I am fairly new to LINQ and can't get my head around some inconsistency in behaviour. Any knowledgeable input would be much appreciated. I see similar issues on SO and elsewhere but they don't seem to help.

I have a very simple setup - a company table and an addresses table. Each company can have 0 or more addresses, and if > 0 one must be specified as the main address. I'm trying to handle the cases where there are 0 addresses, using an outer join and altering the select statement accordingly.

Please note I'm currently binding the output straight to a GridView so I would like to keep all processing within the query.

The following DOES work

IQueryable query =
    from comp in context.Companies
    join addr in context.Addresses on comp.CompanyID equals addr.CompanyID into outer   // outer join companies to addresses table to include companies with no address
    from addr in outer.DefaultIfEmpty()
    where (addr.IsMain == null ? true : addr.IsMain) == true    // if a company has no address ensure it is not ruled out by the IsMain condition - default to true if null
    select new {
        comp.CompanyID,
        comp.Name,
        AddressID = (addr.AddressID == null ? -1 : addr.AddressID), // use -1 to represent a company that has no addresses
        MainAddress = String.Format("{0}, {1}, {2} {3} ({4})", addr.Address1, addr.City, addr.Region, addr.PostalCode, addr.Country)
    };

but this displays an empty address in the GridView as ", , ()"

So I updated the MainAddress field to be

MainAddress = (addr.AddressID == null ? "" : String.Format("{0}, {1}, {2} {3} ({4})", addr.Address1, addr.City, addr.Region, addr.PostalCode, addr.Country))

and now I'm getting the Could not translate expression error and a bunch of spewey auto-generated code in the error which means very little to me.

The condition I added to MainAddress is no different to the working condition on AddressID, so can anybody tell me what's going on here?

Any help greatly appreciated.