResult("...");
    else return ValidationResult.Success;
}

when add,the id is 0,so in db,there is not a id 0 record,so if there is 1 record has same email,the any() return true.
when update,you have a id,so x.id!=user.id can except this record self.