title: Missing DLLImport even though there is a "using InteropServices"
body: <p>I have the following code:</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Runtime.InteropServices;
  using System.Web;
  using System.Web.UI;
  using System.Web.UI.WebControls;
  using MapsApp.DB;
  
  namespace MapsApp
  {
      public partial class _Default : System.Web.UI.Page
      {
          [DLLImport("GeoUrbanApp.exe")]
          public static extern double CalcFigure(double east, double north, double size);
  ...
  </code></pre>
  
  <p>I am trying to call the CalcFigure function from the .exe. I've added it in the references, and trying to import it. All I get is:</p>
  
  <pre><code>The type or namespace name 'DLLImport' could not be found (are you missing a using directive or an assembly reference?)
  
  The type or namespace name 'DLLImportAttribute' could not be found (are you missing a using directive or an assembly reference?)
  </code></pre>
  
  <p>The solution most people find online is the "using System.Runtime.InteropServices;" but I have it.</p>