How to resolve .NET Core package version conflicts

Question

I am migrating from a .NET MVC 5 Web Application to a .NET Core 2.2 Web API project along with five .NET Standard 2.0 projects all housed under one solution.

I am now receiving 28 warnings (MSB3277) regarding package conflicts, which are all within the System namespace. For example, there appears to be a version conflict for System.Collections.Concurrent between Version=4.0.11.0 and Version=4.0.14.0 (see error block below).

Troubleshooting attempted:

1. I tried uninstalling all of the .NET Core SDKs that I do not use, and currently have 2.2.300 installed. That did not work.
2. I tried going into NuGet at the solution level to install the missing assembly, but 4.0.14 isn't even available in the selection list. I can select from the following versions: 4.3 , 4.0.12 , 4.0.10 and 4.0.0 . I did try installing 4.3 in all of the projects, but that did not work.
3. Other solutions on the web appear to either recommend going down a version on the SDK or hard-coding a reference in the *.csproj file - neither of which I prefer to do if I do not have to. I do not like the former because it seems counter-intuitive to be moving backwards and I do not like the latter because that hard codes a fix to permanently use an older package, which seems like a really bad idea because it seems unlikely that it will update to newer versions that get released.

Does anyone know how to resolve these warnings? Any help is greatly appreciated.

Detailed output on build for one of the warnings:

Line 5419: 3>    Dependency "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
Line 5420: 3>        Could not resolve this reference. Could not locate the assembly "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.
Line 5675: 3>    There was a conflict between "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" and "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
Line 5675: 3>    There was a conflict between "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" and "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
Line 5676: 3>        "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was chosen because it was primary and "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was not.
Line 5676: 3>        "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was chosen because it was primary and "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" was not.
Line 5677: 3>        References which depend on "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" [C:\Program Files\dotnet\sdk\NuGetFallbackFolder\netstandard.library\2.0.3\build\netstandard2.0\ref\System.Collections.Concurrent.dll].
Line 5677: 3>        References which depend on "System.Collections.Concurrent, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" [C:\Program Files\dotnet\sdk\NuGetFallbackFolder\netstandard.library\2.0.3\build\netstandard2.0\ref\System.Collections.Concurrent.dll].

Update

More troubleshooting:

https://github.com/do.net/standard/issues/731

https://github.com/do.net/corefx/issues/32561

Is this a Microsoft bug?

Minimal output on build listing all 28 warnings:

3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Collections.Concurrent" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Collections" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Collections.NonGeneric" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.ComponentModel" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Diagnostics.Debug" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Diagnostics.Process" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Diagnostics.Tracing" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.IO.FileSystem" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Linq" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Net.Primitives" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Net.Security" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Resources.ResourceManager" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Runtime" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Runtime.Extensions" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Runtime.InteropServices.RuntimeInformation" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Security.Cryptography.Algorithms" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Security.Cryptography.Csp" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Security.Cryptography.Encoding" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Security.Cryptography.Primitives" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Security.Cryptography.X509Certificates" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Text.Encoding.Extensions" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Text.RegularExpressions" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Threading" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Threading.Tasks" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Threading.ThreadPool" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Threading.Timer" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.
3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3277: Found conflicts between different versions of "System.Memory" that could not be resolved.  These reference conflicts are listed in the build log when log verbosity is set to detailed.

Answer 1

The issue is here strong naming and mixed assembly versions for at least "System.Collections.Concurrent", but probably for all the 28 warnings.

I tried going into NuGet at the solution level to install the missing assembly, but 4.0.14 isn't even available in the selection list.

The versions you are seeing are the NuGet versions, and those aren't the same to the assembly versions. eg NuGet version 4.3.0-preview1-24530-04 has assembly version 4.0.13.0 for .NET standard 1.3:

Please note that in the same package, the assembly version for netcore50 is 4.0.10.0!

Is this a Microsoft bug?

No it's a feature. When an assembly is strong named ., then the full version should match. If not, this will give a warning. When not using the GAC, then only one version of the assembly could be published - so which one if multiple versions are used? Note- all Microsoft's assemblies are strong named.

The problem here:

• You are including an (strong named) assembly that's built to System.Collections.Concurrent assembly version 4.0.14.0
• You are (indirectly) using in your application assembly version 4.0.11.0 for the System.Collections.Concurrent assembly.

So two version for one assembly!

Solutions

There are multiple solutions possible:

1. Use everywhere the same assembly version for System.Collections.Concurrent (you need to find out which NuGet package is assembly version 4.0.14.0). This is most of the time not feasible.
2. Install the version 4.0.11.0 and 4.0.14.0 into the GAC - this ins't a really popular option either these days -> Not possible for .NET Core as there is no GAC for .NET Core. See Is there any GAC equivalent for .NET Core?

3. Use a <bindingRedirect> in your .config. See Redirecting Assembly Versions | Microsoft Docs

   eg for "System.Collections.Concurrent":

    <dependentAssembly> <assemblyIdentity name="System.Collections.Concurrent" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" /> <bindingRedirect oldVersion="4.0.0.0-4.0.14.0" newVersion="4.0.11.0" /> </dependentAssembly> 

Final notes

There are 28 version issues, but there is a big change that multiple issues are solved with a single bindingRedirect. So the best way is to start with one (preferable the most top-level one), and rebuild and repeat until all are resolved.

Please also note that downgrading an assembly is a bit tricky, eg the assembly built on 4.0.13.0 could use a feature that is introduced/changed in 4.0.11.0+. So you could also update your version and use that version in "newVersion" attribute

Answer 2

Binding redirect (bindingRedirect ) for every assembly you can do it manually in config or there is an option in Project file properties. Check it then upgrade.

Answer 3

Short Answer: When migrating from one framework to another, YOU MAY COPY AND PAST YOUR CODE, BUT DO NOT COPY AND PASTE THE USING STATEMENTS AT THE TOP OF THE CLASSES The compiler frowns on the latter. This makes sense because the frameworks in question (.NET Framework to .NET Core and .NET Standard) are different and, without getting into the specifics, Microsoft has moved things around.

Julian's answer pointed to a workaround when copying and pasting the using statements. But, I opted to go ahead and re-migrate the project without the using statements and then manually re-included them with the help of Intellisense. All warnings went away.

Sigh, this is what I get for copying and pasting my code. It appears that Microsoft's efforts to migrate .NET Framework into a .NET Core and .NET Standard implementation has convinced the compiler to drink a fifth of tequila and start speaking Spanish to developers. In my case, it said "¿que es eso?"

My response: You're the compiler, you should know.

Answer 4

@Julian's solution wasn't working for me. In my case, I had a test project referencing another project with business logic. The dependency graph looked like this:

Project.BL.Tests
↳ Project.BL

Project.BL
↳ Microsoft.Extensions.DependencyInjection.Abstractions Version="6.0.0"
↳ ThirdParty.Nuget.Package

ThirdParty.Nuget.Package
↳ Microsoft.Extensions.DependencyInjection.Abstractions Version="3.1.0"

When I tried to create a ServiceCollection instance in tests, the error was thrown during build time, saying that DI.Abstractions v6 and v3 are conflicting. I tried adding

<PropertyGroup>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
    <GenerateBindingRedirectsOutputType>true</GenerateBindingRedirectsOutputType>
</PropertyGroup>

But it didn't fix it.

The solution was to add a ThirdParty.Nuget.Package dependency directly to the test project:

Project.BL.Tests
↳ Project.BL
↳ ThirdParty.Nuget.Package