Why would Entity Framework not be able to use ToString() in a LINQ statement?

Question

This works in LINQ-to-SQL:

var customersTest = from c in db.Customers
                select new
                {
                    Id =          


        

Answer 1

Simply put: LINQ to Entities doesn't know about the conversion from your ID type to a string.

What is the type of c.ID? Is there any reason why it's one type for ID, but another for ReferenzId? If at all possible, make them the same type, at which point you won't have a problem any more. I don't know if there are other ways of performing conversions in LINQ to Entities - there may be - but aligning the types would be cleaner.

By the way, this really looks like it's a join:

var query = from c in db.Customers
            join a in db.Addresses on c.Id equals a.ReferenzId into addresses
            select new { Id = c.Id, Addresses = addresses };

EDIT: To respond to your comment - ToString appears in IntelliSense because the compiler has no real idea what your query is going to mean or how it will be translated. It's perfectly valid C#, and can generate a valid expression tree - it's just that EF doesn't know how to convert that expression tree into SQL.

You could try using Convert.ToString(c.Id) instead of just calling c.Id.ToString()...