Question

Why can’t I return as a new custom class (cms.bo.Site) which implements ISite?

public IQueryable<ISite> GetSites()
{
    return (from site in Db.Sites select new cms.bo.Site(site.id, site.name));
}

Answer 1

Basically LINQ to SQL doesn’t know what that constructor will do – it wants to try to convert that into a SQL query, but doesn’t know how. Do you need to be able to add extra bits to the query afterwards? If not, you could do:

public IEnumerable<ISite> GetSites()
{
    return Db.Sites.Select(x => new { x.id, x.name }) // Project in SQL
                   .AsEnumerable() // Do the rest in process
                   .Select(x => new cms.bo.Site(x.id, x.name))
                   .Cast<ISite>(); // Workaround for lack of covariance
}

EDIT: I had missed the variance aspect, and was assuming the query was failing at execution time. It’s definitely worth trying just the call to Cast<ISite>() as per tvanfosson’s answer – but if that doesn’t work, try the above 🙂