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?

Answer 1

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.