The pain of session variables in LINQ

Linq to Entities rocks. Seriously, but there are a few little bugbears.

One of them is in how errors are reported in a totally misleading manner.

Consider this little snippet of code.


var q=from t in MyEnities.SomeEntitySet
where t.ID==(int)Session["userid']
select t;

if (q.Count() > 0)


q.Count threw an exception that said:

LINQ to Entities does not recognize the method 'System.Object get_Item(System.String)' method, and this method cannot be translated into a store expression.

This drove me mad for hours because I tried everything such as verifying that the session existed, that the variable was real, was of the right type and was of the value I expected. All to no avail.

In the end I solved the problem by first saving the session value into a local variable which I then used in the LINQ expression.

It seems to me that this has to do with the order of how things are evaluated within a LINQ expression and may even constitute a bug in the language. Why? well because it seems obvious now that the direct value from the session variable is somehow used before the cast operation by the LINQ parser and so an exception that seems to have no real relation to actual circumstances is thrown.

I hope that this helps to trap similar errors for people hunting for hours on Google like I did.