How to implement delegates with std::string return type?

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
20/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
cpp, csharp
Domain
tooling

Research direction

Start with the C++ SetCallback declaration and the generated C# delegate signature shown in the issue. Determine whether the C++ std::string callback type can be represented and returned through the generated binding, and document the supported outcome or required scope.

Written by the indexing model from the issue text.

Description

Delegates for types like int just work out of the box.
E.g. __declspec(dllexport) int __cdecl SetCallback(int (*callback)(int arg));

Then in C# I can do

private static int MyCallback(int arg)
{
    return arg + 1;
}

SetCallback(MyCallback);

Now let's say we want strings instead of ints

__declspec(dllexport) void __cdecl SetCallback(std::string (*callback)(std::string arg));

Things get difficult when trying to return

private static unsafe __Internalc__N_std_S_basic_string__C___N_std_S_char_traits__C___N_std_S_allocator__C MyCallback(__Internalc__N_std_S_basic_string__C___N_std_S_char_traits__C___N_std_S_allocator__C arg)
{
    var argBasicString = global::Std.BasicString<sbyte, global::Std.CharTraits<sbyte>, global::Std.Allocator<sbyte>>.__CreateInstance(new global::System.IntPtr(&arg));
    var platformString = global::Std.BasicStringExtensions.Data(argBasicString);
    argBasicString.Dispose();

    var result = DoSomethingWithTheArg(platformString);

    // How to convert the System.string to the type needed or is this possible at all?
    return ...
}

Any hints how to proceed here would be very welcome.

Dominant language
C#
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