Return unique values only in Webapi controller - entity-framework

Question

I have the following line in a WebApi controller;
string Codes = i.Products.FirstOrDefault().Code
As the line states, it gets the code, from the first Product.
But, what I really want it to do is to get all unique codes, and return them as a comma separated string.
So, let's say, there are 6 related products, and they have the following codes:
45
54
45
120
54
45
Right now, the statement just returns "45", given the above data.
But I want the statement above to return "45, 54, 120" (as a string).
How do I do this?
Complete code:
public System.Data.Entity.DbSet<WebAPI.Models.Product> Products { get; set; }
private ApplicationDbContext db = new ApplicationDbContext();
var product = await db.Products.Select(i =>
new ProductDTO()
{
Id = i.Id,
Created = i.Created,
Title = i.Title,
Codes = i.Products.FirstOrDefault().Code
}).SingleOrDefaultAsync(i => i.Id == id);

Answer 1

To convert to a comma separated list:
IEnumerable<string> distinctCodes = i.Products.Select(product => product.Code).Distinct();
return string.Join(",", distinctCodes);
But it's probably better if your controller returns a collection of string instead of a concatenated string.
Edit, after OP code update:
var DBProduct = await db.Products.SingleOrDefaultAsync(i => i.Id == id);
IEnumerable<string> productCodes = DBProduct.Products.Select(p => p.Code).Distinct();
var product = new ProductDTO()
{
Id = DBProduct.Id,
Created = DBProduct.Created,
Title = DBProduct.Title,
Codes = string.Join(",", productCodes)
};

Answer 2

I think Distinctand String.Join works for you.Please try this:
var product = await db.Products.AsEnumerable()//Turn AsEnumarable
.Select(i =>
new ProductDTO()
{
Id = i.Id,
Created = i.Created,
Title = i.Title,
Codes = string.Join(",",
i.Products.Select(l => l.Code).Distinct())
}).SingleOrDefaultAsync(i => i.Id == id);