You must add a reference to assembly 'netstandard, Version=2.0.0.0

I think the solution might be this issue on GitHub:

Try add netstandard reference in web.config like this:"

I realise you're using 4.6.1 but the choice of .NET 4.7.1 is significant as older Framework versions are not fully compatible with .NET Standard 2.0.

I know this from painful experience, when I introduced .NET Standard libraries I had a lot of issues with NUGET packages and references breaking. The other change you need to consider is upgrading to PackageReferences instead of package.config files.

See this guide and you might also want a tool to help the upgrade. It does require a late VS 15.7 version though.

Manually editing the .csproj file and adding the reference below worked for me.

<ItemGroup>
    <Reference Include="netstandard" />
</ItemGroup>

Thank you to Fahad Alshaya who suggested it here.

I had to do a combination of other people's answers on this thread.

Install the NetStandard.Library via NuGet Manually editing the .csproj file and adding the reference. Expanding project-->References in the VS Solution Explorer, right clicking on 'netstandard' and showing the properties page and setting "Copy Local" to true.

Deleting Bin and Obj folders worked for me.

After upgrading from 4.6.1 framework to 4.7.2 we started getting this error:

"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'." and ultimately the solution was to add the "netstandard" assembly reference mentioned above:

<compilation debug="true" targetFramework="4.7.1" >
    <assemblies>
      <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, 
            PublicKeyToken=cc7b13ffcd2ddd51"/>
    </assemblies>
  </compilation>

I have run into this before and trying a number of things has fixed it for me:

Delete a bin folder if it exists

Delete the hidden .vs folder

Make sure the 4.6.1 targeting pack is installed

Last Ditch Effort: Add a reference to System.Runtime (right click project -> add -> reference -> tick the box next to System.Runtime), although I think I've always figured out one of the above has solved it instead of doing this.

Also, if this is a .net core app running on the full framework, I've found you have to include a global.json file at the root of your project and point it to the SDK you want to use for that project:

{
  "sdk": {
    "version": "1.0.0-preview2-003121"
  }
}

https://i.stack.imgur.com/Ibsi6.png

Set Copy Enbale to true in netstandard.dll properties.

Open Solution Explorer and right click on netstandard.dll. Set Copy Local to true.

Although this is an old thread, I had the same issue today, last week I updated some NuGet packages and although the MVC website worked OK on my dev machine when I published to the testing server it failed.

I read numerous posts but none worked. I finally compared the DLL's in my local bin to those in the testing server and found that the netstandard.dll was not uploaded, once uploaded the website worked OK, not sure why VS2017 web deploy did not publish the DLL.

Just something to look out for in case none of the above work for you.

Those who are not having web.config file. Output Type other than web application. update the project file (.csproj) with below give code.

It may cause due to adding/removing the .netframework in improper way or it may broke unexpected way.

  <ItemGroup>
    <Reference Include="netstandard" />
  </ItemGroup>

Output Type

Console application

Class Library

This is where netstandard.dll exists: C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2\Facades\netstandard.dll Add ref to your Project through this.

Might have todo with one of these:

Install a newer SDK. In .csproj check for Reference Include="netstandard" Check the assembly versions in the compilation tags in the Views\Web.config and Web.config.

I experienced this when upgrading .NET Core 1.1 to 2.1.

I followed the instructions outlined here.

Try to remove <RuntimeFrameworkVersion>1.1.1</RuntimeFrameworkVersion> or <NetStandardImplicitPackageVersion> section in the .csproj.

We started getting this error on the production server after deploying the application migrated from 4.6.1 to 4.7.2.

We noticed that the .NET framework 4.7.2 was not installed there. In order to solve this issue we did the following steps:

Installed the .NET Framework 4.7.2 from: https://support.microsoft.com/en-us/help/4054530/microsoft-net-framework-4-7-2-offline-installer-for-windows Restarted the machine Confirmed the .NET Framework version with the help of How do I find the .NET version?

Running the application again with the .Net Framework 4.7.2 version installed on the machine fixed the issue.

