title: Cannot find namespace System.IdentityModel.Services
body: <p>I'm trying to provide user authentication by implementing <code>UserNamePasswordValidator</code>. I have added <code>Microsoft.IdentityModel</code> and <code>System.IdentityModel.Tokens.ValidatingIssuerNameRegistry</code> nuget packages.</p>
  <p>I'm getting and error when I implement <code>UserNamePasswordValidator</code> in my class <code>UserAuthentication</code> saying</p>
  <blockquote>
  <p>The type or namespace name Selectors does not exist in namespace
  <code>System.IdentityModel</code> (are you missing some reference).</p>
  </blockquote>
  <p>Why is this failing?</p>
  <pre><code>public class UserAuthentication : System.IdentityModel.Selectors.UserNamePasswordValidator
  {
      public override void Validate(string userName, string password)
      {
          try
          {
              if (userName == &quot;test&quot; &amp;&amp; password == &quot;test123&quot;)
              {
                  Console.WriteLine(&quot;Authentic User&quot;);
              }
          }
          catch (Exception ex)
          {
              throw new FaultException(&quot;Unknown Username or Incorrect Password&quot;);
          }
      }
  }
  </code></pre>