Sorry, but I don't know how to create a validator for a boolean field if I always need to have a validator assigned.

I can't even assign it to Validator class and use the default "do nothing" field because that field had to change. Since the last example you gave me returned a boolean, the "do nothing" function had to be modified.

So now, it technically does something.

Code:
    public class Validator<T>
    {
        public virtual bool Validate(T data) { return false; }
    }