.NET 5 have been out for a few weeks when writing then. Upgrading from .NET Core 3.1 has been easy, I think. Except when it comes to Azure Functions.

The problem

To keep it short, at the moment of writing this Azure Functions does not support .NET 5. As I understand it, the process that is executing you functions library is built with .NET Core 3.1. And if you are using a library, like Microsoft.Extensions.Logging.Abstractions, that is also used by the main host – but it is using a different version – you get strange errors like:

System.Private.CoreLib: Could not load file or assembly 'Microsoft.Extensions.Logging.Abstractions,
Version=5.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.

The problem is discussed in Issue 6674 in the projects GitHub repository.

The solution

The suggestion workaround so far is to use .NET Standard libraries and use the older version of NuGet-packages. But I find it a bit messy to use different version NuGet-packages. Especially since the problem is detected first when you execute your application, not in compile time. So, I decided to do something else.

In my project file, for my Azure Functions project, I referenced to other projects like this:

Project File 
  <ItemGroup>
    <ProjectReference Include="..\DatabaseLibrary\DatabaseLibrary.csproj" />
    <ProjectReference Include="..\LogicLibrary\LogicLibrary.csproj" />
  </ItemGroup>

I removed this and decided to instead to link to every file in the other projects instead. The code above was then replaced with this:

Project File 
  <ItemGroup>
    <Compile Include="..\DatabaseLibrary\DatabaseHelper.cs" Link="Dependencies\DatabaseLibrary\DatabaseHelper.cs" />
    <Compile Include="..\DatabaseLibrary\DatabaseManager.cs" Link="Dependencies\DatabaseLibrary\DatabaseManager.cs" />
    <Compile Include="..\LogicLibrary\LogicHelper.cs" Link="Dependencies\LogicLibrary\LogicHelper.cs" />
    <Compile Include="..\LogicLibrary\LogicManager.cs" Link="Dependencies\LogicLibrary\LogicManager.cs" />
  </ItemGroup>

This way, I have better control on which NuGet-packages to use (that I instead references directly in my Azure Functions project).

In Visual Studio, you could right click on a project, select Add and then Existing Item. Then you select the files you want to use and use the drop-down menu and select Add As Link. But if you have a lot files this could be a bit tedious. Instead, you could run this PowerShell-script in the folder where you have your library:

PowerShell 
Get-ChildItem *.cs -Recurse `
  | Where-Object { $_ -notlike "*\Debug\*" -and $_ -notlike "*\Release\*" } `
  | ForEach-Object { Write-Output "    <Compile Include=""..\$($_.FullName.SubString(62))"" Link=""Dependencies\$($_.FullName.SubString(62))"" />" } `
  | Set-Clipboard

This code:

  • Get all .cs-files
  • Ignores files in Debug and Release directories.
  • Creates a string for each file that should be in the project file. You might need to adjust the value of the SubString call.
  • Copies everything to the clipboard.

You should then be able to paste everything into your project file.

Summary

Clearly, this solution has some limitations. For instance, you code must be able to be compiled with .NET Core 3.1. Also, if you change the files that you have in your libraries you may also need to change this in the Azure Functions project.

But for me, I decided that this is good enough. Azure Functions will have a preview with .NET 5 support quite soon, so I do not mind having it like this for a while.