title: Error: The type or namespace name 'Interface' does not exist in the namespace (are you missing an assembly reference)
body: <p>I am developing an application using Xamarin and got stuck into this error. The error has been generated from my <strong>.Droid</strong> project while building the solution (Clean Solution is successful). The 'Interface' is a folder which is present inside my <strong>Portable PCL</strong> project. Basically I want to implement an interface (declared inside my Portable project) into my .Droid project. I have already tried to refer that Interface folder with <strong>using</strong> keyword but it says  <em>does not exist</em>. And since the folder is not getting referenced, all the interfaces inside that folder are also not getting referenced.</p>
  <p>The below code is of <strong>.Droid</strong> project file in which interface has to be implemented.</p>
  <pre><code>using ProjectName;
  using ProjectName.Interface;
  using Xamarin.Forms;
  
  [assembly: Dependency(typeof(FileName))]
  namespace ProjectName.Droid
  {
      class FileName : InterfaceName
      {
          ....
      }
  }
  </code></pre>
  <p>The below code is of one of the <strong>Portable</strong> project interface file inside Interface folder.</p>
  <pre><code>using System;
  using ProjectName;
  
  namespace ProjectName.Interface
  {
       public interface InterfaceName
       {
            ....
       }
  }
  </code></pre>
  <p>When I try to <strong>Add Reference</strong> of Portable project into .Droid project it says:</p>
  <blockquote>
  <p>Unable to add reference to Portable project. The current project's target is not one of or compatible with the targets of Portable Project.</p>
  </blockquote>
  <p>Please Note:</p>
  <ol>
  <li>I am using <strong>Visual Studio 2019</strong>.</li>
  <li>My Portable project <strong>Target Frameworks</strong> are: .NET Framework 4.5, ASP.NET Core 1.0, Windows 8, Windows 8.1, Windows Phone Silverlight 8.</li>
  <li>My Droid project Target is <strong>Android 11.0</strong>(API Level 30 -R)</li>
  <li>I have already added <strong>Mono.Android.dll</strong> file inside Portable as well as Droid project.</li>
  <li>I have tried deleting <strong>bin</strong> and <strong>obj</strong> folder of both projects but didn't worked at all.</li>
  <li>I am unable to add Reference of my Portable project into my Droid project.</li>
  <li>Also, all the interfaces inside 'Interface' folder are <strong>public</strong>.</li>
  </ol>
  <p>Any kind of help will be appreciated.</p>