title: Project 'ClassLibrary1.csproj' targets 'netstandard2.1'. It cannot be referenced by a project that targets '.NETFramework,Version=v4.8'
body: <p>I have some class library projects in targets <code>netstandard2.1</code>.</p>
  
  <p>When I referenced that to my WPF project in target <code>.NET Framework v4.8</code>, On building time I get an error:</p>
  
  <blockquote>
    <p>Severity   Code    Description Project File    Line    Suppression State
    Error       Project '..\ClassLibrary1\ClassLibrary1.csproj' targets 'netstandard2.1'. It cannot be referenced by a project that targets '.NETFramework,Version=v4.8'.   WpfApp1     </p>
  </blockquote>
  
  <p>What can I do?  </p>
  
  <p>Update:
  I read <a href="https://stackoverflow.com/questions/53083554/project-classlibrary1-csproj-targets-netcoreapp2-1-it-cannot-be-referenced">this</a> Q/A about referencing <code>netcoreapp2.1</code> and in that answer telling about add <code>netstandar2.1</code>. But my libraries are already in targets <code>netstandard2.1</code>!</p>