title: Does not contain a definition for Where and no extension method?
body: <p>So I've looked around for awhile for the answer to my problem, and I'm seeing to add <code>using System.Linq</code>, except I already have that there so I don't know what my code isn't compiling. In my context of code, <code>_accountreader</code> exists, so why is it saying no definition of it exists?</p>
  
  <p>The line <code>return _accountReader.Where(x =&gt; x.Age);</code> is where the compiler yells at me.</p>
  
  <pre><code>public interface IAccountReader
  {
      IEnumerable&lt;Account&gt; GetAccountFrom(string file);
  }
  
  public class XmlFileAccountReader : IAccountReader
  {
      public IEnumerable&lt;Account&gt; GetAccountFrom(string file)
      {
          var accounts = new List&lt;Account&gt;();
          //read accounts from XML file
          return accounts;
      }
  }
  
  public class AccountProcessor
  {
      private readonly IAccountReader _accountReader;
      public AccountProcessor(IAccountReader accountReader)
      {
          _accountReader = accountReader;
      }
      public IEnumerable&lt;Account&gt; GetAccountFrom(string file)
      {
          return _accountReader.Where(x =&gt; x.Age);
      }
  }
  
  public class Account
  {
      public int Age { get; set; }
  }
  </code></pre>