Build warnings & .Net Standard dependencies

Build warnings & .Net Standard dependencies

Having recently migrated a Windows Azure hosted .Net Framework ASP.net MVC 5 web application over to ASP.net Core (v2.2) I hit a few issues so I’m documenting them here for my future self and anyone else who might find it useful. The application was a multi functional ASP.net MVC site with an Azure Blob Storage Service back-end that I re-wrote in new ASP.Net Core with a mixture of both MVC and Razor Pages. The new site was .Net core 2.2 which referenced supporting projects built to .Net Standard 2.0.

The Problem

The application ran fine locally and as I was re-purposing the same Windows Azure Web Service instance I re-configured it to support .Net Core via the Azure Portal. Its not clear if this step is actually required but it makes sense to set it correctly (see screnshot below).

Next I deployed my Web App via Web Deploy, and then hit the homepage but immediately received a long list of errors such as the one below:

The type 'Attribute' is defined in an assembly that is not referenced. 
You must add a reference to assembly
'System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'. 
[assembly: global::Microsoft.AspNetCore.Razor.Hosting.RazorCompiledItemAttribute(typeof(AspNetCore.Views_Home_Index), @"mvc.1.0.view", @"/Views/Home/Index.cshtml")] ……etc……

It looked like .Net itself was not being found on the target machine, and so after checking the server settings and server logs in the Azure Portal I double checked my project references to confirm I was targeting .Net Core and .Net Standard 2.0 and that’s when I found a build Warning that I had previously missed:

[NU1701] Package 'Microsoft.Azure.KeyVault.Core 1.0.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETStandard,Version=v2.0'. 
This package may not be fully compatible with your project

One of my dependent projects in the solution was referencing the Azure SDK assembly Microsoft.Azure.Storage.Blob (via NuGet package) which in turn was referencing a version of Microsoft.Azure.KeyVault.Core that was not compatible with .NetStandard 2.0. This solution ran fine locally (as the dependent framework is installed on my machine) but when the application was deployed to a server without the .Net Framework Version=v4.6.1 binaries the application failed to load. More on this problem can be found in this GitHub issue.

The Solution

The solution was to reference a later version of the Microsoft.Azure.KeyVault.Core assembly that does support .NetStandard. As my project had no direct reference to Microsoft.Azure.KeyVault.Core (as it was a dependency of Microsoft.Azure.Storage.Blob) I had no dependency reference to update and so instead added a direct dependency to Microsoft.Azure.KeyVault.Core version 3.0.3 in my project file. This didn’t directly resolve my issue though despite reportedly working for other people, but at this point I decided to update all my Azure SDK references which took Microsoft.Azure.Storage.Blob to v11.1.3 which references a .NetStandard 2.0 compliant version of the KeyVault.Core dependency:

  <ItemGroup>
    <PackageReference Include="Microsoft.Azure.Storage.Blob" Version="11.1.3" />
  </ItemGroup>

After upgrading (tweaking a few updated namespaces that has changed in the newer package versions) I deployed the project successfully without error. Issue resolved!

The lesson here I think is to firstly don’t ignore build warnings (we all sometimes – right?) and secondly be careful with dependencies and their children not being .NetStandard compliant. This may be less of a problem in the future as .Net Core and .Net Standard gets more widespread adoption but for now its one to watch for.