Linq Aggregate produces error: "Exception has been thrown by the target of an invocation. // Unhandled Expression Type: 1001"

Question

The following line of code keeps crashing on me with: "Exception has been thrown by the target of an invocation. // Unhandled Expression Type: 1001"

Can anyone tell why just by looking at it?

myList.Aggregate((curmax, x) => (curmax == null || x.LastMonth > curmax.LastMonth ? x : curmax));

What i do get is that curmax is the variable that will be returned and that X is an item being iterated over. but i just can not explain the error.

myList contains a list of a class called: "Period". I am using Linq to Nhibernate. — HerbalMart, Oct 06 '11 at 12:26
But it's not *actually* a `List`, presumably? I suspect this is just something that NHibernate doesn't support. — Jon Skeet, Oct 06 '11 at 12:29

score 1 · Accepted Answer · answered Oct 06 '11 at 12:30

1

I suspect this is just an operation which is unsupported by NHibernate. However, if you're just after "the value with the largest value for LastMonth" you could use:

var latestPeriod = myList.OrderByDescending(x => x.LastMonth)
                         .FirstOrDefault();

That would be considerably simpler, and more likely to be supported.

answered Oct 06 '11 at 12:30

Jon Skeet

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Thanks for your input. I added my own awser also. – HerbalMart Oct 07 '11 at 12:43

score 0 · Answer 2 · edited Oct 07 '11 at 12:45

0

I solved it by calling .Tolist() from Nhiberate. From there on the aggregate extension method from Microsoft Linq is used (in stead of the Nhibernate version) and no more problem.

myList.ToList().Aggregate((curmax, x) => (curmax == null || x.LastMonth > curmax.LastMonth ? x : curmax));

edited Oct 07 '11 at 12:45

Bala R

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answered Oct 07 '11 at 12:43

HerbalMart

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Are you aware that that means you're now pulling down *everything* in `myList` from the database? Does the solution I proposed not work? (It's rather more readable aside from anything else, IMO.) – Jon Skeet Nov 10 '11 at 09:12
Thanks for your comment Jon. At the time i wasn't aware of this. Now... with a better knowledge of concrete classes vs IEnumerables i am. In our case pulling down everything wasn't that bad performance wise. – HerbalMart Mar 28 '12 at 09:18

Linq Aggregate produces error: "Exception has been thrown by the target of an invocation. // Unhandled Expression Type: 1001"

2 Answers2