Problem consuming C# API with default value for parameter

[wallymathieu]

Please provide a succinct description of the issue.

Repro steps

Provide the steps required to reproduce the problem:

1. Load a C# library that defines a method that takes a default value
2. From F# call the method without specifying the value3.

Expected behavior

Expects that the following snippet compiles:

#r "nuget: FluentAssertions.Json, 6.1.0"
open FluentAssertions.Json
open Newtonsoft.Json.Linq
let assertJsonEqual expected actual =
    let expectedToken = JToken.Parse expected
    let actualToken = JToken.Parse actual
    actualToken.Should().BeEquivalentTo(expectedToken) |> ignore

Actual behavior

The member or object constructor 'BeEquivalentTo' does not take 1 argument(s). An overload was found taking 3 arguments.F# Compiler505

Known workarounds

A workaround is to specify the default values explicitly:

#r "nuget: FluentAssertions.Json, 6.1.0"
open FluentAssertions.Json
open Newtonsoft.Json.Linq
let assertJsonEqual expected actual =
    let expectedToken = JToken.Parse expected
    let actualToken = JToken.Parse actual
    actualToken.Should().BeEquivalentTo(expected = expectedToken, because = "") |> ignore

Related information

Provide any related information (optional):

• Operating systems: Windows 11, macOS Monterey 12.4
• .NET Runtime kinds: .NET Core 6.0.300 , 3.1.421
• Editing Tools: Rider 2022.1.2, Visual Studio Community 2022 for Mac Preview (17.3 build 198), Visual Studio Code and Ionide

[dsyme]

Looks like this involves interop with methods that have both omitted optional arguments and ParamArray arguments:

        BeEquivalentTo(class [Newtonsoft.Json]Newtonsoft.Json.Linq.JToken expected,
                       [opt] string because,
                       object[] becauseArgs) cil managed

[github-actions]

🤖 This is an automated response from Repo Assist.

I tested this issue against the current F# 10 SDK (.NET 9.0) and it now compiles successfully without needing to specify the optional parameters explicitly:

open FluentAssertions.Json
open Newtonsoft.Json.Linq

let assertJsonEqual expected actual =
    let expectedToken = JToken.Parse expected
    let actualToken = JToken.Parse actual
    actualToken.Should().BeEquivalentTo(expectedToken) |> ignore  // ✅ now compiles

This was fixed as part of F# support for calling C# methods with optional/default parameters by omitting optional arguments (related to C# interop improvements in F# 8+). The workaround of specifying expected = expectedToken, because = "" is no longer needed.

This issue may be eligible for closure. Could a maintainer verify and close if confirmed?

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