Question

In LINQ to SQL, I get the exception “The query operator ‘ElementAt’ is not supported.” When trying to use the ElementAt extension method on an IQueryable returned from a LINQ to SQL query.

Here is the stack trace:

at System.Data.Linq.SqlClient.QueryConverter.VisitSequenceOperatorCall(MethodCallExpression mc)
   at System.Data.Linq.SqlClient.QueryConverter.VisitMethodCall(MethodCallExpression mc)
   at System.Data.Linq.SqlClient.QueryConverter.VisitInner(Expression node)
   at System.Data.Linq.SqlClient.QueryConverter.ConvertOuter(Expression node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.Table`1.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.ElementAt[TSource](IQueryable`1 source, Int32 index)

Now I realize to get rid of this exception and to use ElementAt I could call ‘.ToList()’ before using the extension method and it will work. That is fine, but I still don’t like the fact that this is a runtime exception (and what seems like LSP violation).

Is there a reason why these methods cannot be supported? Is it just because they cannot be translated easily into SQL? What other IQueryable/IEnumerable extension methods are not supported, is there a list somewhere?

It would be nice to avoid runtime exceptions.

Answer 1

From MSDN, Standard Query Operator Translation (LINQ to SQL) – this article contains the full list of operators that haven’t been translated:

• TakeWhile , SkipWhile
• Reverse
• Last , LastOrDefault
• ElementAt , ElementAtOrDefault
• DefaultIfEmpty

Operators with No Translation

The following methods are not
translated by LINQ to SQL. The most
common reason is the difference
between unordered multisets and
sequences.

Operators

Rationale

…

ElementAt ,
ElementAtOrDefault

SQL queries operate on multisets, not
on indexable sequences.