I am facing Same Problem i do following Setup Now Application Work fine

1-

<compilation debug="true" targetFramework="4.7.1">
      <assemblies>
        <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, 
      PublicKeyToken=cc7b13ffcd2ddd51"/>
      </assemblies>
    </compilation>

2- Add Reference

 **C:\Program Files (x86)\Microsoft Visual
Studio\2017\Professional\Common7\IDE\Extensions\Microsoft\ADL
 Tools\2.4.0000.0\ASALocalRun\netstandard.dll**

3-

Copy Above Path Dll to Application Bin Folder on web server

I was facing this problem when trying to add a .NETStandard dependency to a .NET4.6.1 library, and compiling it in Linux with Mono 4.6.2 (the version that comes with Ubuntu 16.04).

I finally solved it today; the solution requires to do both of these things:

Change v4.6.1 to v4.7.1 in the .csproj file. Upgrade your mono to a newer version. I believe 5.x should work, but to be sure, you can just install Ubuntu 20.04 (which at the time of writing is only in preview), which includes Mono 6.8.0.105.

You can add to your web.config in your project.

It wouldn't work when you add it to projects web.config because it works with MVC.

The solution of Quango in is working but I prefer to resolve it by adding this code in my Web.config like new projects :

<system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:default /nowarn:1659;1699;1701"/>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:default /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
  </system.codedom>

I have the same problem with a net5.0 project, which includes a buildtask written using netstandard2.0

<Project>

  <Target Name="RestoreBuildTasks">
    <MSBuild Projects="../../../BuildTasks/BuildTasks.csproj" Targets="Restore" Properties="Configuration=Release"/>
  </Target>

  <Target Name="BuildBuildTasks">
    <MSBuild Projects="../../../BuildTasks/BuildTasks.csproj" Targets="Build" Properties="Configuration=Release"/>
  </Target>

  <UsingTask
      TaskName="Brick.Build.NugetTask"
      AssemblyFile="AGXBrick/BuildTasks/bin/$(Platform)/Release/netstandard2.0/BuildTasks.dll"/>

  <Target Name="CheckNuGetSources" BeforeTargets="Restore" DependsOnTargets="RestoreBuildTasks;BuildBuildTasks">
      <NuGetTask />
  </Target>

</Project>

Where the BuildTasks.csproj looks like this

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>

I was able to resolve this by explicitly setting TargetFramework=netstandard2.0 in the MSBuild task:

    <MSBuild Projects="../../../BuildTasks/BuildTasks.csproj" Targets="Restore" Properties="Configuration=Release;TargetFramework=netstandard2.0"/>

I got same issue, my project use .net framework 4.6.1, and when I use latest MSBuild(16.10.2) to build solution, I got an error message, e.g. ValueType ... you must reference netstandard 2.0.....

I figured it out by installing .Net SDK item, when you select this item, .NET 5.0 runtime and .NET 3.1 Runtime will be installed as well.

https://i.stack.imgur.com/HGbDc.png

After I installed this item, I can build my project with latest MSBuild and no errors!

I had the same issue that would show up every once and a while, probably due to switching branches.

Using 4.6.1. Visual studio 2019, resharper...

My solution was to manually add a new reference, browse to the netstandard.dll, add it, build (errors galore), remove the reference, build again (no errors).

For me it was as simple as the framework being set wrong in the web.config file.

Solution is 4.8 but the web.config was 4.5.1

From

<httpRuntime requestValidationMode="2.0" maxRequestLength="8192" executionTimeout="110" targetFramework="4.5.1" />

To

<httpRuntime requestValidationMode="2.0" maxRequestLength="8192" executionTimeout="110" targetFramework="4.8" />

In my case I've updated my VS 2019 to the latest version as of 05/21, but still got the same issue.

I resolved it by

carefully looking at the error message The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'system.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a Open .csproj file and under tag add a new tag as below

Follow VS prompt for a project reload, and re-build and I didn't see the same issue.

This issue is based on your installed version of visual studio and Windows, you can follow the following steps:-

Go to Command Window downgraded your PCL by the following command Install-Package Xamarin.Forms -Version 2.5.1.527436 Rebuild Your Project. Now You will able to see the required output