Why doesn't XmlSerializer support Dictionary?

Question

Just curious as to why Dictionary is not supported by XmlSerializer?

You can get around it easily enough by using DataContractSerializer and writing the object to a XmlTextWriter, but what are the characteristics of a Dictionary that makes it difficult for a XmlSerializer to deal with considering it's really an array of KeyValuePairs.

In fact, you can pass an IDictionary<TKey, TItem> to a method expecting an IEnumerable<KeyValuePairs<TKey, ITem>>.

score 33 · Accepted Answer · answered May 26 '10 at 09:18

33

Hashtables need hashcode and equality comparer providers generally. These cant be serialized easily in XML, and definitely will not be portable.

But I think you already found your answer. Just serialize the hashtable as a List<KeyValuePair<K,V>> and then (re)construct it into a hashtable.

answered May 26 '10 at 09:18

leppie

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Or use that code: http://weblogs.asp.net/pwelter34/archive/2006/05/03/444961.aspx – prostynick May 26 '10 at 09:30
4

What methods/properties in Dictionary imply it must be implemented as a hash table? Dictionary and IDictionary are collections of Key/Value pairs and are distinct from the Hashtable implementation. From a serialization perspective our goal seems to be to take data out of volatile memory to be able to load it back in at a later time. Reloading *any* data into a different concrete implementation of an interface is going to result in different behavior. – Jeremy Jun 16 '11 at 12:48
4

Serializing KeyValuePair does not work. You'll end up with an empty tag. See: https://stackoverflow.com/questions/2658916/serializing-a-list-of-key-value-pairs-to-xml – Maxwin Jul 05 '17 at 08:56

score 7 · Answer 2 · edited Jul 19 '18 at 16:19

7

This is waaay late - but I found this question whilst looking for the answer myself, and thought I'd share my eventual answer which was to replace XmlSerializer with a different tool that will serialize everything:

http://www.sharpserializer.com

It worked for me straight out of the box, serializing Dictionaries, and multi-layered custom types, and even generics using interfaces as type arguments. Also has a fully permissive license.

Thank you Pawel Idzikowski!

edited Jul 19 '18 at 16:19

jv42

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answered Sep 22 '11 at 22:40

Nich Overend

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DataContractSerializer will serialize dictionaries as well, and is now part of the .NET Framework. – Eric J. Mar 16 '12 at 18:11

Wojciech Nagórski · Answer 3 · 2018-02-06T10:43:31.107

You can use ExtendedXmlSerializer. If you have a class:

public class TestClass
{
    public Dictionary<int, string> Dictionary { get; set; }
}

and create instance of this class:

var obj = new TestClass
{
    Dictionary = new Dictionary<int, string>
    {
        {1, "First"},
        {2, "Second"},
        {3, "Other"},
    }
};

You can serialize this object using ExtendedXmlSerializer:

var serializer = new ConfigurationContainer()
    .UseOptimizedNamespaces() //If you want to have all namespaces in root element
    .Create();

var xml = serializer.Serialize(
    new XmlWriterSettings { Indent = true }, //If you want to formated xml
    obj);

Output xml will look like:

<?xml version="1.0" encoding="utf-8"?>
<TestClass xmlns:sys="https://extendedxmlserializer.github.io/system" xmlns:exs="https://extendedxmlserializer.github.io/v2" xmlns="clr-namespace:ExtendedXmlSerializer.Samples;assembly=ExtendedXmlSerializer.Samples">
  <Dictionary>
    <sys:Item>
      <Key>1</Key>
      <Value>First</Value>
    </sys:Item>
    <sys:Item>
      <Key>2</Key>
      <Value>Second</Value>
    </sys:Item>
    <sys:Item>
      <Key>3</Key>
      <Value>Other</Value>
    </sys:Item>
  </Dictionary>
</TestClass>

You can install ExtendedXmlSerializer from nuget or run the following command:

Install-Package ExtendedXmlSerializer

Why doesn't XmlSerializer support Dictionary?

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