title: VS2015-The default XML namespace of the project must be the MSBuild XML namespace
body: <p>I am trying to add an existing project to my source project in VS2015, and I am getting the following error:</p>
  
  <blockquote>
    <p>Visual Studio 2015 - The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add 
    xmlns="http://schemas.microsoft.com/developer/msbuild/2003" to the  element. if the project has been authored  in the old 1.0 or 1.2 format, please convert it to MSBuild 2003 fromat.</p>
  </blockquote>
  
  <p>How can I solve this?</p>