Found conflicts between different versions of the same dependent assembly that could not be resolved

eta: There's a killer article on this stuff by SO's own @Nick Craver that you should read

While the other responses say this, they don't make it explicit, so I will....

On VS2013.2, to actually trigger the emission of the cited information, you need to not read the message, which says:

C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1697,5): warning MSB3277: Found conflicts between different versions of the same dependent assembly that could not be resolved. These reference conflicts are listed in the build log when log verbosity is set to detailed.

This is incorrect (or at least it was for some versions of Visual Studio - it seems to be OK on an up to date VS2015 Update 3 or later). Instead turn it to Diagnostic (from Tools->Options->Project and Solutions->Build and Run, set MSBuild project build output verbosity), whereupon you'll see messages such as:

There was a conflict between "Newtonsoft.Json, Version=6.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" and "Newtonsoft.Json, Version=6.0.5.17707, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed". "Newtonsoft.Json, Version=6.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" was chosen because it was primary and "Newtonsoft.Json, Version=6.0.5.17707, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" was not.

Then

Ctrl-Alt-O to go to Build output window

search for "was chosen" to find the drilldown.

...And yes, for those looking at the detail of the [diagnostic] message, it was news to this ignoramus that there's a convention in town whereby all 6.x versions are, internally Assembly Version 6.0.0.0, i.e. only the SemVer Major component goes into the Assembly Version :)

Run msbuild Foo.sln /t:Rebuild /v:diag (from C:\Program Files (x86)\MSBuild\12.0\bin) to build your solution from command line and get a bit more details, then find the .csproj. that logs the warning and check its references and references of other projects that use the same common assembly that differs in version.

Edit: You can also set the build verbosity directly in VS2013. Go to Tools > Options menu then go to Projects and Solutions and set MSBuild verbosity to Diagnostic.

Edit: Few clarifications as I just got one myself. In my case warning was due to me adding a reference using Resharper prompt as opposed to the Add Reference dialog, which did it versionless even though both v4 and v12 are available to choose from.

<Reference Include="Microsoft.Build, Version=12.0.0.0, ..." />
<Reference Include="Microsoft.Build.Framework" />

vs

<Reference Include="Microsoft.Build, Version=12.0.0.0, ..." />
<Reference Include="Microsoft.Build.Framework, Version=12.0.0.0, ..." />

In the MSBuild log with /v:diag verbosity it looked like the following. giving details which two references conflicted:-

  There was a conflict between 
  "Microsoft.Build.Framework, Version=4.0.0.0, ..." and 
  "Microsoft.Build.Framework, Version=12.0.0.0, ...". (TaskId:16)

      "Microsoft.Build.Framework, Version=4.0.0.0, ..." was chosen because it was primary and 
      "Microsoft.Build.Framework, Version=12.0.0.0, ..." was not. (TaskId:16)

      References which depend on "Microsoft.Build.Framework, Version=4.0.0.0, ..." 
      [C:\...\v4.5.1\Microsoft.Build.Framework.dll]. (TaskId:16)

          C:\...\v4.5.1\Microsoft.Build.Framework.dll (TaskId:16)
            Project file item includes which caused reference "C:\...\v4.5.1\Microsoft.Build.Framework.dll". (TaskId:16)
              Microsoft.Build.Framework (TaskId:16)

      References which depend on "Microsoft.Build.Framework, Version=12.0.0.0, ..." 
      [C:\...\v12.0\Microsoft.Build.Framework.dll]. (TaskId:16)

          C:\...\v12.0\Microsoft.Build.dll (TaskId:16)
            Project file item includes which caused reference "C:\...\v12.0\Microsoft.Build.dll". (TaskId:16)
              Microsoft.Build, Version=12.0.0.0, ... (TaskId:16)

          C:\...\v12.0\Microsoft.Build.Engine.dll (TaskId:16)
            Project file item includes which caused reference "C:\...\v12.0\Microsoft.Build.Engine.dll". (TaskId:16)
              Microsoft.Build, Version=12.0.0.0, ... (TaskId:16)

C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1697,5): warning MSB3277: 
Found conflicts between different versions of the same dependent assembly that could not be resolved.  
These reference conflicts are listed in the build log when log verbosity is set to detailed. 
[C:\Users\Ilya.Kozhevnikov\Dropbox\BuildTree\BuildTree\BuildTree.csproj]

I can only support further Ruben's answer with a comparison between the two messages displayed:

https://i.stack.imgur.com/61xPl.png

and the message:

C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1697,5): warning MSB3277: Found conflicts between different versions of the same dependent assembly that could not be resolved. These reference conflicts are listed in the build log when log verbosity is set to detailed.

So, Ruben's right—this is just not true. There are no conflicts whatsoever, just a missing assembly. This is especially boring when the project is an ASP.NET application, since the views are compiled on demand, that is, just before displayed for the first time. This is when it becomes necessary to have the assembly available. (There's an option to pre-compile the views together with the rest of the code, but this is another story.) On the other hand, if you set the verbosity to Diagnostic you get the following output:

C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1697,5): warning MSB3245: Could not resolve this reference. Could not locate the assembly "System.Web.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.

