Missing type forward for JsonResult in ASP.NET Core 3.0?

[martincostello]

Also posted as domaindrivendev/Swashbuckle.AspNetCore#1030

Describe the bug

If an ASP.NET Core 3.0 preview 2 application references Swashbuckle.AspNetCore 4.0.1 and uses the Microsoft.AspNetCore.Mvc.JsonResult anywhere in code, a CS0433 compiler error is generated and the application will not compile.

The type 'JsonResult' exists in both 'Microsoft.AspNetCore.Mvc.Core, Version=3.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60' and 'Microsoft.AspNetCore.Mvc.Formatters.Json, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'

To Reproduce

1. Create a new ASP.NET Core MVC application using the built-in Visual Studio 2019 template using 3.0.0-preview-19075-0444.
2. Add the following reference: <PackageReference Include="Swashbuckle.AspNetCore" Version="4.0.1" />
3. Add the following method to ValuesController.cs: public void Foo(int id) => new JsonResult(new { });
4. Attempt to compile the application.

Expected behavior

The application compiles cleanly.

Additional context

The issue can be worked around by adding the following to the .csproj file of the project as referenced by NuGet/Home#4989 (comment):

<ItemGroup>
  <PackageReference Include="Microsoft.AspNetCore.Mvc.Formatters.Json" Version="2.0.0" Alias="LegacyAspNetCoreJsonFormatter" />
</ItemGroup>
<Target Name="AddPackageAliases" BeforeTargets="ResolveReferences" Outputs="%(PackageReference.Identity)">
  <PropertyGroup>
    <AliasPackageReference>@(PackageReference->'%(Identity)')</AliasPackageReference>
    <AliasName>@(PackageReference->'%(Alias)')</AliasName>
  </PropertyGroup>
  <ItemGroup>
    <ReferencePath Condition="'%(FileName)'=='$(AliasPackageReference)' and '$(AliasName)' != ''">
      <Aliases>$(AliasName)</Aliases>
    </ReferencePath>
  </ItemGroup>
</Target>

Output of dotnet --info:

.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview-010184
 Commit:    c57bde4593

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.0.100-preview-010184\

Host (useful for support):
  Version: 3.0.0-preview-27324-5
  Commit:  63a01b08e5

.NET Core SDKs installed:
  2.0.3 [C:\Program Files\dotnet\sdk]
  2.1.3 [C:\Program Files\dotnet\sdk]
  2.1.100 [C:\Program Files\dotnet\sdk]
  2.1.104 [C:\Program Files\dotnet\sdk]
  2.1.200 [C:\Program Files\dotnet\sdk]
  2.1.201 [C:\Program Files\dotnet\sdk]
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.300-rc1-008673 [C:\Program Files\dotnet\sdk]
  2.1.300 [C:\Program Files\dotnet\sdk]
  2.1.301 [C:\Program Files\dotnet\sdk]
  2.1.302 [C:\Program Files\dotnet\sdk]
  2.1.400 [C:\Program Files\dotnet\sdk]
  2.1.401 [C:\Program Files\dotnet\sdk]
  2.1.402 [C:\Program Files\dotnet\sdk]
  2.1.403 [C:\Program Files\dotnet\sdk]
  2.1.500 [C:\Program Files\dotnet\sdk]
  2.1.502 [C:\Program Files\dotnet\sdk]
  2.1.503 [C:\Program Files\dotnet\sdk]
  2.1.600-preview-009472 [C:\Program Files\dotnet\sdk]
  2.2.100-preview2-009404 [C:\Program Files\dotnet\sdk]
  2.2.100-preview3-009430 [C:\Program Files\dotnet\sdk]
  2.2.100 [C:\Program Files\dotnet\sdk]
  2.2.101 [C:\Program Files\dotnet\sdk]
  2.2.102 [C:\Program Files\dotnet\sdk]
  2.2.103 [C:\Program Files\dotnet\sdk]
  2.2.200-preview-009748 [C:\Program Files\dotnet\sdk]
  3.0.100-preview-010184 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0-rc1-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.0-preview2-35157 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.0-preview3-35497 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0-rc1-final [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.0-preview2-35157 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.0-preview3-35497 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.0-preview-19075-0444 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0-rc1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.0-preview2-26905-02 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.0-preview3-27014-02 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.0-preview-27324-5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.0.0-preview-27325-3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[Eilon]

Presumably due to this breaking change: aspnet/Announcements#325

I don't think a type forwarder can be use because I thought they could be used only when the type moved to an assembly referenced from the original assembly. In this case clearly that can't happen because the whole idea was to move JsonResult to a new, un-referenced assembly, that the developer has to add a reference to on their own. (I'm happy to be wrong about all this, though.)

[davidfowl]

So JsonResult used to be in Microsoft.AspNetCore.Mvc.Formatters.Json and it moved to Microsoft.AspNetCore.Mvc.Core but one of the constructors changed shape as well so it's not binary compatible. Maybe we can do a TypeForwardedFrom on the new type to fix this compile error but I'm not sure if it'll work after that. We should see play around and see what we can do.

[martincostello]

I guess a way to make it a non-breaking compiler change would at least be good if it can't be a non-breaking binary change.

[mkArtakMSFT]

@pranavkm, any suggestions?

[pranavkm]

Type forwarding resolves this specific build error. There are other breaking changes with that specific package, but we expect using JsonResult in user code to work pretty smoothly.