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CS0012: The type 'System.Object' is defined in an assembly that is not referenced.

A fter referencing a .NET Standard 2.0 project in a .NET 4.8 ASP.NET MVC project, the project failed ant runtime with the following error message:

CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

Whoops! Let me explain how I fixed the problem.

The solution – Part I

I first tried to add the NETStandard.Library nuget package to the .NET 4.8 project but that didn’t made the error disappear.(Although I come back to this in Part II below).

So I removed the nuget package again and instead I did the following:

  • I manually edited the csproj file and added the following reference:
<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<Reference Include="netstandard" />
</ItemGroup>
</Project>
view raw Example.csproj hosted with ❤ by GitHub
  • I also updated the reference and set Copy Local=true

After doing that the error disappeared and the application ran successfully on my local machine.

Victory…

… or not? After committing the updated project, a colleague contacted me that he wasn’t able to compile it on his machine as Visual Studio complained that he couldn’t find the netstandard reference.

The solution – Part II

The solution was to move back to my original attempt and also add the NETStandard.Library nuget package to the .NET 4.8 project:

<?xml version="1.0" encoding="utf-8"?>
<packages>
<package id="Microsoft.CodeDom.Providers.DotNetCompilerPlatform" version="2.0.1" targetFramework="net48" />
<package id="Microsoft.NETCore.Platforms" version="1.1.0" targetFramework="net48" />
<package id="NETStandard.Library" version="2.0.3" targetFramework="net48" />
</packages>
view raw Packages.config hosted with ❤ by GitHub

That finally fixed the problem…

More information

NuGet Gallery | NETStandard.Library 2.0.3

.NET Standard - .NET | Microsoft Learn

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