As a result, all you need to do is either:

Add a reference to the assembly manually (locate it on disk, maybe GAC, and add it as a "direct" reference), or Use NuGet package (if published in the gallery) to download it and reference the assembly contained within it.

More about NuGet gallery here. More about precompiling ASP.NET views here.

Changing the build verbosity in visual studio will help to point in the right direction. Follow the below steps to change verbosity in VS

Go to Tools->Options menu in VS Open Projects and Solutions->Build and Run Change the value of the MSBuild project build output verbosity. Pick one from Quiet, Minimal, Normal, Detailed and Diagnostic

Check the output window(Ctrl+Alt+O) in VS to see the changes in the build log.

Reiterating one of the comments from @elshev Right click on the solution -> Manage NuGet packages for solution -> Under Consolidate you can see if there are different versions of the same package was installed. Update the packages there. The conflict error is resolved.

and how do I then make the warning go away?

You are probably going to have to reinstall or upgrade your NuGet packages to fix this.

I'm using Visual Studio 2017 and encountered this when I updated some Nuget packages. What worked for me was to open my web.config file and find the <runtime><assemblyBinding> node and delete it. Save web.config and rebuild the project.

Look at the Error List window. You'll see what looks like a massively long warning about binding conflicts. Double-click it and it will automatically recreate the <runtime><assemblyBinding> block with the correct mappings.

As stated in dotnet CLI issue 6583 the issue should be solved with dotnet nuget locals --clear all command.

I could solve this installing Newtonsoft Json in the web project with nugget packages

Obviously there's a lot of different causes and thus a lot of solutions for this problem. To throw mine into the mix, we upgraded an assembly (System.Net.Http) that was previously directly referenced in our Web project to a version managed by NuGet. This removed the direct reference within that project, but our Test project still contained the direct reference. Upgrading both projects to use the NuGet-managed assembly resolved the issue.

If you made any changes to packages -- reopen the sln. This worked for me!

You could run the Dotnet CLI with full diagnostic verbosity to help find the issue.

dotnet run --verbosity diagnostic >> full_build.log

Once the build is complete you can search through the log file (full_build.log) for the error. Searching for "a conflict" for example, should take you right to the problem.

I found that, sometimes, nuget packages will install (what I'm guessing are) .NET Core required components or other items that conflict with the already-installed framework. My solution there was to open the project (.csproj) file and remove those references. For example, System.IO, System.Threading and such, tend to be added when Microsoft.Bcl is included via some recently installed NuGet package. There's no reason for specific versions of those in my projects, so I remove the references and the project builds. Hope that helps.

You can search your project file for "reference" and remove the conflicts. If they're included in System, get rid of them, and the build should work. This may not answer all cases of this issue - I'm making sure you know what worked for me :)

Example of what I commented out:

I just ran into this and the problem after switching a package from nuget to locally referenced dlls. The issue was old runtime binding stuff in app.config.

I followed the advice of several of the responses here to figure out what was wrong, but none of the answers seemed to explain how to fix it. My issue was that one reference required a different version of a second reference. So Newtonsoft was at version 6, but some other DLL wanted 4.5. Then I upgraded Newtonsoft as one of the other answers suggested and that made things worse.

So I actually downgraded my Newtonsoft install and the warning went away (VS 2017):

Right click References in the solution explorer and select Manage NuGet Packages... Under the "Installed" tab, find Newtonsoft (or whatever your conflict is) On the right side, a dropdown appears next to "Version" that you can change to older versions. It wasn't obvious to me that this dropdown could be used to downgrade.

Please note that I solved this problem by putting the AutoGenerateBindingRedirects right after the TargetFramework in the csproj file:

<TargetFramework>net462</TargetFramework>
<AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>

I have uninstalled Microsoft ASP.NET MVC nuget.org from manage NuGet Packagaes and again re-installed it. While re-installing it resolved all the conflicts related to razor version. Try it .

I changed the MSBuild verbosity to Diagnostic.but couldn't find where the problem was so according to the answers above I had this code in app.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<configSections>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
<sectionGroup name="userSettings" type="System.Configuration.UserSettingsGroup, System, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
<section name="XbimXplorer.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" allowExeDefinition="MachineToLocalUser" requirePermission="false" />
</sectionGroup>
</configSections>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
</startup>

So I just changed the first System, Version from 4.0.0.0 to 12.0.0.0 and my project worked.

As per the other answers, set the output logging level to detailed and search there for conflicts, that will tell you where to look next.

In my case, it sent me off in a few directions looking for the source of the references, but in the end it turned out that the problem was one of my portable class library projects, it was targeting the wrong version and was pulling its own version of the references in, hence the conflicts. A quick re-target and the problem was solved.

I had this warning after migrating to Package Reference. In diagnostic output there was information that library was referenced by the same library itself. It might be a bug of new Package Reference. The solution was to enable AutoGenerateBindingRedirects and delete custom binding redirect.

VS 2017, MVC project

I don't know why, but for me, the solution for this issue was to remove an out parameter from a model method signature that was called from the controller action method. that is very strange behaviour but that was the solution to my problem.

I have installed Newtonsoft.Json v10.0.0.3 and Newtonsoft.Json v11.X.X.X in diferente projects from nuget.org from manage NuGet Packagaes and again re-installed it (same version) . While re-installing it resolved all the conflicts related to razor version. Work for me!