FluentValidation: how to simplify this code? - ef-fluent-api

Question

I'm using FluentValidation library to create validators like this:
Please any one help me to simplify this part of code?
private bool UniqueSimNo(string simNo)
{
MyContext _db = new MyContext();
Sim s = _db.Sims.Where(x => x.SimNo.ToLower() == simNo.ToLower()).SingleOrDefault();
var sim = _db.Sims.Where(x => x.SimNo.ToLower() == s.SimNo.ToLower()).Where(x => x.Id != s.Id).FirstOrDefault();
if (sim == null) return true;
return false;
}

Answer 1

Well, your code doesn't make sense if you don't pass an id as parameter to your method.
I guess you wanna use the same validator for a new item (id = 0) and an existing (id !=0).
This line doesn't do what you think it is doing after your SingleOrDefault test, as SingleOrDefault would raise an exception if there was more than one item with same SimNo :
var sim = _db.Sims.Where(x => x.SimNo.ToLower() == s.SimNo.ToLower()).Where(x => x.Id != s.Id).FirstOrDefault();
So I would do
private bool UniqueSimNo(string simNo, int id = 0) {
var _db = new MYContext());
return !_db.Sims.Any(x => x.Id != id && x.simNo.ToLower() == simNo.ToLower());
}