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copied!<p>Your problem occurs because of during deserialization XmlSerializer first create an object by calling default constructor and then call <strong>ReadXml</strong> method to set property values so <strong>ReadXml</strong> can't cancel object creation. Is it necessary to serialize null values to see it in xml? I mean that you may avoid this by using non-List collection. For example create your custom collection:</p> <pre><code>public class MyCollection : System.Collections.ObjectModel.Collection<MyFrac> { protected override void InsertItem(int index, MyFrac item) { if(item == null) return; base.InsertItem(index, item); } protected override void SetItem(int index, MyFrac item) { if(item == null) { base.RemoveAt(index); } else { base.SetItem(index, item); } } } </code></pre> <p>Use it in your <strong>MyData</strong> class:</p> <pre><code>public class MyData { [XmlArrayItem("Frac")] public MyCollection Fractions; } </code></pre> <p>And then serialization/deserialization works as you want:</p> <pre><code> public static void Main(string[] args) { var data = new MyData(); data.Fractions = new MyCollection(); 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); foreach (var element in data2.Fractions) { Console.WriteLine(element); } } Console.ReadLine(); } </code></pre> <p>Serialized xml:</p> <pre><code><?xml version="1.0" encoding="utf-16"?> <MyData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Fractions> <Frac>1/2</Frac> <Frac>3/6</Frac> </Fractions> </MyData> </code></pre> <p>Outputs:</p> <p><strong>1/2</strong></p> <p><strong>3/6</strong></p> <p><strong>Update</strong></p> <p>Ok, you need a colletion with custom serialization rules. Let's implement it:</p> <pre><code>public class MyCollection<T> : Collection<T>, IXmlSerializable where T: class { public XmlSchema GetSchema() { return null; } public void ReadXml(XmlReader reader) { var serializer = new XmlSerializer(typeof(T)); var wasEmpty = reader.IsEmptyElement; reader.Read(); if (wasEmpty) return; while (reader.NodeType != XmlNodeType.EndElement) { if (reader.IsEmptyElement) { reader.Read(); Items.Add(null); continue; } var item = (T)serializer.Deserialize(reader); Items.Add(item); } reader.ReadEndElement(); } public void WriteXml(XmlWriter writer) { var serializer = new XmlSerializer(typeof (T)); foreach (var myFrac in Items) { serializer.Serialize(writer, myFrac); } } } </code></pre> <p>Usage:</p> <pre><code>public class MyData { public MyCollection<MyFrac> Fractions; } class Program { static void Main(string[] args) { var data = new MyData(); data.Fractions = new MyCollection<MyFrac>(); 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()); Trace.WriteLine(sb.ToString()); using (var reader = new StringReader(sb.ToString())) { var data2 = (MyData)serializer.Deserialize(reader); foreach (var fraction in data2.Fractions) { var output = fraction == null ? "null" : fraction.ToString(); Console.WriteLine(output); Trace.WriteLine(output); } } Console.ReadLine(); } } </code></pre> <p>Output xml:</p> <pre><code><?xml version="1.0" encoding="utf-16"?> <MyData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Fractions> <MyFrac>1/2</MyFrac> <MyFrac xsi:nil="true" /> <MyFrac xsi:nil="true" /> <MyFrac>3/6</MyFrac> </Fractions> </MyData> </code></pre> <p>Output data:</p> <p><strong>1/2</strong></p> <p><strong>null</strong></p> <p><strong>null</strong></p> <p><strong>3/6</strong></p> <p>I think this is what you want.</p>

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