Working with GAEB files in C++ with the Dangl.GAEB & Dangl.AVA Libraries, see https://www.dangl-it.com/articles/create-read-and-edit-gaeb-files-in-cplusplus-with-the-danglgaeb-danglava-libraries/

CreateGaebFile.cpp

Console::WriteLine(L"Creating new GAEB file:");
GaebTargetType targetType = GaebTargetType::GaebXml;
Project^ projectToExport = gcnew Project();
ServiceSpecification^ servSpec = gcnew ServiceSpecification();
servSpec->ExchangePhase = ExchangePhase::Grant;
projectToExport->ServiceSpecifications->Add(servSpec);

Position^ position = gcnew Position();
position->ShortText = "C++ Position";
position->ItemNumber->StringRepresentation = "01";
Dangl::AVA::Contents::ServiceSpecificationContents::PositionExtensions::SetUnitPrice(position, *gcnew System::Decimal(12));
Dangl::AVA::Contents::ServiceSpecificationContents::PositionExtensions::SetQuantity(position, *gcnew System::Decimal(5));
servSpec->Elements->Add(position);

Dangl::AVA::Interop::AvaConverter::ExportToGaeb(projectToExport, args[1], GaebTargetType::GaebXml);
Console::WriteLine(args[1]);

Libraries.cs

public static class AvaConverter
{
    public static Project ConvertGaebToAva(string gaebFilePath)
    {
        using (var fs = File.OpenRead(gaebFilePath))
        {
            var gaebFile = Dangl.GAEB.Reader.GAEBReader.ReadGaeb(fs);
            var avaProject = Dangl.AVA.Converter.Converter.ConvertFromGaeb(gaebFile);
            return avaProject;
        }
    }

    public static Position[] GetPositionsInProject(Project avaProject)
    {
        return avaProject.ServiceSpecifications
            .First()
            .RecursiveElements()
            .OfType<Position>()
            .ToArray();
    }

    public static void ExportToGaeb(Project avaProject, string gaebFilePath, GaebTargetType gaebTargetType)
    {
        var gaebFile = Dangl.AVA.Converter.Converter.ConvertToGaeb(avaProject, destinationType: gaebTargetType.ToDestinationType());
        using (var fs = File.Create(gaebFilePath))
        {
            using (var gaebStream = Dangl.GAEB.Writer.GAEBWriter.GetStream(gaebFile))
            {
                gaebStream.CopyTo(fs);
            }
        }
    }

    private static AVA.Converter.DestinationGAEBType ToDestinationType(this GaebTargetType gaebTargetType)
    {
        switch (gaebTargetType)
        {
            case GaebTargetType.Gaeb90:
                return AVA.Converter.DestinationGAEBType.GAEB90;

            case GaebTargetType.Gaeb2000:
                return AVA.Converter.DestinationGAEBType.GAEB2000;

            default:
                return AVA.Converter.DestinationGAEBType.GAEBXML_V3_2;
        }
    }
}

PrintPositions.cpp

Dangl::AVA::Project^ project = Dangl::AVA::Interop::AvaConverter::ConvertGaebToAva(args[0]);

array<Position^> ^positions = AvaConverter::GetPositionsInProject(project);

Console::WriteLine(L"Printing positions in imported GAEB file:");
Console::WriteLine(L"********************");
for (int i = 0; i < positions->Length; i++) {
	Console::WriteLine(positions[i]->ItemNumber->StringRepresentation);
	Console::WriteLine(positions[i]->ShortText);
	Console::WriteLine(positions[i]->TotalPrice);
	Console::WriteLine(L"********************");
}