c# - Linq "Could not translate expression... into SQL and could not treat it as a local expression."

Question

I started out with this question, which I sort of answered there, and now I'm asking the more fundamental question here. I've simplified the query down to this:

var q = from ent in LinqUtils.GetTable<Entity>()
        from tel in ent.Telephones.DefaultIfEmpty()
        select new {
          Name = ent.FormattedName,
          Tel = tel != null ? tel.FormattedNumber : "" // this is what causes the error
        };

tel.FormattedNumber is a property that combines the Number and Extension fields into a neatly formatted string. And here's the error that results:

System.InvalidOperationException: Could not translate expression 'Table(Entity).SelectMany(ent => ent.Telephones.DefaultIfEmpty(), (ent, tel) => new <>f__AnonymousType0`2(Name = ent.FormattedName, Tel = IIF((tel != null), tel.FormattedNumber, "")))' into SQL and could not treat it as a local expression.

If I change the reference above from FormattedNumber to just plain Number, everything works fine.

But I do want the formatted number to display nicely in my list. What do you recommend as the neatest, cleanest way of doing so?

Answer 1

You could use AsEnumerable on the entity, but that would force it to bring back all the columns (even if not used); perhaps instead something like:

var q1 = from ent in LinqUtils.GetTable<Entity>()
         from tel in ent.Telephones.DefaultIfEmpty()
         select new {
           Name = ent.FormattedName,
           Number = (tel == null ? null : ent.Number),
           Extension = (tel == null ? null : ent.Extension)
         };

var q2 = from row in q1.AsEnumerable()
         select new {
             row.Name,
             FormattedNumber = FormatNumber(row.Number, row.Extension)
         };

where FormatNumber is some method that takes the two and merges them, presumably re-used from your other (property) code.

With LINQ-to-SQL, another option is to expose a UDF on the data-context that does the formatting inside the database; a slightly different example:

var qry = from cust in ctx.Customers // and tel
          select new {
              cust.Name,
              FormattedNumber = ctx.FormatNumber(tel.Number, tel.Extension)
          };

(which will do the work at the database; whether or not that is a good idea ;-p)