Multiple assemblies with equivalent identity imported (mscorlib)

[dazinator]

Steps to Reproduce

Repro available from this branch here: https://github.com/dazinator/Repros/tree/BuildIssue

1. Clone https://github.com/dazinator/Repros
2. git checkout BuildIssue
3. Open Solution file in VS 2017 15.5.3 (i.e the latest)
4. Build Todo.Android

Expected Behavior

Builds.

Actual Behavior

69 errors. First of which:

Severity Code Description Project File Line Suppression State
Error CS1703 Multiple assemblies with equivalent identity have been imported: 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0\mscorlib.dll' and 'C:\Users\darrell.tunnell.nuget\packages\microsoft.netcore.portable.compatibility\1.0.2\ref\netstandard1.0\mscorlib.dll'. Remove one of the duplicate references. Todo.Android C:\Users\darrell.tunnell\Source\Repos\BuildIssue\Repros\Todo.Android\CSC 1 Active

followed by many:

Severity Code Description Project File Line Suppression State
Error CS0518 Predefined type 'System.Object' is not defined or imported Todo.Android C:\Users\darrell.tunnell\Source\Repos\BuildIssue\Repros\Todo.Android\MainActivity.cs 15 Active

and including:

Severity Code Description Project File Line Suppression State
Error CS0731 The type forwarder for type 'System.Object' in assembly 'System.Runtime' causes a cycle Todo.Android C:\Users\darrell.tunnell\Source\Repos\BuildIssue\Repros\Todo.Android\MainActivity.cs 15 Active

Version Information

Microsoft Visual Studio Professional 2017
Version 15.5.3
VisualStudio.15.Release/15.5.3+27130.2020
Microsoft .NET Framework
Version 4.7.02556

Installed Version: Professional

Visual Basic 2017 00369-60000-00001-AA768
Microsoft Visual Basic 2017

Visual C# 2017 00369-60000-00001-AA768
Microsoft Visual C# 2017

Visual C++ 2017 00369-60000-00001-AA768
Microsoft Visual C++ 2017

Visual F# 4.1 00369-60000-00001-AA768
Microsoft Visual F# 4.1

.NET Portability Analyzer 1.1.10808.0
Evaluates portability of assemblies across .NET platforms.

Application Insights Tools for Visual Studio Package 8.10.01106.1
Application Insights Tools for Visual Studio

ASP.NET and Web Tools 2017 15.0.31127.0
ASP.NET and Web Tools 2017

ASP.NET Core Razor Language Services 1.0
Provides languages services for ASP.NET Core Razor.

ASP.NET Web Frameworks and Tools 2017 5.2.51007.0
For additional information, visit https://www.asp.net/

Azure App Service Tools v3.0.0 15.0.31106.0
Azure App Service Tools v3.0.0

Common Azure Tools 1.10
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.

JavaScript Language Service 2.0
JavaScript Language Service

JavaScript Project System 2.0
JavaScript Project System

JavaScript UWP Project System 2.0
JavaScript UWP Project System

JustDecompilePackage2017 Extension 1.0
JustDecompilePackage2017 Visual Studio Extension Detailed Info

Merq 1.1.17-rc (cba4571)
Command Bus, Event Stream and Async Manager for Visual Studio extensions.

Microsoft Azure Tools 2.9
Microsoft Azure Tools for Microsoft Visual Studio 2017 - v2.9.51120.3

Microsoft Continuous Delivery Tools for Visual Studio 0.3
Simplifying the configuration of continuous build integration and continuous build delivery from within the Visual Studio IDE.

Microsoft JVM Debugger 1.0
Provides support for connecting the Visual Studio debugger to JDWP compatible Java Virtual Machines

Microsoft MI-Based Debugger 1.0
Provides support for connecting Visual Studio to MI compatible debuggers

Microsoft Visual C++ Wizards 1.0
Microsoft Visual C++ Wizards

Microsoft Visual Studio Tools for Containers 1.1
Develop, run, validate your ASP.NET Core applications in the target environment. F5 your application directly into a container with debugging, or CTRL + F5 to edit & refresh your app without having to rebuild the container.

Microsoft Visual Studio VC Package 1.0
Microsoft Visual Studio VC Package

Mono Debugging for Visual Studio 4.8.4-pre (3fe64e3)
Support for debugging Mono processes with Visual Studio.

NuGet Package Manager 4.5.0
NuGet Package Manager in Visual Studio. For more information about NuGet, visit http://docs.nuget.org/.

SettingsWindow Extension 1.0
SettingsWindow Visual Studio Extension Detailed Info

SQL Server Data Tools 15.1.61710.120
Microsoft SQL Server Data Tools

TypeScript Tools 15.5.11025.1
TypeScript Tools for Microsoft Visual Studio

Visual Studio Code Debug Adapter Host Package 1.0
Interop layer for hosting Visual Studio Code debug adapters in Visual Studio

Visual Studio Tools for CMake 1.0
Visual Studio Tools for CMake

Visual Studio Tools for Universal Windows Apps 15.0.27130.2020
The Visual Studio Tools for Universal Windows apps allow you to build a single universal app experience that can reach every device running Windows 10: phone, tablet, PC, and more. It includes the Microsoft Windows 10 Software Development Kit.

VisualStudio.Mac 1.0
Mac Extension for Visual Studio

Xamarin 4.8.0.757 (7f9ec2a)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer 4.8.188 (c5813fa34)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin.Android SDK 8.1.3.0 (HEAD/ef47226b7)
Xamarin.Android Reference Assemblies and MSBuild support.

Xamarin.iOS and Xamarin.Mac SDK 11.6.1.2 (6857dfc)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Log File

I'm not on a mac.

I can't add an area label as not visible.

[dazinator]

Issue was caused by presence of this package in the netstandard project:

  <PackageReference Include="Microsoft.NETCore.Portable.Compatibility" Version="1.0.2" />

Removing it, cleared the problems.

This issue was encountered after updating a netstandard1.1 solution to netstandard2.0 - the netstandard1.1 solution was created before the official templates were released for netstandard20.