Unexpected NullReferenceException bug when using async functions from another assembly

Repro:

Create an F# console application and an F# library.
Add a reference from the console application to the library project.
Replace the code Visual Studio generates with the following:

Code for the console application:

[<EntryPoint>]
let main argv = 

  WeirdBug.Bug.reproduce() |> Async.RunSynchronously  

  0 // return an integer exit code

Code for the library:

namespace WeirdBug

module Bug =

  let nums = [|1;2;3|]

  let reproduce () =
    async {
      for n in nums do // Why does this throw a nullreferenceexception?!?!
        printfn "%d" n
    }

Now compile/run and you should get an null reference exception. If you move the code from the library into the console application itself the error disappears.

Tested on VS2013, FSharp.Core 4.3.1.0, .NET 4.5, Windows 8.

I'm not seeing the nullref, it just prints "1 2 3" for me. Can anyone repro the issue?

I can't repro on:

Microsoft Visual Studio Ultimate 2013
Version 12.0.30723.00 Update 3
Microsoft .NET Framework
Version 4.5.51641

Installed Version: Ultimate

On Wed, Aug 6, 2014 at 4:45 PM, Lincoln Atkinson notifications@github.com
wrote:

I'm not seeing the nullref, it just prints "1 2 3" for me. Can anyone
repro the issue?

—
Reply to this email directly or view it on GitHub
#326 (comment).

Can we see more of the exception stack you are seeing perhaps?

I cannot. Vs2013U, FSharp.Core 4.3.1.0, .NET 4.5, Windows 7

It seems that the project type guids got muddled and my library project was actually also a console application (.exe). Changing this to a library (.dll) fixed the problem.

Not entirely sure why this should be an issue though, is there any reason a referenced assembly can't also be a .exe?

You should be able to reproduce the problem if you change the library project to a console application in project settings.

System.NullReferenceException was unhandled
Message: An unhandled exception of type 'System.NullReferenceException' occurred in FSharp.Core.dll
Additional information: Object reference not set to an instance of an object.

Ahh, yes that makes sense. When compiled as a console app, the compiler generates a main method, and initialization of nums is done there. When compiled as a library, initialization of nums is done in a generated static constructor. If you reference the console app like a library, main is never run, so nums is null when accessed. Unfortunately, main is tucked away in an internal class you can't really get to, so you can't work around by calling main explicitly.

You can work around by declaring an explicit dummy main in your exe, though:

module Program =
    [<EntryPoint>]
    let main _ = 0

This forces the module initialization stuff to be put in a static constructor, like it would be if it were a library.

Not sure where this lands in terms of by-design vs bug. I agree it's unintuitive/annoying.

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Unexpected NullReferenceException bug when using async functions from another assembly #326

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Unexpected NullReferenceException bug when using async functions from another assembly #326

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latkin commented on Aug 6, 2014

brodyberg commented on Aug 7, 2014

brodyberg commented on Aug 7, 2014

jamessdixon commented on Aug 7, 2014

endeavour commented on Aug 8, 2014

latkin commented on Aug 8, 2014

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