title: How to resolve .NET Core package version conflicts
body: <p>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. </p>
  
  <p>I am now receiving 28 warnings (MSB3277) regarding package conflicts, which are all within the <code>System</code> namespace. For example, there appears to be a version conflict for <code>System.Collections.Concurrent</code> between <code>Version=4.0.11.0</code> and <code>Version=4.0.14.0</code> (see error block below).</p>
  
  <p><strong>Troubleshooting attempted:</strong></p>
  
  <ol>
  <li>I tried uninstalling all of the .NET Core SDKs that I do not use, and currently have <code>2.2.300</code> installed. That did not work.</li>
  <li>I tried going into NuGet at the solution level to install the missing assembly, but <code>4.0.14</code> isn't even available in the selection list. I can select from the following versions: <code>4.3</code>, <code>4.0.12</code>, <code>4.0.10</code> and <code>4.0.0</code>. I did try installing <code>4.3</code> in all of the projects, but that did not work.</li>
  <li>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.</li>
  </ol>
  
  <p>Does anyone know how to resolve these warnings? Any help is greatly appreciated.</p>
  
  <p><strong>Detailed output on build for one of the warnings:</strong></p>
  
  <pre><code>Line 5419: 3&gt;    Dependency "System.Collections.Concurrent, Version=4.0.14.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
  Line 5420: 3&gt;        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&gt;    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&gt;    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&gt;        "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&gt;        "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&gt;        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&gt;        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].
  </code></pre>
  
  <p><strong>Update</strong></p>
  
  <p>More troubleshooting: </p>
  
  <p><a href="https://github.com/dotnet/standard/issues/731" rel="noreferrer">https://github.com/dotnet/standard/issues/731</a></p>
  
  <p><a href="https://github.com/dotnet/corefx/issues/32561" rel="noreferrer">https://github.com/dotnet/corefx/issues/32561</a></p>
  
  <p>Is this a Microsoft bug?</p>
  
  <p><strong>Minimal output on build listing all 28 warnings:</strong></p>
  
  <pre><code>3&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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&gt;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.
  </code></pre>