how to convert datetime? to datetime
<p>I have filed <code>txtb_dateOfService</code> is required to complete the form but if <code>txtb_dateOfService</code> is emapty return null if not  TryParse the date <code>&gt;</code> I had this error I don't know how to fix it</p>

<blockquote>
  <p>The best overloaded method match for
  'System.DateTime.TryParse(string, out System.DateTime)' has some
  invalid arguments</p>
</blockquote>

<pre><code>    DateTime? dateOfService= null;
    if (string.IsNullOrEmpty(txtb_dateOfService.Text))
    {
        dateOfService = null;

    }
    else
        if (DateTime.TryParse(txtb_dateOfService.Text, out dateOfService))
        {

        }
</code></pre>
