title: Why I couldn't use Microsoft.AspNetCore.Mvc; in my unit testing project?
body: <p>I created an asp.net core mvc project and also created a unit testing project
  below is the code for the unit testing project:</p>
  
  <pre><code>using System;
  using Microsoft.AspNetCore.Mvc;  // error, type or namespace is missing
  using Xunit;
  
  namespace Project.Tests
  {
      public class UnitTest1
      { 
  
          [Fact]
          public void Test1()
          {
            ...
          }
      }
  }
  </code></pre>
  
  <p>I don't quite understand, I used to create unit testing projects without any problem, why I cannot access <code>Microsoft.AspNetCore.Mvc</code> this time?</p>