Interop with C# method with optional argument and params

[lucasteles]

This case happened to me when I was using FluentAssertions, when I try to call a C# method that receives the first argument with a default value and a second params argument I can't omit both parameters.

E.g:

    public class Class1
    {
        public static string Foo(string something = "", params object[] args) => 
            string.Format(something, args);
    }

If I try to use this method in F# I expected to be able to call it in this way:

 let result1 = Class1.Foo()

But this gives me an error :

Error FS0501: 
The member or object constructor 'Foo' takes 2 argument(s) but is here given 0. 
The required signature is 'Class1.Foo(?something: string, [<System.ParamArray>] args: obj[]) : string'.

So, to make it work I need to pass the first argument explicitly which I would expect to be optional :

 let result1 = Class1.Foo("")

Pros and Cons

I don't know if this is bt design, but is very confusing and made the interop harder and noisy in some cases.

For Readers

If you would like to see this issue implemented, please click the 👍 emoji on this issue. These counts are used to generally order the suggestions by engagement.

[baronfel]

This may have been addressed in dotnet/fsharp#12515 - where default arguments were being factored into the logic. It's worth testing though.

[dsyme]

I checked - this hasn't been addressed

[edgarfgp]

Consider closing this as has been implemented by dotnet/fsharp#16688

[vzarytovskii]

Consider closing this as has been implemented by dotnet/fsharp#16688

Good catch, thanks, Edgar!