Could not load file or assembly Microsoft.IdentityModel.Tokens (related to _FunctionsSkipCleanOutput)

A newly-created Azure Functions project fails to load Microsoft.IdentityModel.Tokens even when the .csproj file contains a PackageReference to the Microsoft.IdentityModel.Tokens NuGet package.

Investigative information

N/A, this issue is reproducible locally.

Repro steps

1. Clone my repo: https://github.com/srmagura/FunctionsAssemblyLoadingRepro
2. Open the solution file in Visual Studio 2022.
3. Run the project.
4. Go to http://localhost:7071/api/Function1 in your web browser.

Expected behavior

There should be no errors in the debug window. The browser should display the text “Function executed.”

Actual behavior

An error message is displayed in the debug window:

System.Private.CoreLib: Exception while executing function: Function1. FunctionsAssemblyLoadingRepro: Could not load file or assembly 'Microsoft.IdentityModel.Tokens, Version=6.14.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. The system cannot find the file specified.

Known workarounds

Add <_FunctionsSkipCleanOutput>true</_FunctionsSkipCleanOutput> to the .csproj file. This is a workaround, not a solution, because this property is internal and undocumented.

Related issues

• https://github.com/Azure/azure-functions-host/issues/7601
• https://github.com/Azure/azure-functions-host/issues/5894
• https://github.com/Azure/azure-functions-host/issues/7061
• https://github.com/Azure/azure-functions-host/issues/5756

[srmagura]

Commenting to keep issue open

[TroyWitthoeft]

Also ran into this trying to reference the latest Microsoft.IdentityModel.JsonWebTokens library at version 6.15.0 It can be repro’d by creating a new azure functions project in vs code using the azure functions extension. Select dotnet 6.0 (in-process) and runtime host of v4. If I try and reference any objects from that library I get the dreaded Could not load file or assembly error. I did quite the bit of research. Most roads point to the saga that is #5894, and the _FunctionsSkipCleanOutput suggestion there DID work.

However - as others have said - it is a bit of a club. The whole all or nothing approach to cleaning the output. I took a look at the FunctionsPreservedDependencies feature that was added in pull 6849 in order to address this defeciency, and it didn’t work for me.

Lastly, I did notice that if I pin my Microsoft.IdentityModel.JsonWebTokens version to 6.10.0 everything worked just fine. 🎉

Full example Azure functions csproj below

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <AzureFunctionsVersion>v4</AzureFunctionsVersion>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.IdentityModel.JsonWebTokens" Version="6.10.0" />
    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="4.0.1" />
  </ItemGroup>
  <ItemGroup>
    <None Update="host.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Update="local.settings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <CopyToPublishDirectory>Never</CopyToPublishDirectory>
    </None>
  </ItemGroup>
</Project>

Sharing this pin to 6.10.0 solution for other folks that don’t may not want to use _FunctionsSkipCleanOutput or FunctionsPreservedDependencies. My guess is this works because the azure functions host is currently using this version? Can see it listed in this repo here. Sure would be nice to be able to reference the latest 6.15.0 Microsoft.IdentityModel.JsonWebTokens and it’s security enhancements.

Burned a day researching this. Looks like @anthonychu has got feature request 6650 that might help folks save some research time on this?