C#•2mo ago

Sharing the custom marshalled type across libraries

I followed the guide thoroughly https://learn.microsoft.com/en-us/dotnet/standard/native-interop/tutorial-custom-marshaller and source code is in here https://github.com/dotnet/samples/blob/main/core/interop/source-generation/custom-marshalling/src/custommarshalling/NativeLib.cs so far it works, but things goes south when i try to use the ErrorData outside of host library code used to test it out:

C#
using CustomMarshalling;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace CustomMarshalling2
{
    internal partial class NativeLib2
    {
        private const string LibName = "nativelib2";

        [LibraryImport(LibName)]
        internal static partial void PrintErrorData(ErrorData errorData);

        [LibraryImport(LibName)]
        internal static partial ErrorData GetFatalErrorIfNegative(int code);

        [LibraryImport(LibName)]
        [return: MarshalUsing(CountElementName = "len")]
        internal static partial ErrorData[] GetErrors(int[] codes, int len);

        [LibraryImport(LibName)]
        internal static partial void GetErrorCodes(ErrorBuffer buffer, int[] codes);
    }
}

C#
using CustomMarshalling;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace CustomMarshalling2
{
    internal partial class NativeLib2
    {
        private const string LibName = "nativelib2";

        [LibraryImport(LibName)]
        internal static partial void PrintErrorData(ErrorData errorData);

        [LibraryImport(LibName)]
        internal static partial ErrorData GetFatalErrorIfNegative(int code);

        [LibraryImport(LibName)]
        [return: MarshalUsing(CountElementName = "len")]
        internal static partial ErrorData[] GetErrors(int[] codes, int len);

        [LibraryImport(LibName)]
        internal static partial void GetErrorCodes(ErrorBuffer buffer, int[] codes);
    }
}

PS: unlike the original source code, i've made modification to ErrorData and their marshallers by making everything public just for testing. it generates SYSLIB1051 error

error SYSLIB1051: Runtime marshalling must be disabled in this project by applying the 'System.Runtime.CompilerServices.DisableRuntimeMarshallingAttribute' to the assembly to enable marshalling this type. The generated source will not handle marshalling of parameter 'errorData'. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1051)

Use custom marshallers in source-generated P/Invokes - .NET

Learn how to use the CustomMarshallerAttribute and implement a custom marshaller for use with source generation.

GitHub

samples/core/interop/source-generation/custom-marshalling/src/custo...

Sample code referenced by the .NET documentation. Contribute to dotnet/samples development by creating an account on GitHub.

3 Replies

WhiteFalcon•2mo ago

TLDR:

1) Library A defines publicly ErrorData and custom marshallers
2) Library A imports native library and uses the ErrorData as arguments/return type, everything compiles fine so far
3) Library B links against A
4) Library B imports another native library(or same) that also uses ErrorData; SYSLIB1051 error is generated

1) Library A defines publicly ErrorData and custom marshallers
2) Library A imports native library and uses the ErrorData as arguments/return type, everything compiles fine so far
3) Library B links against A
4) Library B imports another native library(or same) that also uses ErrorData; SYSLIB1051 error is generated

The error still persists if i removed the NativeMarshalling attribute and manually added MarshalUsing to each argument/return

C#
using CustomMarshalling;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace CustomMarshalling2
{
    internal partial class NativeLib2
    {
        private const string LibName = "nativelib2";

        [LibraryImport(LibName)]
        internal static partial void PrintErrorData([MarshalUsing(typeof(ErrorDataMarshaller))] ErrorData errorData);

        [LibraryImport(LibName)]
        [return: MarshalUsing(typeof(ErrorDataMarshaller))]
        internal static partial ErrorData GetFatalErrorIfNegative(int code);
    }
}

C#
using CustomMarshalling;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace CustomMarshalling2
{
    internal partial class NativeLib2
    {
        private const string LibName = "nativelib2";

        [LibraryImport(LibName)]
        internal static partial void PrintErrorData([MarshalUsing(typeof(ErrorDataMarshaller))] ErrorData errorData);

        [LibraryImport(LibName)]
        [return: MarshalUsing(typeof(ErrorDataMarshaller))]
        internal static partial ErrorData GetFatalErrorIfNegative(int code);
    }
}

I guess the solution was to disable runtime marshalling with [assembly: System.Runtime.CompilerServices.DisableRuntimeMarshalling] The issue with such solution for me was that it triggers CA1420: Property, type, or attribute requires runtime marshalling for delegates marked with [UnmanagedFunctionPointer(CallingConvention.Cdecl)] or any use of Marshal.GetFunctionPointerForDelegate with delegates managed types (even if they have NativeMarshaller)

jcotton42•2mo ago

I believe you want UnmanagedCallersOnly nowadays. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/function-pointers

Function pointers - C# feature specifications

This feature specification describes function pointers, which are unmanaged delegates. They are typically used to avoid the allocations necessary to instantiate a delegate object.

WhiteFalcon•2mo ago

that would restrict me from using the managed types aswell as making the function static, right? thank you for the suggestion, i'll do that

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Sharing the custom marshalled type across libraries