F# Unit Test Error: xUnit.net / TestDriven.Net - exception

Question

So the following I got from Pluralsight and the chap presenting the course apparently runs the code without a hitch, however, when I run it, I get the error message:
"Test failed ‘T:Messaging’ : Couldn’t find type with name ‘Messaging’"
Test failed 'T: Couldn't find type with name
Any Ideas (appreciated)?
namespace Ploeh.Samples
open System
module Messaging =
type Envelope<'a> = {
Id : Guid
Created : DateTimeOffset
Item : 'a }
let envelop getId getTime item = {
Id = Guid "1CF889F8-201F-44DF-BC86-77227651D3EE"
Created = DateTimeOffset.MinValue
Item = item }
module MessagingTests =
open Xunit
type Foo = { Text : string; Number : int }
[<Fact>]
let ``enevelope returns correct results`` () =
let getId _ = Guid "1CF889F8-201F-44DF-BC86-77227651D3EE"
let getTime _ = DateTimeOffset( 636322011751405346L,
TimeSpan.FromHours(-4.0) )
let item = { Text = "Bar"; Number = 42 }
let actual = Messaging.envelop getId getTime item
Assert.Equal ( Guid "1CF889F8-201F-44DF-BC86-77227651D3EE",
actual.Id )
Assert.Equal ( DateTimeOffset( 636322011751405346L,
TimeSpan.FromHours(-4.0) ),
actual.Created )
Assert.Equal ( item, actual.Item )

Answer 1

I managed to get it working by separating the tests into a separate file and top-level module. For some reason, anything other than this `setup' does not work, e.g., separate file but in a lower-level module, or namespace and module declarations separately ... I am not sure whether this is something specific to say my target being .Net 4.7 rather than whatever used by the instructor or an issue with Visual Studio 2017, or something else. It seems that TestDriven.Net takes issue with the namespace/module system and gets confused by them ...
In the first file:
namespace Ploeh.Samples
open System
module Messaging =
type Envelope<'a> = {
Id : Guid
Created : DateTimeOffset
Item : 'a }
let envelop getId getTime item = {
Id = Guid "1CF889F8-201F-44DF-BC86-77227651D3EE"
Created = DateTimeOffset.MinValue
Item = item }
And in the second file:
module Ploeh.Samples.MessagingTests
open System
open Messaging
open Xunit
type Foo = { Text : string; Number : int }
[<Fact>]
let ``enevelope returns correct results`` () =
let getId _ = Guid "1CF889F8-201F-44DF-BC86-77227651D3EE"
let getTime _ = DateTimeOffset( 636322011751405346L,
TimeSpan.FromHours(-4.0) )
let item = { Text = "Bar"; Number = 42 }
let actual = Messaging.envelop getId getTime item
Assert.Equal ( Guid "1CF889F8-201F-44DF-BC86-77227651D3EE",
actual.Id )
Assert.Equal ( DateTimeOffset( 636322011751405346L,
TimeSpan.FromHours(-4.0) ),
actual.Created )
Assert.Equal ( item, actual.Item )