[MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1, ArraySubType = UnmanagedType.LPUTF8Str)] results in "Cannot marshal 'parameter #3': Unknown error."

[dlech]

I am trying out the UTF8 pinvoke marshalling. I have the following that works as expected:

    [DllImport ("gio-2.0", CallingConvention = CallingConvention.Cdecl)]
    static extern bool g_application_id_is_valid([MarshalAs(UnmanagedType.LPUTF8Str)] string applicationId);

However, this one does not...

    [DllImport ("gio-2.0", CallingConvention = CallingConvention.Cdecl)]
    static extern void g_application_run (SafeApplicationHandle application,
        int argc, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1, ArraySubType = UnmanagedType.LPUTF8Str)] string[] argv);

It throws a MarshalDirectiveException with the message "Cannot marshal 'parameter dotnet/coreclr#3': Unknown error."

I see that in tests/src/Interop/StringMarshalling/UTF8/UTF8Test.cs, there is not a test for such a case, so I am guessing that perhaps it was overlooked and has not been implemented yet. (Or, if there is a mistake on my part, the error message could be improved.)

[yizhang82]

@dlech Thanks for reporting the issue. This is indeed a missing feature. /cc @tijoytom

[ChrisBirkett]

I observe the same MarshalDirectiveException (message Cannot marshal 'parameter dotnet/coreclr#1': Unknown error.) when I attempt to marshal parameters as UnmanagedType.AnsiBStr:

[DllImport(@"_Libs\vcmowr64.dll")]
public static extern Int32 wcmo_init([MarshalAs(UnmanagedType.AnsiBStr), In] ref string WCMOarg_Handle,
    [MarshalAs(UnmanagedType.AnsiBStr), In] ref string WCMOarg_User,
    [MarshalAs(UnmanagedType.AnsiBStr), Out] out string WCMOarg_DataOut,
    [MarshalAs(UnmanagedType.AnsiBStr), Out] out string WCMOarg_ErrOut);

The marshalling and call above work on Windows (1.0.3 and 1.1.0 tested):
[DllImport(@"_Libs\vcmowr64.dll")]
But produce the error when running in a Linux docker container (base image microsoft/dotnet running 1.0.4)
[DllImport(@"_Libs/libvcmowrap.so")]

I also looked at the tests/src/Interop/StringMarshalling and do not see any ANSI tests, so I am wondering if this is a missing feature as well, and curious to know why it works on Windows but not Linux?

[yizhang82]

@ChrisBirkett This is a different issue. We support "ANSI" marshaling in linux - in that context it simply means UTF-8 marshaling ("ANSI" is such an overloaded term...). We don't support BSTR/AnsiBSTR in linux, though. They are very Windows/COM specific (especially AnsiBSTR which is a VB6 thing). Would it make sense for you to simply switch to UnmanagedType.LPStr?

[ro-jo]

@yizhang82 So hopefully UnmanagedType.LPStr is UTF-8 on both Linux and Windows, can you clarify please?
I am writing common native hosting code for both Windows and Linux, the same code compiles on both - it won't be possible to use multilingual char arrays otherwise (with automatic marshalling).

[yizhang82]

@ro-jo LPStr is UTF-8 in linux but in windows it still needs to be "ANSI" (a misnomer). Changing it to UTF-8 in Windows is a breaking change for everybody else. You probably should use LPUTF8Str.
/cc @luqunl @jeffschwMSFT

[AaronRobinsonMSFT]

@GrabYourPitchforks How passionate are you about this support? I am just trying to understand if this is something we will ever update the built-in system to support or if we should defer to whatever happens with source generated P/Invokes.

[AaronRobinsonMSFT]

Details on what needs to happen in order to support this. The crux of the issue is we rely on OLE VARENUM to indicate the element type of arrays containing strings. There a several layers of indirection but that is how we are conveying the information through at present.

The starting point for this logic would be below. Notice the switch statement doesn't handle NATIVE_TYPE_LPUTF8STR.

runtime/src/coreclr/src/vm/mlinfo.cpp

Lines 5147 to 5189 in 103fa2c

	switch (ntElement)
	{
	case NATIVE_TYPE_DEFAULT:
	#ifdef FEATURE_COMINTEROP
	if (arrayNativeType == NATIVE_TYPE_SAFEARRAY || ms == MarshalInfo::MARSHAL_SCENARIO_COMINTEROP)
	{
	m_vtElement = VT_BSTR;
	}
	else
	#endif // FEATURE_COMINTEROP
	{
	m_vtElement = static_cast<VARTYPE>(isAnsi ? VT_LPSTR : VT_LPWSTR);
	}
	break;
	case NATIVE_TYPE_BSTR:
	m_vtElement = VT_BSTR;
	break;
	case NATIVE_TYPE_LPSTR:
	m_vtElement = VT_LPSTR;
	break;
	case NATIVE_TYPE_LPWSTR:
	m_vtElement = VT_LPWSTR;
	break;
	case NATIVE_TYPE_LPTSTR:
	{
	#ifdef FEATURE_COMINTEROP
	if (ms == MarshalInfo::MARSHAL_SCENARIO_COMINTEROP)
	{
	// We disallow NATIVE_TYPE_LPTSTR for COM or if we are exporting.
	ReportInvalidArrayMarshalInfo(IDS_EE_BADMARSHALPARAM_NO_LPTSTR);
	}
	else
	#endif // FEATURE_COMINTEROP
	{
	// We no longer support Win9x so LPTSTR always maps to a Unicode string.
	m_vtElement = VT_LPWSTR;
	}
	break;
	}
	default:
	ReportInvalidArrayMarshalInfo(IDS_EE_BADMARSHAL_STRINGARRAY);
	}