title: C# XmlSerializer: Deserialize empty GUID value from XML file
body: <p>I have a configuration file which contains XML like this:</p>
  
  <pre><code>&lt;XmlParameter Title="Analysis"&gt;
      &lt;ProcedureName&gt;XmlParameterAnalysisGetFirst&lt;/ProcedureName&gt;
      &lt;ProcedureGroupId/&gt;
      &lt;Returns&gt;Table&lt;/Returns&gt;
      &lt;Params&gt;
          &lt;Param OrderKey="0" Name="xmlParameters"&gt;%xmlParameters%&lt;/Param&gt;
      &lt;/Params&gt;
  &lt;/XmlParameter&gt;
  </code></pre>
  
  <p>I use the <code>XmlSerializer</code> class to deserialize the file during runtime into an object of type <code>XmlParameter</code>:</p>
  
  <pre><code>public class XmlParameter {
      [XmlAttribute]
      public string Title { get; set; }
  
      public string ProcedureName { get; set; }
  
      [XmlElement(IsNullable=true)]
      public Guid? ProcedureGroupId { get; set; }
  
      public string Returns { get; set; }
  
      public List&lt;XmlCLRParam&gt; Params { get; set; }
  }
  </code></pre>
  
  <p>But when I try to deserialize the file with this code block:</p>
  
  <pre><code>var deserializer = new XmlSerializer(typeof(XmlParameter);
  var reader = new StreamReader(this.FilePath);
  var obj  = deserializer.Deserialize(reader);
  </code></pre>
  
  <p>I get an <code>System.InvalidOperationException</code>:</p>
  
  <blockquote>
    <p>Error in XML-Document. ---> Sytem.Format.Exception: Unknown Guid-format.</p>
  </blockquote>
  
  <p>My question is whether there is a way to deserialize <code>nullable</code> GUID values from the XML-file</p>