title: How to make some parameters of `Func<>` optional?
body: <pre><code> var extraRules = new List&lt;Func&lt;string, string, string, bool&gt;&gt;
   {
          // Current letter is alphanumeric
          letter =&gt; Regex.IsMatch(letter, &quot;[a-zA-Z]&quot;),
   }
  </code></pre>
  <p>How do I make the second and third string optional?</p>
  <p>I tried doing something like:</p>
  <p><code>private delegate bool Rules(string letter, string nextLetter = null, string nextNextLetter = null);</code></p>
  <p>and then:</p>
  <pre><code>var extraRules = new List&lt;Rules&gt;
   {
          // Current letter is alphanumeric
          letter =&gt; Regex.IsMatch(letter, &quot;[a-zA-Z]&quot;),
   }
  </code></pre>
  <p>But I get <code>Incompatible anonymous function signature</code></p>
  <p>How can I make some parameters of <code>Func&lt;&gt;</code> optional?</p>