使用.net标准2.0将.net4.0旧库转换为多个目标会产生HtmlString引用错误

Question

I'm porting an old class library so it can multi target both .net4.0 and .net standard. I need to use the same library on a .net core project.

The library relies heavily on System.Web.Mvc and I'm using the Strategy 1 described here: Upgrading ASP.NET Libraries to .NET Standard

The errors I couldn't solve are:

1 - Reference to type 'HtmlString claims it is defined in 'System.Web', but it could no be found

2 - 'MvcHtmlString' does not contain a definition for 'ToHtmlString' and no extension method 'ToHtmlString' accepting a first argument of type 'MvcHtmlString' could be found (are you missing a using directive or an assembly reference?)

I created a new .net standard class library project and copied the files over. This is my csproj file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;net40</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0'">
      <PackageReference Include="Microsoft.AspNet.Mvc" Version="3.0.50813.1" />
  </ItemGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'net40' ">     
    <Reference Include="mscorlib" />
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Web" />
  </ItemGroup>
</Project>

I tried adding this to netstandard2.0 ItemGroup but it didn't help:

<Reference Include="System.Web" />

What do I need to do to fix? I've read some about the subject, but it's still confusing and the links I found didn't help.

---

I ended up using a new code base only for .NET Core . My .csproj is like this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>netstandard2.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Html.Abstractions" Version="2.0.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.DataAnnotations" Version="2.0.2" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.ViewFeatures" Version="1.1.0" />
  </ItemGroup>
</Project>

As @NightOwl888 explained, I needed Microsoft.AspNetCore.Html.Abstractions so I could use HtmlString instead of MvcHtmlString .

The Microsoft.AspNetCore.Mvc.ViewFeatures is necessary so the SelectExtensions like DropDownListFor for example could return a type compatible with HtmlString .

I also had to replace all HtmlHelper with IHtmlHelper and ModelMetaData with ModelExplorer .

ModelMetadata.FromLambdaExpression is now ExpressionMetadataProvider.FromLambdaExpression .

Answer 1

As per the article you linked :

Provided that the library does not invoke any functionality that is beyond what is available in .NET Standard 2.0, compatibility mode will work.

In place of MvcHtmlString on .NET Standard, you can use the Microsoft.AspNetCore.Html.HtmlString type as per this answer .

Since the base class of MvcHtmlString is HtmlString in .NET 4.0, you should return HtmlString and it will work in both cases (although you need to import a different namespace for each).

#if NET40
    using HtmlString = System.Web.HtmlString;
#else
    using HtmlString = Microsoft.AspNetCore.Html.HtmlString;
#endif

You need a reference to Microsoft.AspNetCore.Html.Abstractions in your netstandard2.0 targeted references to use that type.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;net40</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0'">
      <PackageReference Include="Microsoft.AspNetCore.Html.Abstractions" Version="2.0.0" />
  </ItemGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'net40' ">     
    <Reference Include="mscorlib" />
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Web" />
  </ItemGroup>
</Project>

As for ToHtmlString() , I am confused what Microsoft intends to do about it . However, adding an extension method between #if NETSTANDARD2.0 and #endif blocks (or a file that is conditionally included for compile) is a pretty straghtforward thing to do in your library.

However, as John Skeet pointed out in the comments, it may not be the best idea to expose 2 different types through the same API like this. You might be better off making a separate APIs for .NET Framework and .NET Core and put them in separate platform-targeted libraries that reference a .NET Standard library containing the shared business logic.