using custom Marshaller in F#

Question

I've been toying with marshalling in F# recently, and to my surprise I ended up with error FS0980: Custom marshallers cannot be specified in F# code. Consider using a C# helper function.

Only information regarding the topic, that I managed to find, is F# 4.0 Spec (page 300)

When applied to a parameter or return type, specifies the marshalling attribute for a CLI P/Invoke stub declaration. This attribute may be used in both F# and imported assemblies. However, F# does not support the specification of "custom" marshallers.

I'm mostly interested in reasons behind lack of custom marshallers support, as I can "live with" introducing C# assembly to project.

Can anyone clarify on this?

EDIT:

Sample code as requested

[<StructLayout(LayoutKind.Sequential, Pack=1)>]
type BASIC_STRUCT =
  struct
    [<MarshalAsAttribute(UnmanagedType.CustomMarshaler,MarshalTypeRef=typedefof<DateTimeMarshaller>)>]
    val timeVar:DWORD
  end

from what i noticed just setting UnmanagedType.CustomMarshaler is enough to cause compile time exception

Answer 1

Looking at the compiler code, it seems the F# team couldn't really make up their mind about this:

| 0x2C -> 
    error(Error(FSComp.SR.ilCustomMarshallersCannotBeUsedInFSharp(),m))
    (* ILNativeType.Custom of bytes * string * string * bytes (* GUID,nativeTypeName,custMarshallerName,cookieString *) *)
    //ILNativeType.Error  

There's no history around that piece of code on github. My gut feeling is that the explanation would be something along the lines of "uncommon use case with a cheap workaround". Very likely it's been that way from the beginning, since F# 2.0 spec also has the line you quoted.

Try raising an issue on github about it. Maybe someone there can dig up something in Microsoft systems.