.Net Tips – Xml Serialize or Deserialize Dictionary in C#

Yan Cui

I help clients go faster for less using serverless technologies.

If you try to serialize/deserialize a type which uses the generic Dictionary<TKey, TValue> type with the XmlSerializer then you’ll get an InvalidOperationException, for instance:

Here’s my class:

public class MyClass
{
    // need a parameterless constructor for serialization
    public MyClass()
    {
        MyDictionary = new Dictionary<string, string>();
    }
    public Dictionary<string, string> MyDictionary { get; set; }
}

I’ll get an InvalidOperationException with an inner exception of type NotSupportedException when I do this:

var xmlSerializer = new XmlSerializer(typeof(MyClass));

And the error message of the NotSupportedException is:

“Cannot serialize member MyClass.MyDictionary of type System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], because it implements IDictionary.”

The reason why Dictionary<TKey, TValue> is not supported by the XmlSerializer is that types such as Dictionary, HashTable, etc. needs an equality comparer which can’t be serialized into XML easily and won’t be portable anyhow. To get around this problem, you generally have two options, in the order of my personal preference:

Use DataContractSerizalizer

The easiest, and cleanest solution is to switch over to data contract serialization, and the only change required would be to mark your class with the DataContractAttribute and mark the properties you want to serialize with the DataMemberAttribute. My class would now look like this:

[DataContract]
public class MyClass
{
    // need a parameterless constructor for serialization
    public MyClass()
    {
        MyDictionary = new Dictionary<string, string>();
    }
    [DataMember]
    public Dictionary<string, string> MyDictionary { get; set; }
}

And to serialize it into Xml, just use the DataContractSerializer and write the output with a XmlTextWriter like this:

var serializer = new DataContractSerializer(typeof(MyClass));
string xmlString;
using (var sw = new StringWriter())
{
    using (var writer = new XmlTextWriter(sw))
    {
        writer.Formatting = Formatting.Indented; // indent the Xml so it's human readable
        serializer.WriteObject(writer, marketplace);
        writer.Flush();
        xmlString = sw.ToString();
    }
}

Use a custom Dictionary type

The alternative is to create your own Dictionary implementation which is Xml serializable. See the references section for an implementation Paul Welter has created.

References:

XML Serializable Generic Dictionary


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16 thoughts on “.Net Tips – Xml Serialize or Deserialize Dictionary in C#”

  1. theburningmonk

    @Nanek – imagine ‘marketplace’ being an instance of ‘MyClass’, looks like I missed out a line or two of the code snippet

  2. Thanks for the information.

    Just thought I’d add – you need to add a reference to System.Runtime.Serialization for this, and it is only available in .NET 3 upwards.

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  4. Great!
    I’d add that for Dat­a­Con­tract­Ser­iza­l­izer a reference to System.Runtime.Serialization is necessary.
    Thnaks, guy!

  5. In .Net 4.5.1 I am getting

    {“Type ‘System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]’ with data contract name ‘ArrayOfKeyValueOfstringstring:http://schemas.microsoft.com/2003/10/Serialization/Arrays‘ is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types – for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.”}

  6. Here is the class I am trying to serialize

    [DataContract]

    public class KeywordFunctionMap

    {

    [DataMember]

    public Dictionary Map { get; set; }

    // need a parameterless constructor for serialization

    public KeywordFunctionMap()

    {

    Map = new Dictionary();

    }

    }

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