LINQ query to pull a single record from a collection still requires enumercation over results

I wrote a LINQ query in Visual Basic .Net 2008 in order to pull a single record from a Generic List collection. The intent was to use LINQ as an easier alternative to writing a Predicate function for the .Find method of the collection. The code is below. I made what I believe will be a common LINQ mistake. Below is the explanation of the errors I saw, the code that produced those errors, and then the proper way to do it.

A symptom of the problem is that the LINQ query returned a SelectIterator object and in order to see the string I had to enumerate (For Each) over the result in order to obtain the data. I knew that the WHERE condition was sufficient to return only a single result, but LINQ did not so it returned a SelectIterator to be enable me to enumerate through the result set.

If I tried to assign the LINQ result directly to a string, I get the error message "Conversion from type 'd__d(Of MyDataType,String)' to type 'String' is not valid.".

The incorrect code is:

Dim Result as string = string.empty
Dim Filename = From item In myCollection _
Where item.key.ToLower = MenuPath.ToLower _
Select item.MyFilename
For Each s As String In Filename : Result = s : Next
Console.WriteLine("Returned String : " + s)

The correct code is achieved by putting parenthesis around the LINQ query and using the .Single method to inform LINQ that we are expecting a single-value result. The corrected code is :

Dim Result as string = string.empty
Dim Filename = (From item In myCollection _
Where item.key.ToLower = MenuPath.ToLower _
Select item.MyFilename).Single
Console.WriteLine("Returned String : " + Filename)

A special thank you to Bill Wagner of SRT Solutions for his solution to the error in the first listing when I initially blogged it.

Hope that helps.

Joe Kunk
Okemos, MI