Cross platform PInvoke - returning strings to .NET core 2.0

[MattWorkWeb]

I need to have my C++ dll return a string to .NET core 2.0 wrapper. I want .NET to free up string when appropriate, and not have to make explicit call back to C++ to deallocate string. I've been able to do this successfully if I use CoTaskMemAlloc() (in Windows) to allocate the memory for string on C++ side but CoTaskMemAlloc is not supported in Linux. Windows C++ solution with CoTaskMemAlloc:

[.NET code]
[DllImport(EngineDllFullPath, CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string getText();

    public static string GetValue()
    {
        string s = getText();  // No need to free; .NET will take care of it
        return s;
    }

[Unmanaged C++ code]
char* getText()
{
...
char* s = (char*)::CoTaskMemAlloc(size);
...
return s;
}

Is there a cross platform solution to this that doesn't use CoTaskMemAlloc that can compile/run on windows and linux?

Thanks

[jkotas]

Forward CoTaskMemAlloc/Free to a regular malloc/free on non-Windows. E.g. it is what the interop tests do: https://github.com/dotnet/coreclr/blob/fa6222d52a81c24077206cf5082b2f3967e67ebc/tests/src/Common/Platform/platformdefines.h#L139

[AaronRobinsonMSFT]

@jkotas is right. He pointed out the global test definition. In the Interop test directory itself we have also defined it explcitly:

https://github.com/dotnet/coreclr/blob/fa6222d52a81c24077206cf5082b2f3967e67ebc/tests/src/Interop/common/xplatform.h#L119

[AaronRobinsonMSFT]

@MattWorkWeb I have created an issue for documenting or creating an example for this. See dotnet/coreclr#19090.

[AaronRobinsonMSFT]

Moved to dotnet/docs#6629