title: ConcurrentDictionary AddOrUpdate a list
body: <p>I'm trying to use a ConcurrentDictionary to help with a filtering task.</p>
  
  <p>If a number appears in list, then I want to copy an entry from one dictionary to another.</p>
  
  <p>But this part of the AddOrUpdate is not right -  <code>v.Add(number)</code> </p>
  
  <p>I get </p>
  
  <blockquote>
    <p>"Cannot implicitly convert type 'void' to
    'System.Collections.Generic.List'</p>
  </blockquote>
  
  <p>And two more errors.</p>
  
  <pre><code>class Program
  {
      static void Main(string[] args)
      {
          Program p = new Program();
          List&lt;int&gt; filter = new List&lt;int&gt; {1,2};
          p.Filter(filter);
      }
  
  
      private void Filter(List&lt;int&gt; filter)
      {
          Dictionary&lt;string, List&lt;int&gt;&gt; unfilteredResults = new Dictionary&lt;string, List&lt;int&gt;&gt;();
          unfilteredResults.Add("key1", new List&lt;int&gt; { 1,2,3,4,5});
  
          ConcurrentDictionary&lt;string, List&lt;int&gt;&gt; filteredResults = new ConcurrentDictionary&lt;string, List&lt;int&gt;&gt;();
  
          foreach (KeyValuePair&lt;string, List&lt;int&gt;&gt; unfilteredResult in unfilteredResults)
          {
              foreach (int number in unfilteredResult.Value)
              {
                  if (filter.Contains(number))
                  {
                      filteredResults.AddOrUpdate(unfilteredResult.Key, new List&lt;int&gt; { number }, (k, v) =&gt; v.Add(number));
                  }
              }
          }
      }
  }
  </code></pre>