Common Issues

You must add a reference...

Note: Updating to 4.5 or newer should alleviate this issue as it is now multi-targeted to .NET 4.5 and .NET Standard 2.0.

The error message you see is similar to:

The type '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'

This has mostly been seen in new web forms projects targeting .NET 4.7 and 4.7.1.

This appears to be a bug in Visual Studio and has some discussion here: https://github.com/dotnet/standard/issues/542 and here: https://github.com/dotnet/standard/issues/481

Also, an issue in the VS Developer Community: https://developercommunity.visualstudio.com/content/problem/335717/csharpaddmissingreferencecodefixprovider-encounter.html

The easiest way to fix this is to edit your web.config (in ASP.NET WebForms websites) and add this line in system.web/compilation/assemblies:

<add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>

So it will look like this:

<compilation debug="false" targetFramework="4.7">
  <assemblies>
    <add assembly="System.Data.OracleClient, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
    <add assembly="System.Net, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
    <add assembly="System.ServiceProcess, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
    <add assembly="System.Transactions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
    <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>
  </assemblies>
</compilation>