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

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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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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