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]