I have a Project with 2 F# Source Files - e3.fs and e4_exception.fs - e4_excpetion.fs contains the following simple Euler 4 solution:

#light

#I @"..\..\vendor\xunit"

#r @"xunit.dll"

open Xunit

module String =
let rev (s : string) = new string(s.ToCharArray() |> Array.rev)

let (|palindrome|notpalindrome|) (x : string) =
if x = String.rev x then palindrome else notpalindrome

let isPalindrome (x : int) =
match Int32.to_string x with
|palindrome x -> true
|notpalindrome x -> false

let threeDigitMultiples =
[for x in 100 .. 999
for y in 100 .. 999
-> x*y]

let highestPalindrome x =
List.filter (fun y -> isPalindrome y) x
|> List.sort (fun x y -> (Int32.compare x y) * -1)
|> List.hd

printfn "%A" (highestPalindrome threeDigitMultiples)

[<Fact>]
let the_answer_to_e4_is_906609 () =
Assert.Equal(906609, highestPalindrome threeDigitMultiples)

If I build this Project into an exe with the default settings in Visual Studio and then use the xUnit.net test runner to run the test I get the following error:

FAILED: E4_exception.the_answer_to_e4_is_906609: Microsoft.FSharp.Core.InvalidArgumentException : hd
at Microsoft.FSharp.Collections.ListModule.hd[T](List`1 l)
at E4_exception.the_answer_to_e4_is_906609() in C:\Users\charles\Documents\Dev\Eulers\trunk\src\EFT\e4_exception.fs:line 34

I struggled with this for awhile and the came across some interesting behaviour I could not explain - If I rename my files so that E4_exception.fs is no longer the last file in the project and rebuild into a new exe the test passes without error!?! I replicated this problem with another project/different code with the same results so I tried to do a little more digging and found something I think is interesting:

When E4_exception.fs is the last file in the project this the to first few lines of Reflectors disassembled version:

[CompilationMapping(SourceLevelConstruct.Module)]
public static class E4_exception
{
// Methods
public static Choice<Unit, Unit> |palindrome|notpalindrome|(string x);
public static int highestPalindrome(List<int> x);
public static bool isPalindrome(int x);
[Fact]
public static void the_answer_to_e4_is_906609();

When E4_exception.fs is NOT the last file in the project this is what I see:

[CompilationMapping(SourceLevelConstruct.Module)]
public static class E4_exception
{
// Methods
static E4_exception();
public static Choice<Unit, Unit> |palindrome|notpalindrome|(string x);
public static int highestPalindrome(List<int> x);
public static bool isPalindrome(int x);
[Fact]
public static void the_answer_to_e4_is_906609();

It seems like the file is built differently depending on its position in the project? In the 2nd disassembled code block there is a static constructor which is not present in the first disassembled version. Other files in this project show the same lack of static constructor when in the last position - but their tests do not fail in xUnit so I am confused as to what is going on...

Is this a bug or related to code/compiler options/.net behaviour that I do not understand? I would love some insight into and more info about this -

Thanks for any help,
Charles