The "TransformAppSettings" task failed unexpectedly

Question

I have a .NET Core 3 Blazor (server side) application which I recently upgraded to .NET Core 3.0.1 preview 6 version from the preview 5 version. When I build and run it locally, it works fine; but when trying to publish it to a file system folder (in Framework-Dependent mode), it throws this error:

C:\Program Files\dotnet\sdk\3.0.100-preview6-012264\Sdks\Microsoft.NET.Sdk.Publish\targets\TransformTargets\Microsoft.NET.Sdk.Publish.TransformFiles.targets(192,5): Error MSB4018: The "TransformAppSettings" task failed unexpectedly. System.IO.FileNotFoundException: Could not load file or assembly 'Newtonsoft.Json, Version=10.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The system cannot find the file specified. File name: 'Newtonsoft.Json, Version=10.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'

at Microsoft.NET.Sdk.Publish.Tasks.AppSettingsTransform.UpdateDestinationConnectionStringEntries(String destinationAppSettingsFilePath, ITaskItem[] destinationConnectionStrings) at Microsoft.NET.Sdk.Publish.Tasks.TransformAppSettings.TransformAppSettingsInternal() at Microsoft.NET.Sdk.Publish.Tasks.TransformAppSettings.Execute() at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() at Microsoft.Build.BackEnd.TaskBuilder.d__26.MoveNext()

It's perhaps worth mentioning that this error didn't occur in the preview 5 version. Also, I use Visual Studio Enterprise 2019 (Windows).

Things I've tried so far: (with no luck)

• Clean/Rebuild solution
• Reinstall .NET Core 3 preview 6 SDK
• Add Newtonsoft.Json package via Nuget
• Search for related issues raised by the community on github

.csproj file

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <LangVersion>7.3</LangVersion>
    <AspNetCoreHostingModel>InProcess</AspNetCoreHostingModel>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="AutoMapper.Extensions.Microsoft.DependencyInjection" Version="6.1.0" />
    <PackageReference Include="MatBlazor" Version="1.2.0" />
    <PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
    <PackageReference Include="SqlTableDependency" Version="8.5.3" />
    <PackageReference Include="System.DirectoryServices" Version="4.5.0" />
    <PackageReference Include="System.DirectoryServices.AccountManagement" Version="4.5.0" />
    <PackageReference Include="Telerik.UI.for.Blazor" Version="1.1.1" />
  </ItemGroup>

  <ItemGroup>
    <Folder Include="wwwroot\images\" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\{path to project file}" />
  </ItemGroup>

</Project>

Answer 1

In my case, I edit the publish configuration and turned off the Database -> Use this connection at runtime. Then I could publish without this error, however, I had to manually edit the web.config and make the following changes.

modules="AspNetCorModuleV2"  had to become modules="AspNetCodeModule"

hostingModel="InProcess" had to become hostingModel="OutOfProcess" 

<environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" /> had to become value="Production"

These changes were already in my own directories web.config, but they didn't seem to get propagated in the publish anymore.

Answer 2

I had this same error and was able to fix it by editing the .pubxml.user file.

When the error occurred, my publish settings didn't have the "Use this connection at runtime" selected, nor were any database migrations checked. But when I looked in the pubxml.user file and it turns out there was a reference to a non-existent database that wasn't showing up in the Publish Settings UI. I removed that and it solved the problem.