问IQueryable不工作的扩展方法

EN

public static IQueryable<TSource> ConditionalDefaultEmpty<TSource>(this IQueryable<TSource> source, bool condition)
{
    return condition ? source.DefaultIfEmpty() : source ;
}

var q = from sr in myDb.tblStudentsRegInfos
        from de in myDb.tblDisciplinesEvents.Where(e => sr.Serial == e.tblStudentsRegInfoRef 
                                                     && e.tblStudentsRegInfoRef == studentRegId 
                                                     && (!forStudent || e.PublishOnInternet)
                                                     && (!formDate.HasValue || e.RegDate >= formDate) 
                                                     && (!toDate.HasValue || e.RegDate <= toDate))
                    .ConditionalDefaultEmpty(noPrintAll)
        join dt in myDb.tblDisciplinesTitles on de.tblDisciplinesTitlesRef equals dt.Serial
        where sr.Serial == studentRegId
        group sr by new
                {
...
                }
                into std
                select new
                {....
                };