无法在 F# 中编写 CLI 兼容的扩展方法

Question

I have the following F# code which I am attempting to use to write extension methods for C#. I'm using F# 4.5 on .NET Core 2.2 (macOS), with C# 7.3 attempting to call the extension methods.

open System
open System.Runtime.CompilerServices

[<assembly: Extension>]
do ()

[<Extension>]
module Extensions =
    [<CompiledName("Print"); Extension>]
    type System.String with
        [<Extension>]
        static member inline Print(str : System.String) = Console.WriteLine(str)

    [<CompiledName("Blorp"); Extension>]
        type System.Object with
            [<Extension>]
            static member inline Blorp(o : System.Object) = Console.WriteLine("Blorp")

This is mostly taken from the F# 4.1 specification, with the assembly attribute cribbed from here . I'm using JetBrains Rider and the namespace Extensions that it's under is imported properly and the method calls even get highlighted properly in the C# function body where they get called, showing their type. But when I do dotnet run, I get a CS1061 build error for both extensions, saying the instances don't contain a definition for either method and no extension methods were found. The same methods written in C# and VB.NET work fine. What am I doing wrong? Is it no longer possible to write CLI-compatible extension methods in F#?

Answer 1

@dumetrulo in the comment had the solution here. The extension methods need to be let-bound functions in a module, without the enclosing type declaration:

namespace Extensions

open System
open System.Runtime.CompilerServices

[<Extension>]
module Extensions =
    [<Extension>]
    let Print(str : System.String) = Console.WriteLine(str)

    [<Extension>]
    let Blorp(o : System.Object) = Console.WriteLine("Hello World from an F# Extension Method!")

Additionally, the [<assembly: Extension>] attribute appears to be unnecessary