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POCustom XmlSerializer issue with null elements in a List<>
 

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    <p>I have a class that I want a custom serializer for. And I will sometimes use this class within a List, that I also want to serialize. Some of those elements will be null.</p> <p>I can get serialization working:</p> <pre><code>&lt;MyData xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"&gt; &lt;Fractions&gt; &lt;Frac&gt;1/2&lt;/Frac&gt; &lt;Frac xsi:nil="true" /&gt; &lt;Frac xsi:nil="true" /&gt; &lt;Frac&gt;3/6&lt;/Frac&gt; &lt;/Fractions&gt; &lt;/MyData&gt; </code></pre> <p>But de-serialization is not working when null elements are present like above. The List&lt;> serializer appears to be invoking ReadXml() for a element vs. just creating a null element in the list.</p> <p>When I run my example, the de-serialized version is:</p> <pre><code>1/2 / / 3/6 </code></pre> <p>I.e. a MyFrac object was created instead of null in element 1 and 2.</p> <p>Do I have to create a custom serializer for a List sub-class to get around this, or am I missing some other way to get null elements on de-serialize? If custom serializer, any best approach/code?</p> <p>I have a complete example below which shows my current implementation.</p> <pre><code>public class MyFrac : IXmlSerializable { public string N; public string D; public override string ToString() { return N + "/" + D; } System.Xml.Schema.XmlSchema IXmlSerializable.GetSchema() { return null; } void IXmlSerializable.ReadXml(System.Xml.XmlReader reader) { if (reader.IsEmptyElement &amp;&amp; reader.NodeType != XmlNodeType.EndElement) { reader.Read(); return; } reader.ReadStartElement(); string sfrac = reader.ReadString(); try { var m = Regex.Match(sfrac, @"(\d+)/(\d+)"); if (!m.Success) throw new Exception(sfrac + " was not in the correct format"); N = m.Result("$1"); D = m.Result("$2"); } finally { reader.ReadEndElement(); } } void IXmlSerializable.WriteXml(System.Xml.XmlWriter writer) { writer.WriteString(N + "/" + D); } } public class MyData { [XmlArrayItem("Frac")] public List&lt;MyFrac&gt; Fractions; } public static void Run() { var data = new MyData(); data.Fractions = new List&lt;MyFrac&gt;(); data.Fractions.Add(new MyFrac { N = "1", D = "2" }); data.Fractions.Add(null); data.Fractions.Add(null); data.Fractions.Add(new MyFrac { N = "3", D = "6" }); var serializer = new XmlSerializer(typeof(MyData)); StringBuilder sb = new StringBuilder(); using (var writer = new StringWriter(sb)) { serializer.Serialize(writer, data); } // Dump XML Console.WriteLine(sb.ToString()); using (var reader = new StringReader(sb.ToString())) { var data2 = (MyData)serializer.Deserialize(reader); Console.WriteLine(data2.Fractions[0]); Console.WriteLine(data2.Fractions[1]); Console.WriteLine(data2.Fractions[2]); Console.WriteLine(data2.Fractions[3]); } } </code></pre>
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