expression generation problem

Question

Hello ..
I'm having trouble understanding something w/ Linq to Sql ..
Here's a short example that shows my problem:

These both compile, but only the top one works as expected:

 

        static public Expression <Func <Person , bool >> blah()

        {

            DateTime date = DateTime .Parse("10/10/2009" );

            string title = "master of technology" ;

            return (p => p.Titles

                .Any(t => (date == DateTime .MinValue ||

                          (t.StartDate <= date &&

                          (!t.EndDate.HasValue || t.EndDate.Value >= date))) &&

                          t.TitleDescription == title));

        }

        static public Expression <Func <Person , bool >> blah2()

        {

 

            DateTime date = DateTime .Parse("10/10/2009" );

            string title = "master of technology" ;

           

           // Changing this to Expression<Func<Title, bool>> won't compile,

           //   as noted in http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/121ec4e8-ce40-49e0-b715-75a5bd0063dc/?prof=required

            Func <Title, bool > predicate =

                t => (date == DateTime .MinValue ||

                      (t.StartDate <= date &&

                      (!t.EndDate.HasValue || t.EndDate.Value >= date))) &&

                      t.TitleDescription == title;

 

            return (p => p.Titles.Any(predicate);

        }

Later on we do something like:

 

         var people = linqDataContext.People.Select(p => p);

         List <Expression <Func <Person , bool >>> predicates = new List <Expression <Func <Person , bool >>>();

         predicates.Add(blah());

         foreach (Expression <Func <Person , bool >> pred in predicates)

         {

             people = people.Where(pred);

         }

         var peopleList = people.ToList();

In the case of blah2(), everything works as expected, but with blah() we get "NotSupportedException: Unsupported overload used for query operator 'Any'."  How is a different version of "Any()" being used by blah2()?

Any way we can get this to work without using System.Linq.Dynamic (I'd rather keep it type-safe .. otherwise we might as well just create the SQL query ourselves)?

Answer 1

Hi cwa_,

The reason to this problem is that the compiler would generate a subquery when it translates the clause in the keyword 'Any()', and LINQ to SQL can't deal with references to expressions within this subquery.

Well, for you case, I think you have already given a workaround yourself. And if you really want to erase the error, you can use compiled queries to have a try.

For compiled queries, you can refer to:

http://msdn.microsoft.com/en-us/library/bb896297.aspx

---

Best regards,
Charlie Lee

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