title: groupby filtering in linq
body: <p>i need a groupby this query</p>
  
  <pre><code>if (Randomize)
  {
      List&lt;tblInvoice&gt; _Result = db.tblInvoices.ToList();
  
      var _Temp = xxx.Where(p =&gt; (p.isPostalPayment == null || p.isPostalPayment == false)).ToList();
      _Result = _Temp.OrderBy(o =&gt; Guid.NewGuid()).Take(5).ToList();
      _Result = _Result.GroupBy(x =&gt; x.cUserID).ToList();
      return _Result;
  }
  </code></pre>
  
  <p>when i use groupby in query, error
  When I use groupby in this query, get the Under error</p>
  
  <blockquote>
    <p>Cannot implicitly convert type
    'System.Collections.Generic.List>'
    to
    'System.Collections.Generic.List'</p>
  </blockquote>