C++/CLI library .NETFramework to .Net Standard

Question

What follows is a VS2019 project file for a C++/CLI application. The goal is to upgrade it from being a .Net Framework to a .Net Standard 2.0.

It was compiling and linking find with .Net Core 5.0 until I started to deal with the whole log4net reference. In other words, the .Net Core 5.0 was NOT trying to pass an log4net interface into this library and all was good. Once one was passed in, there where errors. The old log4net reference was pulled out of this project and replaced with one to the .Net Standard 2.0. Now there is this error:

warning MSB3268: The primary reference "log4net" could not be resolved because it has an indirect dependency on the framework assembly "netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "log4net" or retarget your application to a framework version which contains "netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51".
AFile.h(3): error C2871: 'log4net': a namespace with this name does not exist

I simply am not seeing any reference in the project file to either .Net Framework or .Net Standard. How do I fix this issue?

<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup Label="ProjectConfigurations">
    <ProjectConfiguration Include="Debug|Win32">
      <Configuration>Debug</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Release|Win32">
      <Configuration>Release</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
  </ItemGroup>
  <PropertyGroup Label="Globals">
    <ProjectGuid>{9FAC29B5-62B2-4323-9469-921B467C8433}</ProjectGuid>
    <TargetFrameworkVersion>v4.6.2</TargetFrameworkVersion>
    <RootNamespace>theCppCliLibraryName</RootNamespace>
    <Keyword>ManagedCProj</Keyword>
    <WindowsTargetPlatformVersion>8.1</WindowsTargetPlatformVersion>
  </PropertyGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <PlatformToolset>v140_xp</PlatformToolset>
    <CharacterSet>Unicode</CharacterSet>
    <CLRSupport>true</CLRSupport>
    <WholeProgramOptimization>true</WholeProgramOptimization>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <PlatformToolset>v140_xp</PlatformToolset>
    <CharacterSet>Unicode</CharacterSet>
    <CLRSupport>true</CLRSupport>
  </PropertyGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
  <ImportGroup Label="ExtensionSettings">
  </ImportGroup>
  <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="PropertySheets">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="PropertySheets">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <PropertyGroup Label="UserMacros" />
  <PropertyGroup>
    <_ProjectFileVersion>14.0.25431.1</_ProjectFileVersion>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
    <OutDir>$(SolutionDir)$(Configuration)\</OutDir>
    <IntDir>$(Configuration)\</IntDir>
    <LinkIncremental>true</LinkIncremental>
    <LinkKeyFile>theCppCliLibraryName.snk</LinkKeyFile>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
    <OutDir>$(SolutionDir)$(Configuration)\</OutDir>
    <IntDir>$(Configuration)\</IntDir>
    <LinkIncremental>false</LinkIncremental>
    <LinkKeyFile>theCppCliLibraryName.snk</LinkKeyFile>
  </PropertyGroup>
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
    <ClCompile>
      <Optimization>Disabled</Optimization>
      <AdditionalIncludeDirectories>../../include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
      <PreprocessorDefinitions>WIN32;_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <WarningLevel>Level3</WarningLevel>
      <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
    </ClCompile>
    <Link>
      <AdditionalDependencies>LicenseLib2015MDd.lib</AdditionalDependencies>
      <AdditionalLibraryDirectories>../../lib;%(AdditionalLibraryDirectories)</AdditionalLibraryDirectories>
      <GenerateDebugInformation>true</GenerateDebugInformation>
      <AssemblyDebug>true</AssemblyDebug>
      <RandomizedBaseAddress>false</RandomizedBaseAddress>
      <DataExecutionPrevention />
      <TargetMachine>MachineX86</TargetMachine>
      <Profile>true</Profile>
      <SubSystem>Windows</SubSystem>
    </Link>
  </ItemDefinitionGroup>
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
    <ClCompile>
      <AdditionalIncludeDirectories>../../include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
      <PreprocessorDefinitions>WIN32;NDEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <WarningLevel>Level3</WarningLevel>
      <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
    </ClCompile>
    <Link>
      <AdditionalDependencies>LicenseLib2015MD.lib</AdditionalDependencies>
      <AdditionalLibraryDirectories>../../lib;%(AdditionalLibraryDirectories)</AdditionalLibraryDirectories>
      <GenerateDebugInformation>false</GenerateDebugInformation>
      <RandomizedBaseAddress>false</RandomizedBaseAddress>
      <DataExecutionPrevention />
      <TargetMachine>MachineX86</TargetMachine>
      <SubSystem>Windows</SubSystem>
      <ShowProgress>LinkVerbose</ShowProgress>
    </Link>
  </ItemDefinitionGroup>
  <ItemGroup>
    <Reference Include="log4net">
      <HintPath>..\3rdPartyLibraries\log4net-2.0.8\bin\net\4.5\release\log4net.dll</HintPath>
    </Reference>
    <Reference Include="System">
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
    </Reference>
    <Reference Include="System.Data">
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
    </Reference>
    <Reference Include="System.Drawing">
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
    </Reference>
    <Reference Include="System.Windows.Forms">
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
    </Reference>
    <Reference Include="System.Xml">
      <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
      <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
    </Reference>
  </ItemGroup>
  <ItemGroup>
    <ClCompile Include="AFile.cpp" />
    <ClCompile Include="BFile.cpp" />
    <ClCompile Include="Stdafx.cpp">
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">Create</PrecompiledHeader>
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">Create</PrecompiledHeader>
    </ClCompile>
  </ItemGroup>
  <ItemGroup>
    <ClInclude Include="AFile.h" />
    <ClInclude Include="BFile.h">
      <FileType>CppForm</FileType>
    </ClInclude>
    <ClInclude Include="resource.h" />
    <ClInclude Include="Stdafx.h" />
  </ItemGroup>
  <ItemGroup>
    <ResourceCompile Include="app.rc" />
  </ItemGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
  <ImportGroup Label="ExtensionTargets">
  </ImportGroup>
  <ProjectExtensions>
    <VisualStudio>
      <UserProperties Spices_ObfuscationOptions="{Anonymizer=High}{AntiILDASM=CompleteImproved}{Members=Full}" />
    </VisualStudio>
  </ProjectExtensions>
</Project>

Answer 1

According to Microsoft C++ CLI projects can't target .Net Standard:

There are some important limitations to porting C++/CLI projects to .NET Core compared to other languages:

C++/CLI projects can't target .NET Standard, only .NET Core (or .NET Framework).

See https://learn.microsoft.com/en-us/dotnet/core/porting/cpp-cli