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When compiling an ArcObjects add-in using Microsoft Visual Studio 2015, the following error may be returned:
Error: "The ValidateAddInXMLTask task failed unexpectedly. System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.VisualStudio.Shell.12.0, Version=126.96.36.199, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified."
ArcGIS Desktop and Engine add-ins have a dependency on the Microsoft Visual Studio Shell 12.0 library.
This error occurs when using Visual Studio 2015 without this library.
Note: If Visual Studio 2013 and 2015 are both installed on the machine, this problem does not occur. Visual Studio 2013 includes the Microsoft Visual Studio Shell 12.0 library.