Loop to LINQ Conversion -

Question

Ok I have the following, set-up and working great. These lines of code should do a conversion from DAL Entity (Subsonic) to a ViewModel.

    IList<ProductOptionModel> OptionsRetData = new List<ProductOptionModel>();

    foreach (var CurProductOption in this.ProductOptions)
    {
        OptionsRetData.Add(CurProductOption.ToDataModel());
    }

    returnData.Options = OptionsRetData.AsEnumerable();

I'd like to convert this to a LINQ single line statment and came up with the following.

returnData.Options = this.ProductOptions.Select(o => o.ToDataModel());

and am recieving the following error.

Server Error in '/' Application.
Sequence contains no matching element

So why does the first statment work but not the LINQ and, what steps can I take to resolve it.

Stack Trace

at System.Linq.Enumerable.First[TSource](IEnumerable1 source, Func2 predicate) at SubSonic.Extensions.Database.Load[T](IDataReader rdr, T item, List1 ColumnNames) at SubSonic.Extensions.Database.ToEnumerable[T](IDataReader rdr, List1 ColumnNames) at SubSonic.Linq.Structure.DbQueryProvider.Execute[T](QueryCommand1 query, Object[] paramValues) at lambda_method(Closure ) at SubSonic.Linq.Structure.DbQueryProvider.Execute(Expression expression) at SubSonic.Linq.Structure.Query1.GetEnumerator()

Maybe this is to do with subsonic?

I dont think 'select' method throw any error. did you used any 'single' method? — cem, May 25 '10 at 15:19
I also have gotten similar error. Double check if your return type is the same for Options collection and your ProductOptions is not null — Junior Mayhé, May 25 '10 at 15:41

Jon Skeet · Accepted Answer · 2010-05-25T15:36:57.017

7

One possibility is that it's not working because you've changed the time at which the query is materialized. Change the code to this instead:

returnData.Options = this.ProductOptions.Select(o => o.ToDataModel()).ToList();

That will force the query to be evaluated at the same time it was before.

EDIT: Your stack trace is showing First() being called somehow, but we haven't got anything about that in the code you've shown... any ideas where that's happening?

EDIT: I've realised the difference - and I'm foolish for not doing so before. You want to force the projection to be done in-process:

returnData.Options = this.ProductOptions
                         .AsEnumerable()
                         .Select(o => o.ToDataModel())
                         .ToList();

That extra call to AsEnumerable means it'll be the Enumerable.Select overload which gets called, making it equivalent to your original code.

edited May 25 '10 at 15:36

answered May 25 '10 at 15:10

Jon Skeet

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@Jon SKeet - Thanks for the reply. Adding the .ToList() results int he same error. (Just on that line now rather than at the VIEW) – LiamB May 25 '10 at 15:14
@Jon Skeet - Sneeky suspicion that this is to do with SubSonic. As I make no actual call to .First(); – LiamB May 25 '10 at 15:34
@Jon Skeet - There is a reason you have such a high rep. Thanks again, this worked. Is there anywhere I can read up a little more about what you have written? – LiamB May 25 '10 at 15:43
@Pino: its weird. sorry for _bad_ suggestions – cem May 25 '10 at 15:49
@cem dont be sorry it wasnt bad, i appriciate you trying to help! – LiamB May 25 '10 at 16:07

score 2 · Answer 2 · answered May 25 '10 at 15:29

2

As i said you're using First method. you may want to change it to FirstOrDefault. it will be solved. or do you able to change?

Stack Trace

at System.Linq.Enumerable.First

answered May 25 '10 at 15:29

cem

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as I said, I am not directly calling that. This seems to be deep in the subsonic files. Maybe thats causing the issue? – LiamB May 25 '10 at 15:35
yep. its why this happening. edit. maybe using try catch statements'll fix this. did you tried? – cem May 25 '10 at 15:40

score 0 · Answer 3 · answered May 25 '10 at 15:11

0

this.ProductOptions.Select(o => o.ToDataModel()).ToList<ProductOptionModel>();

answered May 25 '10 at 15:11

Kevin Le - Khnle

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Same problem when using the example line. – LiamB May 25 '10 at 15:15

Kevin Le - Khnle · Answer 4 · 2010-05-25T15:27:52.727

-1

I think you have to check for the length of this.ProductOptions before the LINQ statement.

So maybe (revised without check for null):

returnData.Options = (this.ProductOptions.Length > 0) ? this.ProductOptions.Select(o => o.ToDataModel()) : new List<ProductOptionModel>().AsEnumerable();

edited May 25 '10 at 15:27

answered May 25 '10 at 15:22

Kevin Le - Khnle

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Maybe no need to check for null. The loop works because with length 0, the flow of the program never enters the loop. – Kevin Le - Khnle May 25 '10 at 15:26
Select shouldn't fail if it doesn't get any values though... it should just yield an empty result. – Jon Skeet May 25 '10 at 15:28

Loop to LINQ Conversion -

4 Answers4