项目文件.csproj文件内容如下：

重点是：不能有其他的

netstandard2;net40;net45;net46;net6

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netstandard2;net40;net45;net46;net6</TargetFrameworks>
   </PropertyGroup>
	
	<ItemGroup Condition="'$(TargetFramework)'=='net40' OR '$(TargetFramework)'=='net45' OR '$(TargetFramework)'=='net46'">
		<Reference Include="System" />
		<Reference Include="System.Core" />
		<Reference Include="Microsoft.CSharp" />
		<Reference Include="System.Data" />
	</ItemGroup>

	<ItemGroup Condition="'$(TargetFramework)'=='net45' OR '$(TargetFramework)'=='net46'">
		<Reference Include="System.ComponentModel.DataAnnotations" />
	</ItemGroup>
	
  <ItemGroup>
    <Folder Include="Http\" />
    <Folder Include="Log\" />
    <Folder Include="Ftp\" />
    <Folder Include="File\" />
    <Folder Include="EncryDecry\" />
    <Folder Include="FileZip\" />
  </ItemGroup>

</Project>